Pulse Radiolysis Studies of Reactions of Primary Species in Water

28 Pulse Radiolysis Studies of Reactions of Primary Species in Water with Downloaded by UNIV OF GUELPH LIBRARY on May 15, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch028

Nucleic Acid Derivatives C. L. GREENSTOCK, M. NG, and J. W. HUNT Department of Medical Biophysics, University of Toronto, Toronto, Canada

Bimolecular rate constants have been measured for the re actions of the solvated electron (e aq) and the hydroxyl free radical (∙OH) with a variety of nucleic acid derivatives. In neutral solution, the e aq reaction rates are diffusion con trolled and the ∙OH reaction rates are only slightly lower. For uracil, the e aq reactivity is reduced as the p H is in creased partly because of the electrostatic repulsion from the negatively charged molecule and partly because of a tautomeric structural change. The ∙OH reactivity shows no distinct changes around the pK value. The sites of attack of e-aq are probably the carbonyl groups in pyrimidines and the imidazole ring in purines. The primary site of attack of ∙OH is the 5,6 double bond in pyrimidines. The reactivity per nucleotide is considerably reduced in long-chain poly nucleotides, the effect increasing with chain length. -

-

-

a

" V I J T h e n a b e a m o f i o n i z i n g r a d i a t i o n hits l i v i n g cells, i m p o r t a n t c e l l f u n c t i o n s m a y b e a l t e r e d o r d e s t r o y e d . I n p a r t i c u l a r , the r e p l i c a t i o n m e c h a n i s m s of t h e c e l l w h i c h are associated w i t h their genetic m a t e r i a l , d e o x y r i b o n u c l e i c a c i d ( D N A ) , are p a r t i c u l a r l y radiosensitive. to a p p r e c i a t e f u l l y the b i o l o g i c a l i m p l i c a t i o n s of this d a m a g e , u n d e r s t a n d i n g at t h e c h e m i c a l l e v e l is n e e d e d .

I n order further

F o r these reasons, w e

are u n d e r t a k i n g a d e t a i l e d s t u d y of t h e reactions of t h e p r i m a r y reactive species i n w a t e r (e~ , • O H ) w i t h n u c l e i c a c i d d e r i v a t i v e s . m

A large n u m b e r of papers h a v e b e e n p u b l i s h e d i n w h i c h t h e reac t i v i t y of e~

aq

a n d - O H r a d i c a l s w i t h o r g a n i c c o m p o u n d s of b i o l o g i c a l

interest h a v e b e e n s t u d i e d i n a n attempt to d e t e r m i n e t h e c o n t r o l l i n g 397 In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

398

RADIATION CHEMISTRY

factors i n s u c h reactions

I n most cases,

(2, 7, 10, 11, 17, 21, 23).

1

e~

m

acts l i k e a n u c l e o p h i l e , a t t a c k i n g positions of l o w electron density. F o r example, e l e c t r o n w i t h d r a w i n g g r o u p s s u c h as — C = = N increase

and

^c=o " ( f

C

QH

the reactivity, w h i l e H , O H , C H , a n d N H 3

2

groups

r e d u c e i t . W h i l e u n s u b s t i t u t e d h e t e r o c y c l i c c o m p o u n d s are f a i r l y u n reactive, t h e i n t r o d u c t i o n of c a r b o n y l or i m i d a z o l e groups greatly i n creases t h e r e a c t i v i t y .

A n o t h e r factor affecting t h e e~

m

reactivity, the

Downloaded by UNIV OF GUELPH LIBRARY on May 15, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch028

charge o n t h e solute m o l e c u l e , has b e e n i n v e s t i g a t e d b y G o r d o n et al. (14)

a n d D a i n t o n et al. (12)

a n d i n organic molecules b y Braams

(10,

11). T h e observations of H a r t et al. (17,18)

a n d Scholes et al. (23)

have

s h o w n that b o t h t h e p u r i n e a n d p y r i m i d i n e bases of n u c l e i c acids react r a p i d l y w i t h b o t h e~

m

and • O H .

