Radiation Chemistry


Radiation Chemistrypubs.acs.org/doi/pdf/10.1021/ba-1968-0081.ch025Similarof chloride—i.e., chloride ions increase the...

1 downloads 82 Views 760KB Size

25 Steady State and Pulse Radiolysis of Aqueous Chloride Solutions of Nucleic

Downloaded by CORNELL UNIV on June 16, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch025

Acid Components J. F. WARD and I. KUO Laboratory of Nuclear Medicine and Radiation Biology of the Department of Biophysics and Nuclear Medicine, School of Medicine, University of California, Los Angeles, Calif.

The pulse radiolysis technique has been used to measure absolute rate constants for reactions of some nucleic acid constituents with Cl2 radicals (the species produced by reaction of O H radicals with chloride ions in acid aqueous solution). The rate of disappearance of the Cl2 absorption spectrum was measured in the absence and presence of the various solutes. Rate constants for the corresponding O H radical reactions are found to be 20 to 200 times greater than the rate constants for the Cl2 radical reactions. Steady state radiolysis showed that in some cases the radicals pro­ duced by reaction of these compounds with Cl2 radicals differ in their subsequent reaction from the corresponding O H radical adduct. -

-

-

-

T T y d r o x y l free r a d i c a l s f o r m e d i n w a t e r r a d i o l y s i s react w i t h c h l o r i d e X X

|

o

n

s

j

n

a c

j j s o l u t i o n ( I , 9 ) , to give a transient p r o d u c t w h i c h has (

b e e n i d e n t i f i e d as the C L " i o n r a d i c a l ( 1 ) . T h i s species differs f r o m the O H free r a d i c a l i n that it reacts r a p i d l y w i t h t h y m i n e b u t n o t w i t h e t h y l alcohol.

Previous work

( 9 ) u s e d this difference

to d e t e r m i n e

a rate

constant f o r the r e a c t i o n of O H r a d i c a l s w i t h c h l o r i d e ions ( r e l a t i v e to the rate constant f o r the r e a c t i o n of O H r a d i c a l s w i t h t h y m i n e ) .

Subse­

q u e n t w o r k ( 8 ) has s h o w n that the presence of c h l o r i d e ions d u r i n g i r r a d i a t i o n of other p y r i m i d i n e s i n o x y g e n a t e d ,

a c i d , aqueous

solution

leads to a m a r k e d d e p a r t u r e f r o m the results o b t a i n e d i n t h e absence of c h l o r i d e — i . e . , c h l o r i d e ions increase the extent of u r a c i l a n d of cytosine d e s t r u c t i o n u p to s i x - f o l d , b u t decrease G ( - b a s e ) f o r p u r i n e s , nucleosides, 368 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

25.

WARD AND KUO

Nucleic

Acid

369

Components

a n d nucleotides. T h e s e changes i n y i e l d s s h o w a p H d e p e n d e n c e w h i c h closely f o l l o w s that c a l c u l a t e d for C l ~ f o r m a t i o n . It seems that r e a c t i o n 2

of C l ~ w i t h the p y r i m i d i n e or p u r i n e is i m p o r t a n t i n d e t e r m i n i n g the 2

extent of base d e s t r u c t i o n . T h e present i n v e s t i g a t i o n was u n d e r t a k e n to d e t e r m i n e the rates of r e a c t i o n of some n u c l e i c a c i d d e r i v a t i v e s w i t h C l ~ 2

b y p u l s e r a d i o l y s i s a n d to attempt to correlate these w i t h o b s e r v e d steady state r a d i o l y s i s results.

Downloaded by CORNELL UNIV on June 16, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch025

