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Hydration-Shell Transformation of Thermo-Sensitive Aqueous Polymers Kenji Mochizuki, and Dor Ben-Amotz J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00363 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017
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Hydration-Shell Transformation of Thermo-Sensitive Aqueous Polymers Kenji Mochizuki∗,† and Dor Ben-Amotz‡ †Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan ‡Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States E-mail:
[email protected]
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The influence of hydration shell structure on the conformational stability and function of aqueous polymers and biomolecules (i.e., proteins and DNA) 1–7 is of both fundamental and practical interests. 8,9 Numerous previous studies, aimed at understanding the relationship between water structure and polymer collapse, have left open the question as to whether or not water structure drives polymer collapse. 10–15 More specifically, although recent computational studies suggested that the collapse of idealized hydrophobic polymers is induced by dewetting, 16,17 no previous experiments have either detected the associated water structure changes or determined if such changes precede or follow polymer collapse and aggregation. Here we do so by probing the hydration-shell of aqueous poly(N-isopropylacrylamide) (PNIPAM, inset in Fig.1a) – a stimuli-sensitive polymer 1,18–21 that undergoes clouding upon heating (see the photos in Figs.1b and 1c), and is regarded as a simple model of proteins. 22,23 We measure the hydration shell vibrational spectrum of PNIPAM above and below the cloud point using Raman multivariate curve resolution (Raman-MCR) spectroscopy 24–27 to detect polymer-induced perturbations in the vibrational spectra of water. Our results reveal that a dramatic change in hydration-shell water structure follows rather than precedes the clouding of long-chain PNIPAM aqueous solutions. Moreover, we find that the structural transformation does not occur either upon clouding of short-chain PNIPAM and PPO solutions or in aqueous solutions of PNIPAM monomers. Thus, our results imply that the water structural transformation occurs after the coil-globule transition, upon ripening of the resulting mesoscopic oil-rich domains composed of long-chain PNIPAM polymers. Although the observed hydration-shell transformation bears some resemblance to previously predicted 28 and observed 25 hydrophobic crossover phenomena, it is not yet clear how the associated water structure transformations are related to each other. Figure 1a shows the measured Raman spectra of pure water and an aqueous solution of long-chain PNIPAM (Mn ∼20,000-40,000 g/mol) at 30 ◦ C. The resulting Raman-MCR solute-correlated (SC) spectrum shown in Fig.1b reveals features arising from both the PNIPAM CH stretch band and PNIPAM-induced perturbations of the OH band of water
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temperature above the cloud point. The transparency of the solutions (Fig. 2a), obtained from the back-scattered Raman intensity of the water OH band (as indicated above Fig. 2a), shows that long-chain PNIPAM (Mn ∼20,000–40,000) solutions become cloudy abruptly at ∼34 ◦ C. Above the cloud-point temperature the CH stretch peak frequency of PNIPAM decreases (Fig. 1b), consistent with the less polar environment around the CH groups in the polymer-rich mesoscopic aggregates. 29,30 The hydration-shell OH area of long-chain PNIPAM (Fig. 2c) initially decreases with increasing temperature below 34 ◦ C and then at higher temperatures, above the cloud point, the hydration-shell dramatically transforms to the less-ordered and more weakly H-bonded structure (Fig. 2c and 2d). These results, obtained from well-equilibrated solutions show that clouding of long-chain PNIPAM solutions is accompanied by a water structure transformation. Similar experiments performed using lower PNIPAM concentrations (down to 5 mg/mol) confirm that the hydration shell transformation always takes upon clouding in these long-chain PNIPAM solutions, although lower concentration solutions have a significantly higher transparency (up to 85%), as further described in Fig. S1 in the Supporting Information (SI), thus the water structure transformation reflects the structure of the PNIPAM aggregates, but is not directly correlated with the transparency of the solution. To further explore whether the hydration shell transformation proceeds or follows the long-chain PNIPAM collapse, we performed Raman-MCR measurement in micro-fluidic flow system (further described in the SI methods, and shown in Fig. S2). Briefly, a PNIPAM solution that is initially at 23 ◦ C flows through a glass capillary surrounded by a copper block that heated at 40 ◦ C. Raman spectra are obtained through a window located 6 mm after the point at which the fluid enters the block, at velocities ranging from 2.3 mm/sec to 3.8 mm/sec. The temperature of the fluid at the measurement port was determined from the shape of the OH band of pure water (as further described in the SI methods). 31–33 At a flow velocity of 3.2 mm/sec, when the probed temperature is ∼34.0◦ C, we estimate that the fluid has been above the observed onset of clouding (at ∼33.7◦ C) for less than 0.1 sec (∼0.05
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sec, assuming a linear temperature gradient between the point at which the fluid enters the copper block and the probe point). Figure 2e shows that the hydration shell transformation has already occurred at 34.0 ◦ C, which is less than 0.1 sec after the solution has reached 33.7 ◦ C. The onset of clouding in our microfluidic cell occurred when the temperature at the observation port reached 33.7 ◦ C (see the inset in Fig. 2e). At this temperature we found that the transparency of the solution fluctuated with time, varying from ∼50% to ∼85% on a time scale of seconds to minutes (see Fig.S3). No such fluctuations were observed at either higher or lower temperatures (corresponding to slower or faster flow rates, respectively). In order correlated these fluctuations in transparency with changes in the hydration-shell spectra, we performed time dependent Raman measurements of both pure water and 25 mg/ml aqueous PNIPAM at 33.7 ◦ C, with an integration time of 10 sec per point. The resulting Raman-MCR