I n this p a p e r , t h e r e a c t i o n rates of these

p r i m a r y species w i t h a v a r i e t y of n u c l e i c a c i d derivatives h a v e

been

m e a s u r e d u n d e r different c h e m i c a l c o n d i t i o n s i n a n attempt to u n d e r s t a n d the factors responsible f o r variations i n t h e i r reactivities. Experimental A 30 M e v . l i n e a r accelerator p r o d u c i n g 0.25-2/xsec. electron pulses d e l i v e r i n g a dose p e r p u l s e of 5 0 - 5 0 0 rads w a s u s e d as t h e r a d i a t i o n source. A S y l v a n i a D X M 250 w a t t q u a r t z i o d i d e l a m p o r a P h i l i p s S P 1000, 1000 w a t t h i g h pressure m e r c u r y a r c was u s e d as the a n a l y z i n g l i g h t source. T h e l i g h t w a s passed f o u r times t h r o u g h a 6 c m . l o n g d e t e c t i o n c e l l a n d w a s b e a m e d b y 8 - i n c h d i a m e t e r m i r r o r s to a n adjacent r o o m w h e r e the d e s i r e d s i g n a l was d e t e c t e d b y a B a u s c h a n d L o m b 50 c m . f o c a l length monochromator a n d D u m o n t 7664Q photomultiplier. T h e output was d i s p l a y e d o n a H e w l e t t P a c k a r d 180A oscilloscope a n d p h o t o g r a p h e d . U p o n o p e n i n g t h e c a m e r a shutter, t h e oscilloscope w a s t r i g g e r e d f o u r times to r e c o r d o n t h e same p h o t o g r a p h t h e zero l e v e l , t h e t r a n s m i t t e d l i g h t l e v e l , a n d also another base l i n e a n d t h e a b s o r p t i o n s i g n a l at h i g h e r sensitivity ( 1 5 ) . T h e l i n a c p u l s e i n t e n s i t y w a s m o n i t o r e d w i t h a secondary emission m o n i t o r (19, 26) w h i c h has t h e same aperture size as t h e i r r a d i a t i o n c e l l . E a c h rate constant d e t e r m i n a t i o n represents t h e average of three d e c a y curves w h i c h w e r e r e a d f r o m t h e p h o t o g r a p h s a n d c o n v e r t e d i n t o d i g i t a l f o r m b y a n O s c a r K 7 7 c u r v e reader. T h e rate constants a n d c o n fidence l i m i t s w e r e c a l c u l a t e d f r o m these d a t a b y m a k i n g a regression fit to a n e x p o n e n t i a l d e c a y w i t h the assistance of a G e n e r a l E l e c t r i c D a t a N e t computer. T h e c o m p o u n d s , cytosine, t h y m i n e , u r a c i l , u r i d i n e , u r i d y l i c a c i d ( U M P ) , u r i d y l y l - ( 3 ' - » 5')-uridine ( U p U ) , oligouridylic acid (oligo U ) , a d e n i n e , a n d adenosine ( C a l b i o c h e m . I n c . ) w e r e c h r o m a t o g r a p h i c a l l y p u r e . T h e s u b s t i t u t e d p y r i m i d i n e s a n d p u r i n e s , 1 , 3 - d i m e t h y l u r a c i l , 2,4diethoxypyrimidine, dihydrouracil, a n d imidazole were obtained from Sigma Chemical Corp. a n d Cyclochemical Corp. T h e C H O H (Fisher) w a s spectroscopic grade a n d K C N S , H 0 , H C 1 0 , K H P 0 , K H P 0 , 3

2

2

4

2

4

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

2

4

28.

Nucleic

GREENSTOCK E T A L .