Experimental P u l s e r a d i o l y s i s experiments w e r e c a r r i e d out i n c o o p e r a t i o n w i t h L . M . T h e a r d at G u l f G e n e r a l A t o m i c , Inc., S a n D i e g o , C a l i f . , u s i n g apparatus p r e v i o u s l y d e s c r i b e d ( 7 ) . T y p i c a l l y a 10-nsec. p u l s e g i v i n g a dose of 2 0 0 - 3 0 0 rads w a s u s e d . T h e f o r m a t i o n a n d d e c a y of the C l ~ transient was f o l l o w e d at 360 n . m . T h e e x t i n c t i o n coefficient of this species ( I ) at 360 n . m . is m u c h h i g h e r t h a n those q u o t e d f o r p y r i m i d i n e a n d p u r i n e transients ( 5 ) . Results o b ­ t a i n e d u s i n g other w a v e l e n g t h s of the C l " a b s o r p t i o n s p e c t r u m , s h o w e d close agreement w i t h those o b t a i n e d at 360 n . m . y - I r r a d i a t i o n s w e r e c a r r i e d out i n a C o source at a dose rate of 6 X 10 e.v. m l . m i n . " ( c a l i b r a t e d b y ferrous sulfate d o s i m e t r y ) . G values ( G = n u m b e r of molecules c h a n g e d p e r 100 e.v. a b s o r b e d ) w e r e c a l c u l a t e d f r o m y i e l d dose plots. Solutions f o r y - i r r a d i a t i o n w e r e satu­ rated w i t h oxygen. M a t e r i a l s . A l l n u c l e i c a c i d c h e m i c a l s w e r e A grade ( C a l b i o c h e m ) i r r a d i a t e d d i s s o l v e d i n t r i p l y d i s t i l l e d water. O t h e r c h e m i c a l s w e r e re­ agent grade. F o r p u l s e r a d i o l y s i s , solutions w e r e e q u i l i b r a t e d w i t h the r e q u i r e d gas b e f o r e i n t r o d u c t i o n to the r a d i a t i o n c e l l , the same gas b e i n g u s e d to d r i v e the s o l u t i o n i n the s a m p l e c h a n g i n g arrangement. U n l e s s o t h e r w i s e stated a r g o n was u s e d . 2

2

6 0

16

- 1

Results and

1

Discussion

P r e v i o u s w o r k (1,9)

has s h o w n that the r e a c t i o n of O H free r a d i c a l s

w i t h c h l o r i d e ions shows a

first

order d e p e n d e n c e

on hydrogen ion

concentration and chloride ion concentration. O H ' + CI" + H 0 3

+

-» Cr + 2H 0

(1)

2

( T h e c h l o r i n e a t o m a p p a r e n t l y reacts i m m e d i a t e l y w i t h a c h l o r i d e i o n to g i v e C l " i n a r e a c t i o n w h i c h is not rate l i m i t i n g ( 1 ) . ) 2

T h e t h i r d order

r e a c t i o n rate constant for R e a c t i o n 1 o b t a i n e d i n this l a b o r a t o r y u s i n g a steady state m e t h o d is o n l y h a l f that f o u n d b y A n b a r a n d T h o m a s u s i n g p u l s e r a d i o l y s i s (1).

U s i n g p u l s e r a d i o l y s i s w e o b t a i n results close

those of the latter w o r k e r s .

to

T h e rate w e o b t a i n e d for R e a c t i o n 1 w a s

1.5 ± 0.3 X 1 0 M " sec." . W e f o u n d no consistent v a r i a t i o n of this v a l u e 1 0

2

1

w i t h c h a n g i n g p H (0.8 to 3.4) to 1 0 " ) . 1

T h e rate constant

or c h a n g i n g c h l o r i d e m o l a r i t y ( 3 X

10"

4

d e t e r m i n e d i n the steady state w o r k w a s

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

370

RADIATION

CHEMISTRY-

o b t a i n e d f r o m a m e a s u r e d r e l a t i v e rate constant, u s i n g a p u b l i s h e d rate constant as a reference. T h e m e t h o d u s e d i n d e t e r m i n i n g rate constants for C l " reactions is 2

essentially the same as that u s e d to d e t e r m i n e h y d r a t e d e l e c t r o n rate constants: T h e rate of d i s a p p e a r a n c e of the C l ~ a b s o r p t i o n w a s m e a s u r e d 2

i n the presence of v a r i o u s concentrations of solute.

T h e d e c a y of C l ~ 2

i n the absence of r e a c t i n g solute seemed to be second order.

Increasing

the dose to 850 rads a n d r e m o v i n g h y d r a t e d electrons b y saturation of the s o l u t i o n w i t h nitrous o x i d e at p H 3.1 gave a g o o d second o r d e r p l o t for C l " decay. T h i s suggests that C l " is d e c a y i n g b y R e a c t i o n 2 : Downloaded by CORNELL UNIV on June 16, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch025

2

2

C l " + C l " -> (2C1- + C l ) 2

2

(2)

2

T h e rate constant for this r e a c t i o n was f o u n d to be 1.4 ± sec."

