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Assessing the indirect photochemical transformation of dissolved combined amino acids through the use of systematically designed histidine-containing oligopeptides Chiheng Chu, Rachel A. Lundeen, Michael Sander, and Kristopher McNeill Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03498 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015
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Environmental Science & Technology
Manuscript
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Assessing the indirect photochemical transformation of dissolved combined amino acids
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through the use of systematically designed histidine-containing oligopeptides
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Chiheng Chu, Rachel A. Lundeen, Michael Sander, and Kristopher McNeill *
7 8
Institute of Biogeochemistry and Pollutant Dynamics (IBP), Department of Environmental Systems Science, ETH Zurich, 8092 Zurich, Switzerland
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*Corresponding author
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Kristopher McNeill
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Tel. +41 (0)44 6324755; Fax. +41 (0)44 6321438
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Email:
[email protected]
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Number of Figures:
4
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Number of Tables:
0
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Total word count:
6991
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Abstract
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Photooxidation is an important abiotic transformation pathway for amino acids (AAs) in
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sunlit waters. While dissolved free AAs are well studied, the photooxidation of dissolved
24
combined AAs (DCAAs) remains poorly investigated. This study was a systematic investigation
25
of the effect of neighboring photostable AA residues (i.e., aliphatic, cationic, anionic, aromatic
26
residues) on the environmental indirect photochemical transformation of histidine (His) in His-
27
containing oligopeptides. The pKa values of His in the peptides were found to be between 4.3
28
and 8.1. Accordingly, the phototransformation rate constants of the His-containing peptides were
29
highly pH-dependent in an environmentally relevant pH range with higher reactivity for neutral
30
His than for the protonated species. The photostable AA residues significantly modulated the
31
photoreactivity of oligopeptides either through altering the accessibility of His to
32
photochemically produced oxidants or through shifting the pKa values of His residues. In
33
addition, the influence of neighboring photostable AA residues on the sorption-enhanced
34
phototransformation of oligopeptides in solutions containing chromophoric dissolved organic
35
matter (CDOM) was assessed. The constituent photostable AA residues promoted sorption of
36
His-containing peptides to CDOM macromolecules, through electrostatic attraction, hydrophobic
37
effect, and/or low-barrier hydrogen bonds, and subsequently increased the apparent
38
phototransformation rate constants by up to two orders of magnitude.
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INTRODUCTION
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Dissolved free and combined amino acids (AAs) make up the largest fraction of the
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identifiable pool of dissolved organic nitrogen1,2 and a major source of bioavailable nitrogen for
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aquatic microorganisms.3,4 While biotic and abiotic transformation processes of dissolved AAs
43
are of great biogeochemical importance, these processes are currently not well understood.
44
Uptake into microbial cells is expected to be an important environmental fate for free AAs. By
45
comparison, dissolved combined AAs (DCAAs; e.g., peptides and proteins) are structurally
46
more complex5,6 and require further hydrolysis to liberate smaller AAs before they can be
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assimilated by microorganisms. DCAAs are susceptible to abiotic transformation processes
48
occurring in surface waters, including photochemical transformation1 and sorption to inorganic
49
particles7 and natural organic matter.8,9
50
The photochemical transformation of photolabile free AAs, through either direct or indirect
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photooxidation, has been assessed in sunlit waters.10-14 Yet few studies have examined the
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susceptibility of these AAs to phototransformation when present as DCAAs.5,6 Recent studies on
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dissolved AAs photochemistry have focused on examining the photooxidation of free histidine
54
(His)10,15 and His residues in intact proteins.16 His does not undergo direct phototransformation
55
by sunlight but rather the photoreactivity of His in natural waters is dominated by reaction with
56
singlet oxygen (1O2).10 In addition, the 1O2-mediated phototransformation of free His is pH-
57
dependent with higher reactivity of neutral His than for the protonated species.15,17 This pH-
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dependence modulates His reactivity in natural waters because the imidazole moiety has a pKa of
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6.0 and hence His undergoes protonation/deprotonation reactions in the environmentally relevant
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pH range.
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Studies examining abiotic processes (e.g., photochemical transformation and sorption) of free
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His provide a suitable foundation to further our understanding of His photooxidation in the
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context of higher order structures (i.e., DCAAs). Previous studies suggested that the photolabile
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residues exclusively dominated the photoreactivity of oligopeptides.18-20 However, other studies
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argued that the neighboring photostable residues and the relative position of photolabile residues
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within the sequence also influenced the reactivity of oligopeptides.16,21-24 For instance, the
67
neighboring photostable AA residues may induce a shift in the pKa value of a photolabile residue,
68
which subsequently affects the photoreactivity of DCAAs. Shifts in the pKa values of AAs and
69
the underlying mechanisms that result in these shifts have been extensively studied by
70
biochemists.25-29 The conformation, functionality and stability of many oligopeptides and
71
proteins are tied to structure-dependent alterations of AA pKa values.30-34 Likewise, the
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environmental photoreactivities of DCAAs are expected to vary depending on the pKa values of
73
their constituent photolabile residues, especially for AAs that have environmentally relevant pKa
74
values, namely histidine.
75
The indirect phototransformation of DCAAs in chromophoric dissolved organic matter
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(CDOM) solutions is of particular interest because their phototranformation rates might be
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greatly enhanced through sorption to CDOM. In natural waters, CDOM acts both as the major
78
sensitizer of photochemically produced reactive intermediates (PPRI)35 and as an important
79
sorbent.15 PPRI play an important role in various biological and chemical transformation
80
processes. Molecules associated with CDOM have been shown to be exposed to high PPRI
81
concentrations within and near the CDOM macromolecules and undergo enhanced
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phototransformation rates.36-42 For free His, moderate (3- to 4-fold) enhancements in apparent
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1
O2 reaction rate constants were observed in CDOM solutions at pH values below 6.0, due to
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sorption of the protonated His to CDOM that overcompensated the lower reactivity of this
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species with 1O2.15 The 1O2 reaction rate enhancements of His-containing DCAAs resulting from
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sorption to CDOM are expected to be much higher if the His residue in the CDOM-associated
87
DCAAs remains in its more reactive neutral form. The sorption-enhanced phototransformation
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of DCAAs is of particular interest because sorption of DCAAs to CDOM occurs universally and
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via numerous association processes.43-45 To our knowledge, a systematic assessment of the
90
abiotic transformation processes of DCAAs has not been conducted.
91
In this study, we aim to assess the indirect photochemical transformation of His residues with
92
1
93
heptapeptides, as representative DCAAs having only primary protein structure (Figure 1). Each
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oligopeptide contained one photooxidizable His residue that was combined with up to six
95
photostable AA residues with varying physicochemical properties in a variety of combinations.
96
We conducted 1O2-mediated photooxidation studies with the His-containing oligopeptides in
97
order (i) to examine the effects of neighboring residues (i.e., steric or charge interactions) on the
98
rates of His residue photochemical transformation and (ii) to determine how charged or
99
hydrophobic residues influence sorption-enhanced phototransformation of His-containing
100
oligopeptides in solutions containing CDOM. For aim (i), we examined a series of His-
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containing oligopeptides that also contained either neighboring aliphatic, cationic, anionic, or
102
aromatic moieties (Figure 1) to help understand how the physicochemical properties of
103
neighboring AAs influence the photooxidation of His in DCAAs. For aim (ii), we systematically
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designed His-containing oligopeptides with AAs that varied in charge or hydrophobicity (Figure
105
1) to help investigate the enhancements in phototransformation rates in CDOM solutions due to
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sorption of the oligopeptides to the CDOM macromolecules via various interactions.