Acid

399

Derivatives

N a C O , N a H C 0 , N a O H , a n d N a S 0 w e r e " A n a l a R " grade ( B r i t i s h D r u g Houses). A l l these c o m p o u n d s w e r e u s e d w i t h o u t f u r t h e r purification. T h e solutions w e r e p r e p a r e d u s i n g t r i p l y d i s t i l l e d w a t e r o b t a i n e d f r o m a q u a r t z s t i l l b a s e d u p o n a d e s i g n b y K . S c h m i d t ( 2 2 ) . Since t h e r e a c t i o n rates of the c o m p o u n d s s t u d i e d w e r e v e r y d e p e n d e n t o n p H , t h e p H w a s s t a b i l i z e d b y c o m p a r a t i v e l y l o w concentrations (0.5 X 1 0 " M ) of u n r e a c t i v e i n o r g a n i c buffers. A list of t h e buffers u s e d t o c o n t r o l t h e p H are s h o w n i n T a b l e I, together w i t h t y p i c a l e~ lifetimes o f t h e d e o x y g e n a t e d matrices ( b l a n k solutions m i s s i n g o n l y t h e reactive solutes). T o e l i m i n a t e changes i n r e a c t i v i t y as a f u n c t i o n of the i o n i c e n v i r o n m e n t , the e~ studies w e r e done w i t h solutions m a i n t a i n e d at 0.1 i o n i c strength w i t h t h e u n r e a c t i v e salt, N a S 0 . M e t h a n o l ( 1 0 " M ) w a s a d d e d as a n • O H scavenger, a n d t h e solutions w e r e d e o x y g e n a t e d b y b u b b l i n g f o r 15 m i n u t e s w i t h extra h i g h p u r i t y n i t r o g e n ( O h i o C h e m i c a l C o r p o r a t i o n ) , u s i n g H a r t ' s syringe t e c h n i q u e (16). W h e n the e " r e a c t i o n rates w e r e b e i n g m e a s u r e d , t h e c o n c e n t r a t i o n of solute w a s chosen to give a n e~ h a l f - l i f e of a p p r o x i m a t e l y 1 /xsec. H e n c e , at l o w p H values, t h e c o r r e c t i o n t o t h e absolute rate constant because of t h e finite e~ l i f e t i m e i n t h e m a t r i x is as large as 2 0 % , b u t this falls t o b e l o w 1 % at the highest p H (see T a b l e I ) . 2

H

3

2

4

3


m

m

2

2

4

aq

m

m

Table I. Buffers Used to Stabilize p H for e~ Measurements and Typical Half-Lives ( r l / 2 ) Observed in These Deoxygenated Matrices Containing 10"2M Methanol, 5 X 10~4M Buffer and Na 2 S0 4 to 0.1 Ionic Strength m

Buffer KH P0 K HP0 Na C0 N a H C O '3 , NaOH 2

4

2

4

2

3

pH Range

e qrl /2 (fisec.) of Matrix

5- 6 6- 8 8- 9 9 - 11 11-13

5-10 10-20 20-30 30-50 50-150

a

B e c a u s e t h e O H a b s o r p t i o n s p e c t r u m is i n t h e u l t r a v i o l e t r e g i o n of the s p e c t r u m (28), w h e r e a l l n u c l e i c a c i d derivatives absorb strongly, • O H r e a c t i o n rates i n these experiments w e r e m e a s u r e d u s i n g a m o d i fication of the c o m p e t i t i o n m e t h o d of A d a m s et al. (1, 2, 3, 4, 5). I n this m e t h o d , t h e c o m p e t i t i v e solute ( C N S " ) forms a l o n g l i v e d species (P) a b s o r b i n g at 500 m/x f o l l o w i n g its r e a c t i o n w i t h - O H at a rate k . T h e solute ( S ) competes w i t h C N S " f o r - O H at a rate k. n r e d u c i n g t h e y i e l d of P ( R e a c t i o n s 1 a n d 2 ) . 2

0

k-OH

• O H + S —» non-absorbing product

(i)

k • O H + C N S " - » absorbing product ( P )

(2)

2

In the absence of solute, for a given dose, the optical density of the absorbing species is O D . W h e n solute is added, the final optical density 0


400

RADIATION CHEMISTRY

1

( O D ) of the a b s o r b i n g species is r e d u c e d b y the f r a c t i o n of - O H r a d i cals w h i c h n o w react w i t h the solute. T h i s change i n a b s o r p t i o n is g i v e n by:

If k is k n o w n , the v a l u e of k.on m a y be d e t e r m i n e d f r o m the o p t i c a l d e n s i t y change. 2

I n s t u d y i n g reactions w i t h • O H a n e~ scavenger m u s t b e a d d e d to e l i m i n a t e interference f r o m e\ a d d i t i o n p r o d u c t s . N i t r o u s o x i d e is o f t e n u s e d f o r this p u r p o s e , b u t at the h i g h concentrations of C N S " u s e d i n these experiments, u n u s u a l s c a v e n g i n g kinetics w e r e o b s e r v e d , suggesting that the intermediates N 0 " or O " m i g h t be i n t e r f e r i n g w i t h the r e a c t i o n . I n these experiments, a n alternative e l e c t r o n scavenger, H 0 , w a s u s e d . T h e H 0 c o n c e n t r a t i o n m u s t be l o w e n o u g h to p r e v e n t the r e a c t i o n of • O H w i t h H 0 f r o m c o m p e t i n g w i t h Reactions 1 a n d 2. A l t e r n a t i v e l y , i t m u s t be h i g h e n o u g h to ensure efficient e~ s c a v e n g i n g . T h e i m p o r t a n t reactions w h i c h m u s t be c o n s i d e r e d i n the C N S " c o m p e t i t i o n are: m


LX

2

2

2

2

2

2

2

m

k-OH

OH + S

->

products

(4)

• O H + C N S " -> C N S - + O H -

CNS- + CNS ^

(CNS)"

1.4 X 1 0 e

a q

e~

aq

+ H 0 2

+ S

—>

q

OH + OH-

products

1.9 X 1 0 e\

2

(7) (8)

10

->

+ 0

(6)

2

10

->

2

(5)

O -

(9)

a

2 2 X 10~ OH + H 0 -» H0 - + H 0 (10) F o r - O H r e a c t i o n studies, aerated solutions c o n t a i n i n g 2 X 1 0 " M K C N S a n d 1 0 " M H 0 w e r e u s e d . A t the highest concentrations (4 X 1 0 ~ M ) of a t y p i c a l solute, u r a c i l , 3 0 % of the e~ react w i t h u r a c i l , 7 0 % w i t h H 0 , a n d o n l y 1 % w i t h the o x y g e n present i n aerated solutions ( R e a c t i o n s 7, 8, a n d 9 ) . T h e t h e o r e t i c a l l y expected - O H y i e l d s h o u l d consist of almost e q u a l c o n t r i b u t i o n s f r o m the p r i m a r y y i e l d a n d the c o m p o n e n t a r i s i n g f r o m the c o n v e r s i o n of e~ to • O H t h r o u g h R e a c t i o n 7. H o w e v e r , i n the presence of 4 X 1 0 " M u r a c i l , the - O H y i e l d is r e d u c e d to 8 5 % of the e x p e c t e d y i e l d b y R e a c t i o n 8. F o r a l l u r a c i l concentrations, the r e a c t i o n of - O H w i t h H 0 ( R e a c t i o n 10) o n l y reduces the - O H 2

2

2

2

3

2

2

2

3

m

2

2

m

3

2

2


28.