=

1

Reference

1).

c e n t l y m e a s u r e d rate constants 2k

2

M

_ 1

L a n g m u i r and H a y o n (3)

h a v e re­

of C l " reactions b y a flash p h o t o l y s i s 2

T h e y m e a s u r e d the rate constant for R e a c t i o n 2 a n d f o u n d

1.25 — 1.51

=

1 0

It was also s h o w n that this d e c a y rate was

i n d e p e n d e n t of p H f r o m 0.9 to 3.2. method.

10

2

2

10,000 (see

0.3 X

( T h e e x t i n c t i o n coefficient used for C l " at 360 n . m . was

2k

X

10

1 ( )

M

_ 1

sec." , w h i c h is i n g o o d agreement 1

with

the v a l u e o b t a i n e d here. A l l of the c o m p o u n d s e x a m i n e d for r e a c t i o n w i t h C l " react r a p i d l y 2

w i t h O H free r a d i c a l s ( 5 ) . sec."

T h u s , rate constants l o w e r t h a n 5 X

c o u l d not be d e t e c t e d :

1

10 M 6

I n c r e a s i n g the solute c o n c e n t r a t i o n

c i e n t l y to c o m p e t e w i t h R e a c t i o n 2 for the C l ~ , scavenges

_ 1

suffi­

O H free

2

r a d i c a l s f r o m r e a c t i n g to f o r m C l ~ . 2

C h l o r i d e ions are u s e d i n the F r i c k e dosimeter as h y d r o x y l r a d i c a l scavengers (2).

Since a d d i n g c h l o r i d e ions does not decrease the y i e l d of

f e r r i c i o n ( e x c e p t i n the presence

of o r g a n i c i m p u r i t i e s ) , it c a n

be

a r g u e d that c h l o r i n e atoms a n d hence C l " r a d i c a l ions react to o x i d i z e 2

ferrous i o n . U s i n g the present t e c h n i q u e w e h a v e m e a s u r e d the effect of ferrous ions o n the rate of d e c a y of the C l " transient. 2

F e r r o u s ions

i n c r e a s e d this rate, a n d a rate constant for R e a c t i o n 3 was d e t e r m i n e d fc = 3

3.8 ±

0.3 X 1 0 W "

1

Fe

sec." at p H 2.1 1

2 +

+ C l " -> F e 2

3 +

+ 2C1"

T h e effect of salt c o n c e n t r a t i o n ( s o d i u m p e r c h l o r a t e ) of C l " transient i n the presence of 1 0 " M F e 3

2

2 +

(3) o n the d e c a y rate

was e x a m i n e d . A m a r k e d

salt effect was o b s e r v e d , the p s e u d o first o r d e r rate constant b e i n g r e d u c e d f r o m 3.8 X

10

strength 0.25.

4

sec." at i o n i c strength 0.04, to 2.5 X 1

10

4

sec.

-1

at i o n i c

R e a c t i o n 3 w o u l d be e x p e c t e d to s h o w a negative

salt

effect, b u t the o b s e r v e d decrease i n rate constant w i t h i n c r e a s i n g i o n i c strength is s o m e w h a t less t h a n e x p e c t e d .

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

25.

WARD AND KUO

Nucleic

Acid

371

Components

T a b l e I shows rate constants for r e a c t i o n of some n u c l e i c a c i d c o m ­ ponents w i t h C l ~ a n d w i t h O H radicals. 2

m i n e d b y Scholes et al. (5).

T h e latter are the data deter­

A l s o s h o w n are our results f r o m steady state

w o r k i n w h i c h the G - v a l u e for base d e s t r u c t i o n , G ( - b a s e ) , was m e a s u r e d for i r r a d i a t i o n s c a r r i e d out i n the presence a n d i n the absence of

10 M l

s o d i u m c h l o r i d e at p H 2.7. Table I. Second Order Rate Constants for Reactions of C l ~ with Pyrimidines, Purines and Deoxynucleotides at p H 2.7 in 1 0 M Sodium Chloride Solution 2

Downloaded by CORNELL UNIV on June 16, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch025

_1

Rate Constant X lO'M seer 1

Cli

+ Base

Steady State Value G(-Base)

1

OH + Base

With Cl~

a

Without

PYRIMIDINES Thymine Uracil Cytosine

12 ± 1.0 4.1 ± 0.3 9.1 ± 0.7

310 290 185

2.4 9.5 5.6

2.4 2.6 2.7

PURINES Adenine Guanine