hydration-shell spectra were found to change in a way that correlated with percent transparency of the PNIPAM solution, as indicated by the hatched region in Figure 2e, whose lower bound OH band spectrum pertains to solutions of ∼85% transparency and upper bound pertains to solutions of ∼50% transparency. These results imply that the water structural transformation has not yet occurred when the transparency of these solutions is ∼85% and only begins to occur when the transparency ∼50%. However, the PNIPAM coil-globule transition has already occurred in all these solutions, as indicated by the fact that the CH stretch frequency of PNIPAM in all the 33.7◦ C solution spectra is ∼3 cm−1 lower than that at 33.1◦ C, below the cloud point. Thus, our results clearly indicate that the water structure transformation follows, rather than precedes, the coil-globule and clouding transition. More specifically, at the onset of clouding (at ∼85% transparency) the tetrahedrally enhanced hydration shell structure is maintained, although the hydration-shell OH band area is significantly smaller than that at 33.1 ◦ C, suggesting that some hydration-shell water molecules are expelled to the bulk when the PNIPAM aggregates are initially formed. 34,35 When the clouding has icreased (to ∼50% transparency) the transform to a less-ordered hydration-shell structure begins, as evidenced
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by the lower intensity at 3250 cm−1 (indicating lower tetrahydral order) and higher average OH frequency (indicating weaker H-bonding). It is also important to note that previous temperature jump studies have shown that the onset of clouding occurs in less than 10 ms in PNIPAM solutions of comparable chain length and concentration as those used in our studies. 36 This further supports our conclusion that the water structure transformation that we have observed does not take place in the initially formed PNIPAM aggregates, but only occurs after the aggregates have ripened on a time scale that is long compared to the onset of clouding. The fact that clouding may be decoupled from the water structure transformation is further evidenced by the results obtained from solutions containing short-chain PNIPAM polymers (Mn ∼2000, Fig. 2f) or PNIPAM monomers (Fig. 2g). At ∼20◦ C (below the cloud point), the hydration-shells of equilibrated short-chain PNIPAM solutions (Fig. 2f) resemble those of long-chain PNIPAM solutions (Fig. 1b), indicating that both chains have hydrationshells of similar structure prior to clouding. However, upon clouding of the short-chain PNIPAM solutions (at T≥28◦ C) the PNIPAM hydration-shell spectra simply decrease in area with increasing temperature (and increased clouding) without showing any evidence of undergoing the sort of structural transformation that takes place in clouded long-chain PNIPAM solutions (Figs. 2d and 2e). The decrease in OH area with increasing temperature that is evident in Fig. 2f is reminiscent of that seen in aqueous alcohol solutions with increasing temperature 25 and increasing aggregation (at constant temperature). 34 The same behavior is also observed in aqueous solutions of short-chain PPO (Mn ∼1000) (Fig. S4). Solutions containing PNIPAM monomers (N-isopropylacrylamide), which do not exhibit a clouding transition, yield hydration-shell spectra (Fig.2g) that again simply decrease in area with increasing temperature, like the short-chain PNIPAM and alcohol solutions, without undergoing the transformation that takes place in the clouded long-chain PNIPAM solutions. The fact that the hydration-shell OH area increases above the cloud point in long-chain PNIPAM solutions (as is evident in Figs. 2d and 2e) indicates that the number of water
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molecules that undergo the transformation is comparable (or greater than) the number of water molecules in the hydration-shell of the polymer below the cloud point. In other words, the PNIPAM polymer chains remain essentially fully hydrated above the cloud point. This conclusion, as well as our finding that the PNIPAM aggregates ripen on a time scale that is long compared to the initial onset of clouding, may perhaps be related to previous evidence of a step-wise conformational change associated with the coil-globule transition of PNIPAM. 18,37 However, this connection remains to be elucidated, as does the quantitative connection between our measured hydration-shell spectra and the associate water structure changes. It is also noteworthy that while the hydration shell of a PNIPAM monomer shows clear evidence of a dangling (non-H-bonded) OH population (giving rise to the sharp peak near 3660 cm−1 ), 26 no such peak is present in the hydration-shell spectra of PNIPAM, either above or below the cloud point. This suggest that the hydration-shell of PNIPAM has fewer broken H-bond defects than the hydration-shell of the PNIPAM monomer. Our findings provide clear evidence that, subsequent to clouding, the hydration-shell of long-chain PNIPAM polymers transforms to a structure that has less tetrahedral order and weaker H-bonding than bulk water. This transformation is not evident in the early stages of clouding and does not occur in aqueous solutions of PNIPAM monomers, or clouded short-chain PNIPAM and PPO solutions. Thus, our results are inconsistent with the notion that the observed water structural transformation drives the coil-globule transition. It remains unclear how the observed transformation may be related to the hydrophobic crossover/dewetting transition predicted to take place in the hydration-shells of hydrophobic solutes larger than ∼1 nm, 28 or the experimentally observed water structural transition that takes place in the hydration-shells of isolated (non-aggregated) alcohols with hydrocarbon tails longer than ∼1 nm. 25
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Acknowledgement K.M. thanks Dr. S.R.Zukowski for useful discussions. K.M. was supported by Grant-in-Aids for Scientific Research of Japan Society for the Promotion of Science (15H05474) and Wesco Scientific Promotion Foundation (Okayama city), Japan. D.B.-A. was supported by the National Science Foundation (CHE-1464904).
Supporting Information Available Experimental details and supporting figures This material is available free of charge via the Internet at http://pubs.acs.org/.
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