Nucleic


Acid

401

Derivatives

y i e l d b y 1 - 2 % a n d is, therefore i n significant i n the c o m p e t i t i o n for - O H compared with uracil and C N S . A c o r r e c t i o n w a s a p p l i e d for this r e d u c t i o n i n O H y i e l d to e a c h p o i n t i n the c o m p e t i t i o n plot. If this c o r r e c t i o n w e r e not a p p l i e d , it w o u l d l e a d at the w o r s t to a 1 7 % o v e r e s t i m a t i o n i n the absolute rate constant f o r • O H attack o n u r a c i l . T h e transient a b s o r p t i o n spectra o b t a i n e d i m m e d i a t e l y after the p u l s e f r o m aerated K C N S solutions at p H 2, 7 a n d 13.5 r e s p e c t i v e l y , are s h o w n i n F i g u r e 1. T h e s e spectra are i n g o o d agreement w i t h those r e p o r t e d b y A d a m s et al. (3, 4, 5 ) . B o t h H 0 a n d N 0 are f o u n d to d o u b l e the y i e l d of the a b s o r b i n g species (3, 4, 5) as w o u l d b e e x p e c t e d f r o m R e a c t i o n 7. T h i s s p e c t r u m was o r i g i n a l l y t h o u g h t to b e c a u s e d b y the C N S • r a d i c a l ( R e a c t i o n 5 ) , b u t r e c e n t l y B a x e n d a l e et al. ( 9 ) h a v e suggested that the spectra are i n fact c a u s e d b y ( C N S ) " f o r m e d b y R e a c t i o n 6.


2

2

2

2

i

•

1

300

•

400

1

•

500

r

600

X (m//) Figure 1. Transient absorptions in aerated 2 X 10~ M CNS" solutions containing 10 M H 0 at pH values of 2, 7, and 13.5. The dotted curve shows the absorption spectrum obtained by Adams et al. (3) normalized to the curve obtained at pH 7. Dose per pulse approximately 500 rads 3

2

2

2

T h e d i r e c t o b s e r v a t i o n of O H r e a c t i o n rates b y the d i s a p p e a r a n c e of the 5, 6 d o u b l e b o n d i n u r a c i l w a s also u s e d i n these experiments. T h i s m e t h o d has m a n y advantages since it is not subject to the uncertainties w h i c h p l a g u e the c o m p e t i t i o n experiments. H o w e v e r , s u c h a d i r e c t m e t h o d is c o m p l i c a t e d b y the necessity to use a r e l a t i v e l y h i g h c o n c e n -


402

RADIATION CHEMISTRY

1

t r a t i o n of u r a c i l ( a b o u t 1 0 " M ) so that the r e a c t i o n is fast, w h i l e o b t a i n i n g a reasonable l i g h t t r a n s m i s s i o n i n the r e g i o n of the 5 , 6 d o u b l e b o n d absorption (e 9 = 8.20 X 1 0 M c m . " ) . T o a c c o m p l i s h this, t w o l i g h t passes t h r o u g h a 5 m m . d e t e c t i o n c e l l w e r e u s e d . S c a t t e r e d l i g h t correc tions u n d e r these c o n d i t i o n s w e r e less t h a n 1 0 % . I n a d d i t i o n , a large r a d i a t i o n p u l s e ( 1 - 4 K r a d ) w a s u s e d to c h a n g e a m e a s u r a b l e f r a c t i o n ( a b o u t 1 0 % ) of the u r a c i l i n each p u l s e . D a r k c u r r e n t f r o m the l i n a c , w h i c h m i g h t h a v e r e d u c e d the c o n c e n t r a t i o n of u n r e a c t e d u r a c i l , w a s e l i m i n a t e d b y a d d i n g a p n e u m a t i c a l l y o p e r a t e d b l o c k i n the b e a m l i n e w h i c h m o v e d out of the w a y 1/2 s e c o n d b e f o r e the p u l s e . T h e samples w e r e c h a n g e d after each p u l s e b y a r e m o t e l y - c o n t r o l l e d flow system. S e v e r a l matrices w e r e t r i e d f o r the d i r e c t - O H r e a c t i o n t e c h n i q u e , b u t o x y g e n a t e d , d e o x y g e n a t e d , a n d N 0 b u b b l e d solutions s h o w e d ab s o r b i n g species i n the 2 5 0 to 2 7 0 m/x r e g i o n , so n o clear-cut measurements of the 5 , 6 d o u b l e b o n d d i s a p p e a r a n c e c o u l d be m a d e . F o r t u n a t e l y , h o w ever, n o i n t e r f e r i n g species w e r e present at 2 7 0 m/x i n solutions of H 0 and uracil ( e = 5.6 X 1 0 M c m . " ) . A t a c o n c e n t r a t i o n of 0 . 5 X 1 0 " M H 0 , e~ are r a p i d l y s c a v e n g e d a n d c o n v e r t e d to * O H , a n d w e l l d e f i n e d traces m a y be o b t a i n e d s h o w i n g increases i n l i g h t t r a n s m i s s i o n c a u s e d b y • O H attack of the 5 , 6 d o u b l e b o n d . 4