O2 in DCAAs. We designed His-containing oligopeptides, including di-, hexa- and
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MATERIALS AND METHODS
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Chemicals, CDOM and oligopeptides. Sources and preparation methods of chemicals,
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CDOM and oligopeptides are detailed in the Supporting Information (SI). Suwannee River
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Natural Organic Matter (SRNOM, 1R101N) was chosen as the model CDOM.
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The studied His-containing oligopeptides were ascribed to five subgroups, depending on the
112
numbers and properties of constituent photostable AA residues (Figure 1): (i) charged
113
dipeptides, including HR and HH (H = histidine, R = arginine), (ii) hydrophobic dipeptides,
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represented by HF (F = phenylalanine; F is aromatic and among the most hydrophobic amino
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acids), (iii) charged heptapeptides, including DDDHDDD, AAAHAAA, RAAHAAR,
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RARHRAR, and RRRHRRR (A = alanine, D = aspartic acid), (iv) heptapeptides that varied in
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hydrophobicity, including AAAHAAA, FAAHAAF, FAFHFAF, FFFHFFF (see Table S2 for
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pepetide hydrophobicity), and (v) polyfunctional peptides, including carnosine (β-AH),
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FHGTVK and AGAHLK (K = lysine, G = glycine, T = threonine, V = valine, L = leucine).
120
Carnosine is highly concentrated in muscle and brain tissues where it acts as scavenger of
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reactive oxygen species46-48 and likely to be an important His-containing peptide in natural
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waters. FHGTVK and AGAHLK were previously studied in our group as peptides generated by
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trypsin digestion of glyceraldehyde-3-phosphate dehydrogenase (GAPDH).16
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Photolysis of His-containing oligopeptides in lumichrome- and SRNOM-sensitized
125
systems. The sensitized photolyses of His-containing oligopeptides were carried out in separate
126
experimental setups over a wide pH range using either lumichrome (10 µM) or SRNOM (11.4
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mg C/L) as 1O2-sensitizers, that resulted in homogeneous and microheterogeneous distributions
128
of 1O2 in solutions, respectively.15,37 Dipeptide solutions were prepared with the following pH-
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buffering species: sodium acetate (pH 4.0-6.0), phosphate (pH 6.0-7.8), or borate (above pH 7.8).
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Hexapeptide and heptapeptide solutions were prepared with the following pH-buffering species:
131
ammonium acetate buffer (pH 4.0-6.0) or Tris-acetate buffer (pH 6.5-8.5). Each photolysis
132
solution contained one of the 15 oligopeptides studied at an initial concentration of 40 µM, a pH
133
buffering species (5 mM), furfuryl alcohol (FFA; initial concentrations of 40 µM), and either
134
lumichrome or SRNOM as the 1O2 sensitizer. Solutions containing oligopeptides and SRNOM
135
were stored overnight in the dark at 4 °C to allow for attainment of apparent sorption
136
equilibrium before starting the photolysis (see Section S7 for the evidence of sorption
137
equilibrium attainment). Sensitizer-free solutions prepared at pH 8 served as controls to assess
138
direct phototransformation of the oligopeptides. All photolyses were conducted in borosilicate
139
test tubes using a photochemical reactor (Rayonet) equipped with 365 nm bulbs (Southern New
140
England Ultraviolet Co., RPR-3500 Å). Aliquots were taken from the sample solution at certain
141
time points and split for quantification of FFA and oligopeptide (described below).
142
Analysis of oligopeptides. Aliquots from the photolysis experiments of individual synthetic
143
oligopeptides were sampled and the concentrations of oligopeptides were analyzed using one of
144
two methods: (i) Dipeptides were derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl
145
carbamate (AQC) using previous methods15,16,49 and subsequently analyzed by ultra high-
146
pressure liquid chromatography (UPLC, Waters ACQUITY) coupled to a fluorescence detector
147
(see SI for further details); (ii) Aliquots of hexa- or heptapeptides photolysis solutions were
148
immediately mixed with a mixture of peptide internal standards and subsequently analyzed on a
149
Waters ACQUITY nanoUPLC coupled to an Orbitrap high resolution mass spectrometry
150
(HRMS) detector equipped with electrospray ionization (ESI-HRMS, Thermo Exactive). All
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analyses were carried out in positive ionization mode. Previously published nanoUPLC-ESI-
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HRMS methods16 were followed, as detailed in the SI.
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Determination of steady-state 1O2 concentration. FFA was used as a probe to determine
154
the steady-state 1O2 concentration in the bulk aqueous phase ([1O2]aq). FFA concentrations were
155
determined using a Waters ACQUITY UPLC coupled with a photodiode array detector. Detailed
156
information on sample preparation, separation by UPLC and detection are provided in the SI. A
157
steady-state 1O2 concentration was obtained by dividing the pseudo-first-order rate constant of
158
FFA FFA degradation by its reaction rate constant with 1O2 ( krxn = 8.3 × 107 M–1s–1).50 In SRNOM-
159
containing solutions with a microheterogeneous distribution of 1O2, the FFA-measured 1O2
160
concentration reflects the apparent 1O2 concentration in aqueous phase.
161
Calculation of 1O2 reaction rate constants of oligopeptides. The apparent reaction rate
162
constants (krxn, M–1s–1) of oligopeptides were calculated by dividing each pseudo-first-order
163
phototransformation rate constant by the FFA-measured 1O2 concentration. In lumichrome-
164
containing systems with homogenous 1O2 distributions, the determined second-order rate
165
constants for oligopeptides represent true bimolecular rate constants. In the SRNOM-containing
166
solutions with microheterogeneous distributions of 1O2, sorbed oligopeptides experienced higher
167
local 1O2 concentration than the FFA-measured 1O2 concentration in bulk solution. Thus, the
168
apparent krxn are higher than the intrinsic oligopeptide rate constants.
169
Calculation of pH-dependent 1O2 reaction rates of oligopeptides in lumichrome-
170
sensitized systems. The pH-dependent reaction rate constants in lumichrome-sensitized systems
171
( k aqpred ) were modeled considering the respective reaction rate constants of deprotonated and
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protonated His residues in oligopeptides ( krxn and
His0
His+ rxn
k
), and their respective fractions (
f
0
His
and 8
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f
+ His
) (Equation 1). We adopted a 3700-fold higher 1O2 reaction rate for the neutral His than
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protonated species in His-containing oligopeptides from a previous study (i.e.,
175
3700).15 His0
aq
0
His rxn
k
His+
krxn,calc = krxn fHis + krxn fHis 0
176 177
()
+
The fractions of deprotonated or protonated His residues (
His+
/ krxn =
f
0
His
and
f
+ His
) were calculated
based on the pKa of imidazole sidechains and the solution pH (Equation 2 and 3).
fHis =
1 + 1+[H ]/Ka
()
fHis =
1 + 1+Ka /[H ]
()
0
+
178
The measured oligopeptide phototransformation rate constants in lumichrome-sensitized
179
systems ( k aqrxn ) at various pH values were fit to Equation 1. The resulting fit allowed for
180
aq calculation of oligopeptide reaction rate constants ( krxn,calc ) as a function of pH as well as the
181
His acquisition of krxn and the pKa values of His imidazoles in these oligopeptides (Figure 1).
0
182 183
RESULTS AND DISSUSSION
184
Phototransformation rates of His-containing oligopeptides in lumichrome-sensitized
185
systems. The photochemical transformation rate constants of His-containing oligopeptides in
186
lumichrome-sensitized systems were assessed as a function of solution pH. The steady-state
187
concentrations of 1O2 in lumichrome-sensitized systems are summarized in the SI (Table S3 and
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S4).