3


25

- 1

1

2

2

3

2 7 0

3

2

2

Results and

_ 1

2

1

m

Discussion The

Solvated Electron Reaction Rates. PURINES A N D PYRIMIDINES. r e a c t i o n rates of e~

aq

w i t h p u r i n e s a n d p y r i m i d i n e s at n e u t r a l p H are

s h o w n i n T a b l e II. A l l are v e r y reactive, the r e a c t i o n rates b e i n g close to diffusion controlled.

H o w e v e r , the p y r i m i d i n e cytosine, w h i c h has

an

a m i n o g r o u p at the C - 4 p o s i t i o n , is s o m e w h a t less reactive t h a n t h y m i n e a n d u r a c i l w h i c h h a v e c a r b o n y l groups at this p o s i t i o n . A d e n i n e , w h i c h also has a n a m i n o g r o u p i n this p o s i t i o n , has a v e r y h i g h r e a c t i v i t y , b u t this is p r o b a b l y because of the presence of the p o s i t i v e l y c h a r g e d i m i d a zole ring. A l s o i n T a b l e I I are l i s t e d the r e a c t i o n rates of e'

m

w i t h a series of

c o m p o u n d s h a v i n g s i m i l a r substituents or s t r u c t u r a l groups to the p u r i n e s and pyrimidines. Benzene

(ke~

1.4 X

=

m

10 M 7

1

sec." ) is v e r y u n r e 1

active, b u t the i n t r o d u c t i o n of a h e t e r o c y c l i c n i t r o g e n i n t o the b e n z e n e r i n g increases the r e a c t i v i t y to 1 0 M 9

_ 1

sec. . 1

I n t r o d u c t i o n of c a r b o n y l g r o u p s onto carbons

2 and 4, brings

the

r e a c t i v i t y of the resultant c o m p o u n d , u r a c i l , u p to t h a t of a d i f f u s i o n controlled reaction

(1.5 X

has b e e n s h o w n (7)

that a d d i n g - O H groups to b e n z e n e does not increase

10

1 0

M

1

sec." ). 1

I n b e n z e n e d e r i v a t i v e s , it

the r e a c t i v i t y , b u t a d d i n g t w o c a r b o n y l groups to f o r m

benzoquinone

increases the r e a c t i v i t y a p p r e c i a b l y to 1.3 X

1

10 M 9

_ 1

sec. .

When

the

5 , 6 d o u b l e b o n d i n u r a c i l is saturated to f o r m d i h y d r o u r a c i l , a t h r e e - f o l d decrease i n r e a c t i v i t y occurs.

T h i s suggests that the presence of the 5,6

d o u b l e b o n d is at least p a r t l y necessary f o r the u r a c i l to s h o w h i g h r e a c t i v i t y . H o w e v e r , the h i g h r e a c t i v i t y of c y c l o h e x a n o n e

(0.8 X


10 ) 1 0

28.

Nucleic


Acid

403

Derivatives

suggests that other changes i n the d i h y d r o u r a c i l m o l e c u l e m a y be m o r e important.

A l l these observations p o i n t to the c a r b o n y l groups of

p y r i m i d i n e m o l e c u l e as the most reactive I n p u r i n e s , s u c h as a d e n i n e

(see

the

sites.

Table II),

a n i m i d a z o l e r i n g is

a t t a c h e d to a p y r i m i d i n e r i n g . I m i d a z o l e itself is reactive, b u t less reac t i v e t h a n the p u r i n e m o l e c u l e . p u r i n e s is caused b y e~

m

T h i s suggests that the h i g h r e a c t i v i t y of

attack o n the i m i d a z o l e r i n g , or b y the i n d u c t i v e

effect of the i m i d a z o l e g r o u p o n the p y r i m i d i n e r i n g .


M o r e d e t a i l e d studies w i t h s u b s t i t u t e d p u r i n e s a n d p y r i m i d i n e s at different p H values c o n f i r m the above

findings.

F i g u r e 2 shows the rate

constants as a f u n c t i o n of p H as w e l l as the i o n i z a b l e groups i n u r a c i l a n d u r i d i n e a n d their p K

a

values.

A s the p H is r a i s e d above each p K

a n d the molecules b e c o m e n e g a t i v e l y c h a r g e d , the e~

0

r e a c t i o n rates f a l l .

m

T h i s is to be expected because of the electrostatic r e p u l s i o n b e t w e e n the n e g a t i v e l y c h a r g e d m o l e c u l e a n d the n e g a t i v e l y c h a r g e d

e~ . m

F r o m the slope of the c u r v e at p H 12.5, i t c a n b e seen that the re a c t i v i t y is not so d r a s t i c a l l y c h a n g e d b y i o n i z a t i o n of the ribose sugar i n u r i d i n e as it is b y the i o n i z a t i o n of the p y r i m i d i n e r i n g . T h e D e b y e e q u a t i o n (13)

for a d i f f u s i o n c o n t r o l l e d r e a c t i o n p r e d i c t s

a p p r o x i m a t e l y a t w o f o l d decrease i n r e a c t i v i t y of e~ the m o l e c u l e b e i n g attacked.

T h e values of r ! = r a d i i of e~ 2

sec."

1

u p o n i o n i z a t i o n of

3A. and r

2

=

3 A . w e r e u s e d as the

a n d u r a c i l respectively, a n d the values of D

m

cm.

atl

( I n the D e b y e e q u a t i o n :

±

and D

2

|[ H

+

405

Acid Derivatives

ke~ (M a g

_ I

sec.' ) 1

Reference

6

8 X 10

9

7

6 X 10

9

17

7

4.5 X 1 0

9

This work

6

3 X 10

10

6 X 10

3

25

6

3.4 X 1 0

9

This work

7

1.7 X 1 0

1 0

t o m e r i s m at n e u t r a l p H , the reactivities of e~

m

with

1 0

This work

17

1,3-dimethyluracil

and 2,4-diethoxypyrimidine were measured. T h e results s h o w n i n T a b l e I I I i n d i c a t e that the exclusive e n o l f o r m of the 2 a n d 4 c a r b o n y l groups i n 2 , 4 - d i e t h o x y p y r i m i d i n e leads to a five f o l d decrease i n r e a c t i v i t y . A l t e r n a t i v e l y , 1 , 3 - d i m e t h y l u r a c i l , w h i c h is i n


406

RADIATION CHEMISTRY

the d i k e t o f o r m s i m i l a r to u r a c i l , has the same r e a c t i v i t y as

1

uncharged

u r a c i l , the m e t h y l groups a p p a r e n t l y h a v i n g l i t t l e influence o n the re a c t i v i t y . I n 1 , 3 - d i m e t h y l u r a c i l a n d 2 , 4 - d i e t h o x y p y r i m i d i n e , the m e a s u r e d rate constants are the same at p H 7 a n d p H 11 because the p o t e n t i a l l y i o n i z a b l e groups h a v e b e e n b l o c k e d . F r o m these observations, it w o u l d be p r e d i c t e d that a p a r t i a l ketoe n o l t a u t o m e r i s m of the c a r b o n y l groups (last entry i n T a b l e I I I )

such

as is f o u n d i n the s i n g l y i o n i z e d u r a c i l m o l e c u l e , w o u l d l e a d to a t w o Downloaded by UNIV OF GUELPH LIBRARY on May 15, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch028

a n d a h a l f - f o l d decrease i n r e a c t i v i t y . e~

aq

H e n c e , the

fivefold

decrease i n

r e a c t i v i t y u p o n i o n i z a t i o n of the p y r i m i d i n e r i n g m a y b e

accounted

for b y a t w o f o l d decrease because of the negative c h a r g e a n d a t w o a n d a h a l f - f o l d decrease because of the p a r t i a l t a u t o m e r i z a t i o n . URAC —.