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experiments, followed first-order kinetics at all investigated pH values (Figure 2a, shows
190
AAAHAAA as an example). The pseudo-first-order phototransformation rate constants (kobs, s–1)
191
of AAAHAAA increased strongly from pH 4.1 to pH 8.0. Control experiments with no 1O2
192
sensitizer showed no measurable direct phototransformation of AAAHAAA (Figure 2a). The
193
apparent 1O2 reaction rate constants ( k rxn , M–1s–1) of AAAHAAA were calculated by dividing
194
kobs by the FFA-measured 1O2 concentrations (Figure 2c and 2d).
195
The phototransformation of each His-containing oligopeptide, determined in separate
aq
Following the same approach as described above for AAAHAAA, the
aq rxn
k of each His-
196
containing oligopeptide was determined in lumichrome-sensitized systems at various pH values
197
(Figure 2). The
198
pH. The increases were well described by Equation 1 (Figure 2b-e) and therefore followed the
199
pH dependence of the reactivity of free His with 1O2, where the neutral imidazole in His0
200
exhibited much higher reactivity than the protonated imidazolium form, His+.15 Furthermore,
201
none of the His-containing oligopeptides underwent direct phototransformation in sensitizer-free
202
solutions (data shown only for AAAHAAA in Figure 2a), consistent with previous studies on
203
free His photochemistry.15 The good fits of experimental
204
investigated oligopeptides validate using
205
reaction rate constants in lumichrome-sensitized systems (Table S2 and S3).
k values of all His-containing oligopeptides with 1O2 increased with solution aq rxn
k
aq rxn
data by Equation 1 for
aq to represent the 1O2-mediated oligopeptide krxn,calc
206
Effect of neighboring photostable AAs on the photoreactivity of His in oligopeptides. The
207
data in Figure 2 show that the photoreactivity of His residues in the studied oligopeptides is
208
differentially modulated by neighboring residues. Neighboring photostable AAs may have three
209
effects on photoreactivity of His in oligopeptides: (i) shifts in the pKa of His alter the ratio of 10 ACS Paragon Plus Environment
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more reactive (His0) to less reactive (His+) forms; (ii) electronic effects may influence the
211
intrinsic reactivity of His toward 1O2; and (iii) the accessibility of His to 1O2 may be depressed
212
by aggregation or steric hinderance.
213
To assess the first possibility, the pKa values for the His imidazole residues were extracted for
214
each studied oligopeptide from fits of phototransformation rates to Equation 1. The inflections
215
in Figure 2 indicate the shifts of pKa values of His in the oligopeptides, which ranged from 4.3
216
(FFFHFFF) to 8.1 (DDDHDDD) (Figure 1).
217
The pKa values of His residues neighbored by negatively and positively charged AAs
218
increased and decreased, respectively, relative to free His (pKa = 6.0). The shifts in the pKa
219
values can be rationalized on the basis of electrostatic effects. For instance, the imidazolium
220
form of His in DDDHDDD (pKa = 8.1) was stabilized by the negatively charged carboxylate
221
groups of D, thereby requiring high pH values larger than 6.0 to achieve deprotonation. The
222
opposite was true for RRRHRRR in which the positively charged guanidinium in R
223
electrostatically hindered protonation of His (pKa = 4.7). The low pKa values of 4.6 and 4.3 for
224
FAFHFAF and FFFHFFF were likely due to formation of peptides aggregates (discussed in
225
more detail below). Protonated His is known to be stabilized by hydrogen bonding with water.51
226
This stabilization effect is diminished for His in aggregates due to limited ability of water to
227
interact with the imidazolium sidechain. We attribute the high pKa values of 7.4 and 7.6 for HR
228
and HF, respectively, to an internal hydrogen-bond between the imidazolium group and the C-
229
terminal carboxylate (see insert). While hydrogen-bond is, in principle, possible also for HH, the
230
observed pKa value of His in HH (6.2) is believed to depict only the C-terminal His imidazole
231
group, which cannot participate in internal hydrogen-bonding with the C-terminal carboxylate.
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Internal hydrogen-bond in HR, HF, and HH 232
The results suggest that the electrostatic effects and the microenvironment induced by
233
neighboring AA residues may significantly shift the pKa values of His in oligopeptides, thereby
234
also affecting the protonation equilibrium and thus its reactivity towards 1O2. For example, at pH
235
5.0, the
236
RAAHAAR, (2.4 ± 0.1) × 107 M–1s–1 for RARHRAR, and (4.7 ± 0.4) × 107 M–1s–1 for
237
RRRHRRR, which corresponds to a 23-fold range in the reactivity in this series (Figure 2c).
k values were (0.2 ± 0.1) × 107 M–1s–1 for DDDHDDD, (0.8 ± 0.1) × 107 M–1s–1 for aq rxn
238
In addition to changes in the reactivity induced by shifts in the His pKa value, there was also
239
evidence for electronic effects that modulated the intrinsic photoreactivity of the His residue in
240
the oligopeptides. To separate the electronic effects from the pKa effect,
241
for comparison (Figure 1). In this study, the four most reactive oligopeptides with
242
more than 50% greater than free His (6.5 × 107 M–1s–1) were all dipeptides: HH (1.3 × 108 M-1 s-
243
1
244
2e). The high 1O2 reactivity of HH can be rationalized by two photoreactive sites in HH;
245
however, this rationalization does not hold for the other dipeptides. The cause of the high
246
reactivity of these His-containing dipeptides is not known, nor is it known whether it is a general
247
phenomenon for other His-containing dipeptides.
248 249
0
His rxn
k
values were used 0
His rxn
k
values
), HF (1.3 × 108 M-1 s-1), HR (1.4 x 108 M-1 s-1), and carnosine (1.1 x 108 M-1 s-1) (Figure 1 and
The photochemical data also indicates evidence for steric effects reducing the reactivity of certain oligopeptides. In particular, variations of 1O2-mediated
0
His rxn
k
were observed in the 12
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0
His rxn
k
value of FAAHAAF (6.6 × 107 M–1s–1) was
250
hydrophobic heptapeptide series. While the
251
similar to free His (6.5 × 107 M–1s–1), FAFHFAF and FFFHFFF showed much lower reactivity
252
(1.3 × 107 M–1s–1 and 8.4 × 106 M–1s–1, respectively) (Figure 1 and 2d). We subsequently
253
hypothesized that intra- and interpeptide aggregation for FAFHFAF and FFFHFFF occurred
254
between the F residues when peptides were dissolved in water (as F is among the most
255
hydrophobic AAs). In an aggregate, the encapsulated His residues could be significantly
256
protected from 1O2 oxidation, similar to the protective effect observed where His residues were
257
buried within a folded protein structure.16
258
To test the hypothesis of aggregation-induced decrease in 1O2 reactivity, the 1O2-mediated
259
photolyses of AAAHAAA, FAAHAAF, FAFHFAF and FFFHFFF (separately) were further
260
conducted in ethanol, in which aggregation through the hydrophobic effect should be strongly
261
diminished. Photolysis results show that much higher
262
FFFHFFF ((5.5 ± 0.1) × 107 M–1s–1 and (5.4 ± 0.3) × 107 M–1s–1, respectively) in ethanol than in
263
water ((1.0 ± 0.1) × 107 M–1s–1 and (9.3 ± 0.1) × 106 M–1s–1, respectively), consistent with
264
aggregation-induced decrease in 1O2 reactivity in water. By comparison, AAAHAAA and
265
FAAHAAF showed no evidence of forming aggregates in water and their measured
266
were quite close in water ((7.0 ± 0.2) × 107 M–1s–1 and (6.7 ± 0.2) × 107 M–1s–1, respectively) and
267
in ethanol ((6.3 ± 0.8) × 107 M–1s–1 and (6.2 ± 0.1) × 107 M–1s–1, respectively). Interestingly,
268
while the
269
compared to water, they were still ~10% lower than the
270
suggesting that F residues directly adjacent to His in the oligopeptide sequence decreased the
values were found for FAFHFAF and
0
His rxn
k
values
0
His rxn
k
0
His rxn
k
values of FAFHFAF and FFFHFFF measured in ethanol were enhanced greatly 0
His rxn
k
of AAAHAAA and FAAHAAF,
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reactivity (Figure 2d). These slightly smaller values may reflect limited effect on accessibility of
272
His to 1O2 due to steric hindrance caused by the large neighboring AAs.
273
Sorption-enhanced phototransformation of His-containing oligopeptides in SRNOM1
274
sensitized systems. The pH-dependent
275
oligopeptides were established in lumichrome-sensitized systems. Based on those results, the
276
pH-dependent photochemistry of His-containing oligopeptides was investigated separately in
277
SRNOM-sensitized systems, in which sorption to the sensitizer could play a role. Below we
278
discuss the phototransformation results based on the type of constituent photostable AA residues
279
in the designed oligopeptides: aliphatic, cationic, anionic, aromatic, and polyfunctional (see
280
Figure 1 for definitions and Section S5).
281
Phototransformation
of
His
in
O2 reaction rate constants of His-containing
aliphatic
AA-containing
The
oligopeptides.
282
phototransformation of AAAHAAA was investigated to assess the effect of aliphatic residues on
283
peptide photoreactivity in SRNOM solutions. At solution pH above 6.0, the experimental
284
values for AAAHAAA obtained in SRNOM-sensitized systems were in good agreement with
285
the
286
AAAHAAA was experiencing the bulk 1O2 steady-state concentration and its reactivity was not
287
affected by the presence of SRNOM. At solution pH below 6.0, a large enhancement on the
288
AAAHAAA k CDOM was observed over rxn
289
constant is consistent with sorption of AAAHAAA to SRNOM. The pH-dependent enhancement
290
was in accordance with the protonation of His residue in AAAHAAA, suggesting that the
291
sorption was driven by electrostatic attraction between the positively charged His imidazole and
292
negatively charged sites in SRNOM (e.g., carboxylate moieties). This enhancement is
aq
krxn,calc in lumichrome-sensitized systems (c.f., Figure
k
CDOM rxn
2c and S1), strongly suggesting that
aq
krxn,calc (Figure S1). The enhanced apparent reaction rate
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CDOM aq particularly apparent when computing krxn / krxn,calc which increased from a ratio of one at pH
294
values above the pKa to a ratio of 53 at solution pH values below the pKa of the His residue
295
(Figure 3b and S1). The sorption and concomitantly subsequent enhanced peptide
296
phototransformation rates below pH 6.0 were likely caused by a combined effect of increasing
297
positive charge in the pepetide and decreasing negative charge of SRNOM, due to protonation of
298
carboxylate moieties.15
299
Phototransformation of His in cationic AA-containing oligopeptides. SRNOM-sensitized
300
phototransformation of His-containing oligopeptides that contain cationic residues was
301
investigated
302
phototransformation rates of dipeptides (Figure 3a) and heptapeptides (Figure 3b). The
303
15 CDOM aq dipeptide HH had a higher krxn / krxn,calc ratio than free His. This is particularly interesting
304
because it implies that one His in HH may be in the protonated form, which acts to promote
305
sorption to SRNOM, while the other His in HH may be in the neutral form, acting as the
306
photoreactive moiety. By contrast, free His protonation increased association with SRNOM but
307
decreased the 1O2 reactivity of His because the His+ moiety is much less reactive than His0. The
308
CDOM aq dual roles of His moieties in HH lead to a higher krxn / krxn,calc ratio than free His. The dipeptide
309
CDOM aq HR, which contains a positively charged guandinium moiety, also showed higher krxn / krxn,calc
310
ratios than free His (Figure 3a). Droge et al.52 showed that cationic amine heterocycles, in
311
which the positive change was delocalized, were superior to simple amine cations in promoting
312
sorption to natural organic matter. We hypothesize that the guanidinium group of arginine is
313
more similar to cationic heterocycles than simple amines in this regard. While the R sidechain
314
pKa values may be affected by other residues in peptides and shifted to lower pH values than that
to
assess
the
effect
of
electrostatic
interactions
with
SRNOM
on
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of free R (pKa = 12.0, Figure 1), they were likely to be higher than tested pH (4.1-7.9) and
316
arginine sidechains were protonated at all pH values. Thus, the enhanced
317
observed over the entire measured pH range.
k
CDOM rxn
values were
318
CDOM aq The krxn / krxn,calc ratios for heptapeptides with increasing number of cationic arginine
319
residues were compared at various solution pH values (Figure 3b). At solution pH above 6.0,
320
RARHRAR and RRRHRRR showed enhanced phototransformation in SRNOM-sensitized
321
systems relative to
322
higher than RARHRAR (i.e., 6.9 at pH 7.8), due to the higher number of positively charged
323
guanidinium groups in RRRHRRR. Conversely, no enhancement of the phototransformation rate
324
CDOM aq was observed for RAAHAAR; where, RAAHAAR had nearly identical krxn / krxn,calc ratios
325
CDOM aq with AAAHAAA. At pH values below 6.0, the krxn / krxn,calc ratio for RAAHAAR and
326
RARHRAR increased. While protonation decreased the intrinsic His reactivity, it increased the
327
CDOM aq association to SRNOM and resulted in a higher 1O2 rate enhancement. The krxn / krxn,calc ratio
328
of RRRHRRR was relatively stable over all investigated pH values even at low pH values. The
329
CDOM aq differences in the krxn / krxn,calc ratios among the heptapeptides were smaller at low pH
330
CDOM aq compared to the krxn / krxn,calc ratio variations at high pH. At pH 4.1, nearly identical
331
krxn
332
CDOM aq enhancements on phototransformation rates (Figure 3b). The pH independent krxn / krxn,calc
333
CDOM aq ratios for RRRHRRR and identical krxn / krxn,calc ratios for these heptapeptides at low pH
334
suggest a maximum (around 50) of rate enhancement caused by sorption to SRNOM. This
335
1 CDOM aq maximum krxn / krxn,calc ratio is in good agreement with the concentration gradient of O2 intra-
CDOM
aq
krxn,calc .
CDOM aq The krxn / krxn,calc ratio for RRRHRRR (i.e., 47 at pH 7.8) was
aq / krxn,calc ratios of around 50 were obtained for all heptapeptides, illustrating substantial
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CDOM macromolecules observed by Latch et al., where a 101-102 times higher concentration of
337
1
338
(estimated at CDOM concentrations around 10 mg C/L).37
O2 was generated in intra-CDOM aggregates than the bulk aqueous 1O2 concentration
339
Phototransformation of His in anionic AA-containing oligopeptide. Figure 3e shows the
340
photolysis data of DDDHDDD in SRNOM solutions at various pH values. At pH 7.9, the
341
krxn
342
CDOM aq SRNOM. Decreasing the solution pH resulted in pronounced increases in krxn / krxn,calc ,
343
suggesting that DDDHDDD associated with SRNOM despite its repulsive charge and polarity,
344
both of which were expected to hinder sorption. We propose that low barrier hydrogen bonds
345
(LBHB) may have resulted in significant association of DDDHDDD with SRNOM at low
346
solution pH. It is known that strong LBHB form when sorbent and sorbate have comparable pKa
347
values and when solution pH approaches their pKa values.53-56 In the case of DDDHDDD,
348
negative-charge-assisted LBHB (i.e., [—O•••H•••O—]-) may form between the carboxylate
349
groups in the aspartic acid residues (Figure 1) and analogous carboxylates in SRNOM at pH
350
values between 4.0 and 6.0. At higher pH values (e.g., at pH 7.9), both the carboxylate groups in
351
the D sidechain and SRNOM were deprotonated and thus, no enhancement on
352
phototransformation was observed. Further support for the LBHB-mediated sorption affinity of
353
DDDHDDD to SRNOM was demonstrated by observed reversible association of DDDHDDD to
354
carboxylate-terminated self-assembled monolayers using a quartz crystal microbalance, as
355
detailed in the SI.
CDOM
aq / krxn,calc ratio was close to unity, suggesting weak sorption (if any) of DDDHDDD to
356
Phototransformation of His in aromatic AA-containing oligopeptides. SRNOM-sensitized
357
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358
residues was investigated to assess the effect of hydrophobic binding on peptide
359
CDOM aq phototransformation rates (Figure 3c and 3d). The dipeptide HF shows similar krxn / krxn,calc
360
ratios as free His over the measured pH range, reaching a maximum ratio of 10 at the lowest
361
studied solution pH. This suggests one apolar AA is not enough to change the binding
362
mechanism and electrostatic interactions remain dominant (Figure 3c). At solution pH values
363
above 6.0, FAAHAAF and AAFHFAA showed no enhancement in 1O2 reaction rates in
364
SRNOM solutions, which was similar to the reactivity of AAAHAAA (Figure 3d). By contrast,
365
CDOM aq apparent enhancements ( krxn / krxn,calc > 1) were observed for FAFHFAF and FFFHFFF, these
366
enhancements were identical between pH 5.0 and pH 8.0. The observed enhancements on
367
reaction rates were likely due to a combination of effects, namely hydrophobic binding to
368
SRNOM promoted by the F residues and de-aggregation of the heptapeptides. In the later case,
369
association of hydrophobic peptides with SRNOM might facilitate de-aggregation because
370
SRNOM can act as a co-solvent, an analogous role to ethanol in lumichrome-sensitized
371
photolyses described above (Figure 2d). The photoreactivity of His residues might be either
372
enhanced with increased accessibility to reactants or depressed with raised pKa of His sidechain
373
upon de-aggregation.
374
CDOM aq The krxn / krxn,calc ratios of FAAHAAF and AAFHFAA increased as the solution pH
375
decreased below 6. This finding is consistent with the protonation of His imidazole sidechain
376
(the fitted pKa value for both FAAHAAF and AAFHFAA was 6.3), which induced electrostatic
377
attraction of FAAHAAF and AAFHFAA to SRNOM and subsequently enhanced
378
CDOM aq phototransformation. For FAFHFAF and FFFHFFF, the krxn / krxn,calc ratios increased at pH
379
values below 5.0 (pKa values were 4.6 for FAFHFAF and 4.3 for FFFHFFF). The increased
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380
ratios are also in accordance with the protonation of imidazole sidechain, suggesting the sorption
381
mechanisms to be a combination of electrostatic interaction (between positively charged
382
imidazole and SRNOM) and the hydrophobic effect (between hydrophobic F residues and
383
SRNOM). In the latter case, the hydrophobic effect was enhanced at low solution pH with higher
384
SRNOM hydrophobicity. Interestingly, similar to the results obtained for cationic heptapeptides,
385
CDOM aq a maximum value of the krxn / krxn,calc ratios around 80 was obtained for aromatic heptapeptides
386
at pH 4.1 (Figure 3b and 3d), lending further support to the idea of a maximum threshold on the
387
sorption-induced enhancement of oligopeptides phototransformation rates in SRNOM solutions.
388
Phototransformation of His in polyfunctional oligopeptides. Polyfunctional oligopeptides
389
were studied to assess the sorption-enhanced phototransformation in more representative
390
DCAAs. At pH values above 6.0, no enhancement on
391
oligopeptides (Figure 3f), indicating no sorption of oligopeptides to SRNOM despite the
392
presence of hydrophobic residues (F and V in FHGTVK and L in AGAHLK). As solution pH
393
CDOM aq decreased, increased krxn / krxn,calc ratios were again observed for the polyfunctional
394
oligopeptides in accordance with the protonation of His residue, indicating the electrostatic
395
attraction between the His imidazole sidechain and SRNOM. Notably, all studied polyfunctional
396
oligopeptides had identical magnitudes of enhancements on reaction rates, despite the fact that
397
FHGTVK and AGAHLK also contained a basic amine moiety in the K sidechain. Thus, the
398
enhanced
399
CDOM aq interaction of protonated His residue with SRNOM. The krxn / krxn,calc ratios obtained in
400
SRNOM solutions indicate that electrostatic interaction dominated the sorption of polyfunctional
401
oligopeptides to SRNOM.
k
CDOM rxn
k
CDOM rxn
was observed for the polyfunctional
of FHGTVK and AGAHLK were exclusively attributed to electrostatic
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402
Environmental implications. The goal of this study was to use these systematically designed
403
oligopeptides as model DCAAs to study the relationship between their physicochemical
404
properties and their photochemical reactivity. Herein we examined the effect of photostable
405
constituent AA residues (i.e., aliphatic, anionic, cationic, aromatic AA residues, Figure 1) on the
406
photoreacitivity of His-containing oligopeptides under various environment conditions (e.g.,
407
acidic and basic pH). A visualization of the magnitude of the reactivity differences observed
408
under different conditions is presented in Figure 4. In this figure, we compared the half-lives
409
(assuming [1O2]aq of 1 pM) of the studied heptapeptides, focusing on two pH values, pH 5.0 ±
410
0.1 and pH 7.9 ± 0.2 (Figure 4).
411
In general, neighboring amino acids affected the reactivity of the His sidechain in three ways.
412
First, they led to a shift in the pKa value of the protonated His+, which in turn shifted the
413
distribution of reactive His0 and unreactive His+ forms. Second, the neighboring residues
414
affected the intrinsic reactivity of the His, as evidenced by a shift in the reaction rate constant of
415
neutral His sidechain ( krxn ). Finally, the neighboring residues affected the affinity of the
416
oligopeptides for SRNOM, which led to an apparent enhancement in the reactivity. Apparent
417
phototransformation rate constants of His-containing oligopeptides were enhanced by up to two
418
orders of magnitude upon sorption to SRNOM, where the sorption mechanisms were
419
electrostatic interaction, hydrophobic effect and/or LBHB.
0
His
420
The end result of these effects was an observed factor of approximately 400 in the half-lives
421
of the heptapeptides under lumichrome-sensitized conditions (Figure 4). Under SRNOM-
422
sensitized conditions, a similar factor of approximately 500 in reactivity was observed, but the
423
heptapeptides were generally ten or more times more reactive than under lumichrome-sensitized
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424
conditions. All of the heptapeptides were significantly more reactive at pH 5.0 with SRNOM
425
than with lumichrome and four heptapeptides, namely RARHRAR, RRRHRRR, FAFHFAF, and
426
FFFHFFF, were also more reactive at pH 7.9 with SRNOM than with lumichrome. This result
427
has wide implications for nutrient availability in surface waters, illustrating that the
428
photochemical half-life of the same oligopeptide could vary by orders of magnitude depending
429
on environmental conditions. The results of this study also have implications for predicting the
430
environmental photochemical reactivity of AA-based molecules.
431
We expect CDOM-association to play a more important role for environmental
432
phototransformation of DCAAs in the context of higher order structures. Previous studies have
433
demonstrated that both hydrophobic effect and electrostatic interactions act as important sorption
434
mechanisms of proteins to lipid bilayers, nanoparticles and humic substances.7-9,57-61 Thus, we
435
expect sorption-enhanced photochemistry for larger peptides or proteins in natural waters to be
436
driven by various sorption mechanisms. In addition to sorption-enhanced photooxidation by
437
CDOM, photoreactive AA residues might be protected from photooxidation due to antioxidant
438
properties of CDOM.13,62 For both cases, we suggest that the sorptive affinity of biomolecules to
439
CDOM needs to be considered when studying the photochemistry of AA-based biomolecules in
440
aquatic environments.
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441
Supporting Information Available
442
Supporting figures, tables, detailed experimental methods and additional experiments described
443
within the manuscript are provided. This material is available free of charge via the Internet at
444
http://pubs.acs.org.
445
Acknowledgements
446 447 448 449 450
This work was financially supported by a grant from the Swiss National Science Foundation (Project numbers 200021_138008 and 200020_159809). The authors gratefully acknowledge Betsy L. Edhlund and Lauren C. Kennedy for conducting preliminary experiments. We thank Armanious Antonius and Elisabeth M.-L. Janssen for helpful discussions and experimental support.
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His
cationic
aliphatic NH
+H
+H
CO 2–
3N
+H
pKa 6.0
krxn = 6.5
x 10 7
CO 2– +H
His (H) His 0
NH
CO 2– Ala (A)
3N
CO 2-
3N
+H
3N
β−Ala (βA)
M -1s-1
R1
NH 3N
O
+H
CO 2-
CO 2 Arg (R)
Asp (D)
pK a 12.0
pK a 3.9
3N
R2
R3 N H
H N
CO 2–
3N
Phe (F)
R5 N H
O NH
O
H N
R6
R7 N H
CO 2-
N 0
peptide
pKa (His)
His krxn (10 7 M -1s-1)
-H -R -F
HH HR HF
6.2 ± 0.1 7.4 ± 0.1 7.6 ± 0.3
12.5 ± 0.7 14.4 ± 1.6 12.7 ± 3.9
His 0
pKa (His) krxn (10 7 M -1s-1)
βAH (carnosine) 6.6 ± 0.0 6.1 ± 0.1 FHGTVK 6.2 ± 0.1 AGAHLK
O
O
R2
polyfunctional peptides
451 452 453 454 455 456 457 458
O
H N O
R2
peptide
+H
heptapeptides
N
+H
CO 2–
3N
–
dipeptides
H N
aromatic
NH 2+
H 2N
N
anionic
10.6 ± 0.3 7.3 ± 0.8 7.8 ± 0.4
R1 R 2R 3 R 5R 6R7
peptide
0
His pKa (His) krxn (10 7 M -1s-1)
DDDAAARAARARRRR-
-DDD -AAA -AAR -RAR -RRR
DDDHDDD AAAHAAA RAAHAAR RARHRAR RRRHRRR
8.1 ± 6.6 ± 6.1 ± 5.3 ± 4.7 ±
0.2 0.1 0.1 0.0 0.0
AAAFAAAAFFAFFFF-
-AAA -AAF -FAA -FAF -FFF
AAAHAAA F AAHAAF AAF HF AA F AF HF AF F F F HF F F
6.6 ± 6.3 ± 6.3 ± 4.6 ± 4.3 ±
0.1 0.0 0.1 0.1 0.1
5.8 ± 1.1 7.3 ± 0.2 6.9 ± 0.4 6.8 ± 0.2 6.7 ± 0.1 7.3 ± 6.6 ± 6.0 ± 1.0 ± 0.95 ±
0.2 0.3 0.3 0.1 0.03
Figure 1. Structures, pKa values, and 1O2-mediated reaction rate constants for the histidine (His)-containing oligopeptides employed in this study. The heptapeptides are categorized by their incorporation of charged residues, including anionic D and cationic R residues, or hydrophobic F residues. 1O2-mediated reaction rate constants of neutral His residue ( krxnHis ) and the pKa values of the His residue for each His-containing oligopeptide were obtained from fitting their observed 1O2 photochemical kinetic data using Equation 1. 0
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459 460 461
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Figure 2. (a) 1O2-mediated phototransformation of His-containing oligopeptides sensitized by lumichrome at pH 4.1-8.0 (showing AAAHAAA as an example). Oligopeptide 24 ACS Paragon Plus Environment
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462 463 464 465 466 467 468 469 470 471 472 473 474 475 476
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phototransformation (natural logarithm of the ratio of oligopeptide concentration at time point t and initial concentration, ln([peptide]t/[peptide]0)) was normalized to a representative 1O2 steady state concentrations of 20.0 pM in the lumichrome-sensitized photolysis experiments and plotted versus photolysis time (t). Sensitizer-free photolysis of AAAHAAA, which served as a direct photolysis control at pH 8.0, is also plotted. Solid lines represent the first-order reaction fits of the experimental data (symbols). Error bars represent the range in ion intensities of AAAHAAA measured at two ionization states in the mass spectra (singly and doubly charged). (b)-(e) Plot of lumichrome-sensitized 1O2 reaction rate constants of His-containing (b) dipeptides, (c) charged heptapeptides, (d) heptapeptides varying in degrees of hydrophobicity, and (e) polyfunctional peptides as a function of solution pH. Solid lines represent the fits of the reaction rate constants using Equation 1. Peptide photolyses results conducted in ethanol (EtOH) at pH 8.0 were also plotted in panel (d) (grey insert). The phototransformation data of AAAHAAA is plotted twice in panels (c) and (d) for comparison purposes. Error bars represent the standard deviation of fitting curve of oligopeptide phototransformation. When error bars are not visible, they are contained within the marker symbols.
477
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478 26 ACS Paragon Plus Environment
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479 480 481 482 483 484 485 486 487 488
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Figure 3. CDOM-sensitized phototransformation gave higher than calculated rate constants, herein visualized as 1O2-mediated reaction rate constants of His-containing oligopeptides in CDOM aq SRNOM-sensitized photolyses divided by their calculated rate constants, krxn , at / krxn,calc various pH values, for (a) positively charged dipeptides, (b) positively charged heptapeptides, (c) hydrophobic dipeptide, (d) heptapeptides that vary in degrees of hydrophobicity, (e) negatively charged heptapeptide, and (f) polyfunctional oligopeptides. Errors bars represent the ratio of the CDOM standard deviation of the fitted curve of the experimental krxn in SRNOM solutions to the aq calculated rate constants in lumichrome-sensitized systems ( krxn,calc ). When error bars are not visible, they are contained within the marker symbols. Free His photolysis data from Chu et al.15 were re-plotted (in panel (a) and (c)) for reference.
489
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490 491 492 493 494 495 496 497 498 499
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Figure 4. Half-lives of 1O2-mediated (assuming [1O2]aq = 1 pM) phototransformation of Hiscontaining heptapeptides. The effects of photostable constituent neighboring AA residues (e.g., aliphatic, cationic, anionic, and aromatic AA residues) on half-lives of His-containing oligopeptides were assessed in groups at acid (pH 5.0 ± 0.1) and basic (pH 7.9 ± 0.2) solution pH in lumichrome-sensitized systems and SRNOM-sensitized systems. The 1O2 half-lives of free His at pH 5 and pH 8 under lumichrome-sensitized conditions are plotted for comparison (dashed lines). Errors bars represent the calculated half-life errors based on the errors of experimental reaction rate constants.
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500
References
501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544
1. Bronk, D. A., Dymamics of DON, In biogeochemistry of marine dissolved organic matter. Academic Press: USA, 2002; p 153-249. 2. Keil, R. G.; Kirchman, D. L. Dissolved combined amino-acids in marine waters as determined by a vapor-phase hydrolysis method. Mar. Chem. 1991, 33(3), 243-259. 3. Jorgensen, N. O. G.; Tranvik, L.; Edling, H.; Graneli, W.; Lindell, M. Effects of sunlight on occurrence and bacterial turnover of specific carbon and nitrogen compounds in lake water. Fems Microbiol Ecol 1998, 25(3), 217-227. 4. Keil, R. G.; Kirchman, D. L. Dissolved combined amino-acids - chemical form and utilization by marine bacteria. Limnology and Oceanography 1993, 38(6), 1256-1270. 5. McCarthy, M.; Pratum, T.; Hedges, J.; Benner, R. Chemical composition of dissolved organic nitrogen in the ocean. Nature 1997, 390(6656), 150-154. 6. Aluwihare, L. I.; Repeta, D. J.; Pantoja, S.; Johnson, C. G. Two chemically distinct pools of organic nitrogen accumulate in the ocean. Science 2005, 308(5724), 1007-1010. 7. Puddu, V.; Perry, C. C. Peptide adsorption on silica nanoparticles: evidence of hydrophobic interactions. Acs Nano 2012, 6(7), 6356-6363. 8. Tomaszewski, J. E.; Madliger, M.; Pedersen, J. A.; Schwarzenbach, R. P.; Sander, M. Adsorption of insecticidal Cry1Ab protein to humic substances. 2. Influence of humic and fulvic acid charge and polarity characteristics. Environ. Sci. Technol. 2012, 46(18), 9932-9940. 9. Sander, M.; Tomaszewski, J. E.; Madliger, M.; Schwarzenbach, R. P. Adsorption of insecticidal Cry1Ab protein to humic substances. 1. Experimental approach and mechanistic aspects. Environ. Sci. Technol. 2012, 46(18), 9923-9931. 10. Boreen, A. L.; Edhlund, B. L.; Cotner, J. B.; McNeill, K. Indirect photodegradation of dissolved free amino acids: The contribution of singlet oxygen and the differential reactivity of DOM from various sources. Environ. Sci. Technol. 2008, 42(15), 5492-5498. 11. Jori, G. Photosensitized reactions of amino-acids and proteins. Photochem. Photobiol. 1975, 21(6), 463-467. 12. Davies, M. J. The oxidative environment and protein damage. Biochim. Biophys. Acta, Proteins Proteomics 2005, 1703(2), 93-109. 13. Janssen, E. M.; Erickson, P. R.; McNeill, K. Dual roles of dissolved organic matter as sensitizer and quencher in the photooxidation of tryptophan. Environ. Sci. Technol. 2014, 48(9), 4916-24. 14. Pattison, D. I.; Rahmanto, A. S.; Davies, M. J. Photo-oxidation of proteins. Photochem. Photobiol. Sci. 2012, 11(1), 38-53. 15. Chu, C.; Lundeen, R. A.; Remucal, C. K.; Sander, M.; McNeill, K. Enhanced indirect photochemical transformation of histidine and histamine through association with chromophoric dissolved organic matter. Environ. Sci. Technol. 2015, 49(9), 5511-9. 16. Lundeen, R. A.; McNeill, K. Reactivity differences of combined and free amino acids: quantifying the relationship between three-dimensional protein structure and singlet oxygen reaction rates. Environ. Sci. Technol. 2013, 47(24), 14215-23. 17. Matheson, I. B. C.; Lee, J. Chemical reaction rates of amino-acids with singlet oxygen. Photochem. Photobiol. 1979, 29(5), 879-881. 18. Miskoski, S.; Garcia, N. A. Influence of the peptide-bond on the singlet molecular oxygen-mediated(O2 [1 delta g]) photooxidation of histidine and methionine dipeptides. A kinetic study. Photochem. Photobiol. 1993, 57(3), 447-452. 29 ACS Paragon Plus Environment
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19. Bertolotti, S. G.; Garcia, N. A.; Arguello, G. A. Effect of the peptide-bond on the singletmolecular-oxygen-mediated sensitized photooxidation of tyrosine and tryptophan dipeptides - a kinetic study. J. Photochem. Photobiol., B 1991, 10(1-2), 57-70. 20. Michaeli, A.; Feitelson, J. Reactivity of singlet oxygen toward amino-acids and peptides. Photochem. Photobiol. 1994, 59(3), 284-289. 21. Lundeen, R. A.; Janssen, E. M.; Chu, C.; Mcneill, K. Environmental photochemistry of amino acids, peptides and proteins. Chimia 2014, 68(11), 812-817. 22. Criado, S.; Marioli, J. M.; Allegretti, P. E.; Furlong, J.; Nieto, F. J. R.; Martire, D. O.; Garcia, N. A. Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals. J. Photochem. Photobiol., B 2001, 65(1), 74-84. 23. Reis, A.; Fonseca, C.; Maciel, E.; Domingues, P.; Domingues, M. R. M. Influence of amino acid relative position on the oxidative modification of histidine and glycine peptides. Anal. Bioanal. Chem. 2011, 399(8), 2779-2794. 24. Michaeli, A.; Feitelson, J. Reactivity of singlet oxygen toward large peptides. Photochem Photobiol 1995, 61(3), 255-60. 25. Naor, M. M.; Jensen, J. H. Determinants of cysteine pK(a) values in creatine kinase and alpha 1-antitrypsin. Proteins-Structure Function and Bioinformatics 2004, 57(4), 799-803. 26. Harris, T. K.; Turner, G. J. Structural basis of perturbed pK(a) values of catalytic groups in enzyme active sites. Iubmb Life 2002, 53(2), 85-98. 27. Edgcomb, S. P.; Murphy, K. P. Variability in the pK(a) of histidine side-chains correlates with burial within proteins. Proteins 2002, 49(1), 1-6. 28. Wang, P. F.; McLeish, M. J.; Kneen, M. M.; Lee, G.; Kenyon, G. L. An unusually low pK(a) for Cys282 in the active site of human muscle creatine kinase. Biochemistry 2001, 40(39), 11698-11705. 29. Cederholm, M. T.; Stuckey, J. A.; Doscher, M. S.; Lee, L. Histidine pK(a) shifts accompanying the inactivating Asp121- Asn substitution in a semisynthetic bovine pancreatic ribonuclease. Proc. Natl. Acad. Sci. U.S.A 1991, 88(18), 8116-8120. 30. Poi, M. J.; Tomaszewski, J. W.; Yuan, C. H.; Dunlap, C. A.; Andersen, N. H.; Gelb, M. H.; Tsai, M. D. A low-barrier hydrogen bond between histidine of secreted phospholipase A(2) and a transition state analog inhibitor. J Mol Biol 2003, 329(5), 997-1009. 31. Ash, E. L.; Sudmeier, J. L.; DeFabo, E. C.; Bachovchin, W. W. A low-barrier hydrogen bond in the catalytic triad of serine proteases? Theory versus experiment. Science 1997, 278(5340), 1128-1132. 32. Tobin, J. B.; Whitt, S. A.; Cassidy, C. S.; Frey, P. A. Low-barrier hydrogen-bonding in molecular-complexes analogous to histidine and aspartate in the catalytic triad of serine proteases. Biochemistry 1995, 34(21), 6919-6924. 33. McDougal, O. M.; Granum, D. M.; Swartz, M.; Rohleder, C.; Maupin, C. M. pK(a) determination of histidine residues in alpha-Conotoxin MII peptides by H-1 NMR and constant pH molecular dynamics simulation. J Phys Chem B 2013, 117(9), 2653-2661. 34. Everhart, D.; Cartier, G. E.; Malhotra, A.; Gomes, A. V.; McIntosh, J. M.; Luetje, C. W. Determinants of potency on alpha-conotoxin MII, a peptide antagonist of neuronal nicotinic receptors. Biochemistry 2004, 43(10), 2732-2737. 35. Zepp, R. G., Environmental photoprocesses involving natural organic matter. Environmental Research Laboratory, Office of Research and Development, US Environmental Protection Agency: 1987. 30 ACS Paragon Plus Environment
Page 31 of 33
591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635
Environmental Science & Technology
36. Burns, S. E.; Hassett, J. P.; Rossi, M. V. Mechanistic implications of the intrahumic dechlorination of mirex. Environ. Sci. Technol. 1997, 31(5), 1365-1371. 37. Latch, D. E.; McNeill, K. Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 2006, 311(5768), 1743-1747. 38. Grandbois, M.; Latch, D. E.; McNeill, K. Microheterogeneous concentrations of singlet oxygen in natural organic matter isolate solutions. Environ Sci Technol 2008, 42(24), 9184-90. 39. Zhang, T.; Hsu-Kim, H. Photolytic degradation of methylmercury enhanced by binding to natural organic ligands. Nat. Geosci. 2010, 3(7), 473-476. 40. Fleck, J. A.; Gill, G.; Bergamaschi, B. A.; Kraus, T. E. C.; Downing, B. D.; Alpers, C. N. Concurrent photolytic degradation of aqueous methylmercury and dissolved organic matter. Sci. Total Environ. 2014, 484, 263-275. 41. Hou, W. C.; Stuart, B.; Howes, R.; Zepp, R. G. Sunlight-driven reduction of silver ions by natural organic matter: formation and transformation of silver nanoparticles. Environ. Sci. Technol. 2013, 47(14), 7713-7721. 42. Blough, N. V. Electron-paramagnetic resonance measurements of photochemical radical production in humic substances .1. Effects of O-2 and charge on radical scavenging by nitroxides. Environ. Sci. Technol. 1988, 22(1), 77-82. 43. Tomaszewski, J. E.; Schwarzenbach, R. P.; Sander, M. Protein encapsulation by humic substances. Environ. Sci. Technol. 2011, 45(14), 6003-6010. 44. Tan, W. F.; Koopal, L. K.; Weng, L. P.; van Riemsdijk, W. H.; Norde, W. Humic acid protein complexation. Geochim. Cosmochim. Acta 2008, 72(8), 2090-2099. 45. Hsu, P.-H.; Hatcher, P. G. Covalent coupling of peptides to humic acids: Structural effects investigated using 2D NMR spectroscopy. Org. Geochem. 2006, 37(12), 1694-1704. 46. Egorov, S. Y.; Kurella, E. G.; Boldyrev, A. A.; Krasnovsky, A. A. Quenching of singlet molecular oxygen by carnosine and related antioxidants. Monitoring 1270-nm phosphorescence in aqueous media. Biochem. and Mol. Biol. International 1997, 41(4), 687-694. 47. Yoshikawa, T.; Naito, Y.; Tanigawa, T.; Yoneta, T.; Kondo, M. The antioxidant properties of a novel Zinc-carnosine chelate compound, N-(3-Aminopropionyl)-L-Histidinato Zinc. Biochimica Et Biophysica Acta 1991, 1115(1), 15-22. 48. Boldyrev, A. A.; Stvolinsky, S. L.; Tyulina, O. V.; Koshelev, V. B.; Hori, N.; Carpenter, D. O. Biochemical and physiological evidence that carnosine is an endogenous neuroprotector against free radicals. Cellular and Mol. Neurobiol. 1997, 17(2), 259-271. 49. Remucal, C. K.; McNeill, K. Photosensitized amino acid degradation in the presence of riboflavin and its derivatives. Environ. Sci. Technol. 2011, 45(12), 5230-5237. 50. Latch, D. E.; Stender, B. L.; Packer, J. L.; Arnold, W. A.; McNeill, K. Photochemical fate of pharmaceuticals in the environment: cimetidine and ranitidine. Environ. Sci. Technol. 2003, 37(15), 3342-3350. 51. Li, S.; Hong, M. Protonation, tautomerization, and rotameric structure of histidine: a comprehensive study by magic-angle-spinning solid-state NMR. J. Am. Chem. Soc. 2011, 133(5), 1534-1544. 52. Droge, S. T. J.; Goss, K.-U. Ion-exchange affinity of organic cations to natural organic matter: influence of amine type and nonionic interactions at two different pHs. Environ. Sci. Technol. 2013, 47(2), 798-806. 53. Xiao, F.; Pignatello, J. J. Effect of adsorption non linearity on the pH-adsorption profile of ionizable organic compounds. Langmuir 2014, 30(8), 1994-2001.
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54. Ni, J.; Pignatello, J. J.; Xing, B. Adsorption of aromatic carboxylate ions to black carbon (biochar) is accompanied by proton exchange with water. Environ. Sci. Technol. 2011, 45(21), 9240-9248. 55. Gilli, P.; Pretto, L.; Bertolasi, V.; Gilli, G. Predicting hydrogen-bond strengths from acid-base molecular properties. The pK(a) slide rule: toward the solution of a long-lasting problem. Accounts Chem Res 2009, 42(1), 33-44. 56. Gilli, G.; Gilli, P. Towards an unified hydrogen-bond theory. J Mol Struct 2000, 552, 115. 57. Beschiaschvili, G.; Seelig, J. Peptide binding to lipid bilayers. Nonclassical hydrophobic effect and membrane-induced pK shifts. Biochemistry 1992, 31(41), 10044-53. 58. Dekroon, A. I. P.; Soekarjo, M. W.; Degier, J.; Dekruijff, B. The role of charge and hydrophobicity in peptide lipid interaction - a comparative-study based on tryptophan fluorescence measurements combined with the use of aqueous and hydrophobic quenchers. Biochemistry 1990, 29(36), 8229-8240. 59. Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Recent progress in understanding hydrophobic interactions. Proc. Natl. Acad. Sci. U.S.A 2006, 103(43), 15739-15746. 60. Sammond, D. W.; Yarbrough, J. M.; Mansfield, E.; Bomble, Y. J.; Hobdey, S. E.; Decker, S. R.; Taylor, L. E.; Resch, M. G.; Bozell, J. J.; Himmel, M. E.; Vinzant, T. B.; Crowley, M. F. Predicting enzyme adsorption to lignin films by calculating enzyme surface hydrophobicity. Journal of Biological Chemistry 2014, 289(30), 20960-20969. 61. Jacak, R.; Leaver-Fay, A.; Kuhlman, B. Computational protein design with explicit consideration of surface hydrophobic patches. Proteins-Structure Function and Bioinformatics 2012, 80(3), 825-838. 62. Janssen, E. M. L.; McNeill, K. Environmental photoinactivation of extracellular phosphatases and the effects of dissolved organic matter. Environ. Sci. Technol. 2015, 49(2), 889-896.
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