r

,

°

ill: 1

V

°N

V

1

i

P K, 10"

1

,

o 0

10

, IL

i 6

i 8

1

10

—* PK 1

. 2

1

12

14

PH 1

10

1

8

URIDINE

>

P K, 10"

i

i

i

8

10

pK i 12

2

J

14

pH Figure 2. The effect of pH on the rate constants for the reactions of e~ with uracil and uridine. Also shown are the ionizable groups of uracil and uridine and their pK values. The unionizable OH groups on the ribose molecules are not shown ao

a


28.


Table III.

Nucleic

Acid

407

Derivatives

Effect of Keto-enol Tautomerism on e q Reactivity of U r a c i l

Molecule

a

pH

ke~ (M

Structure

og

1

sec. *)

0


Uracil

1.5 X 1 0

3 X 109

11 O^N

CH 1,3-Dimethyluracil

W

'

3 \ "IT

1.65 X 1 0

1 0

1.45 X 1 0

1 0

i CH,

11

N o change

0C H 2

2,4-Diethoxypyrimidine

N

7

C^H 0^ 5

11

5

2.8 X 10° N

I

0


I

0 = P-0"

0"

0"

OH

OH

V

0

1

1

0 • P-0~

I

N

0~

0

I

0 = P-OH

0

OH

K ?

2'

0 = P-0~

pK,= 5.9

pK =9.4 (3 charges)

/v v

A . 0

(2 charges)

0"

0"

k

u

OH

3 0

0"

ok X

N

1

2' 3-CYCLIC U M P

MONOPHOSPHATE(UMP)

3-URIDINE

CHEMISTRY

0

pK =I2.5

2

pK =9.4

pK = l2.5

(2 charges)

(3 charges)

2

3

(4 charges)

V /V 3

Figure 3. Ionizable groups and pK values in 3'-UMP and 2',3'-cyclic UMP. Note that the second ionization which occurs in 3'-UMP at pH 5.9 is absent in 2',3'-cyclic UMP. The unionizable OH groups on the ribose molecule are not shown &

1

id

Mononucleotides

1

1

_

0

:

*^ = 8

* k\ 5 8

3*

\\\ ^ V N ) 2',3-cyclic UMP | ;

t , UMP i

\

\

2'3-UMP(mixed)

UMP "

, 5-UMP

3

i

i 10

i

i 14

PH Figure 4. The effect of pH on the rate constants of e~ with the mononucleotides of uracil. Note UMP does not show a change in reactivity around UMP (mixed) and 5'-UMP which become doubly above this pH aa

for the reactions that 2\3'-cyclic pH6 as do 2',3'ionized (UMP ~) 2

N U C L E O T I D E S . P h o s p h a t e groups m a y b e attached to the ribose g r o u p of u r i d i n e at t h e 2', 3', or 5 ' positions to give 2', 3', or 5 ' - u r i d y l i c a c i d (UMP).

T h e i o n i z a b l e groups i n 3 ' - U M P are s h o w n i n F i g u r e 3. T h e s e


28.


Nucleic

Acid

409

Derivatives

groups are n e g a t i v e l y c h a r g e d at p H 5 c a u s i n g a 1.5 f o l d decrease i n the e~

m

r e a c t i v i t y c o m p a r e d w i t h u r a c i l or u r i d i n e ( F i g u r e 4 ) . T h e p h o s p h a t e

g r o u p becomes d o u b l y i o n i z e d at a p K decrease i n the e~

rate constant.

m

( t w o f o l d ) t h a n for 2', 3 ' - U M P p e c t e d i f the e~

m

of 5.9 w h i c h results i n a f u r t h e r

a

T h i s decrease is larger f o r 5 ' - U M P

(mixed)

(one a n d a h a l f - f o l d ) as

ex

attacks p r i n c i p a l l y the p y r i m i d i n e r i n g , since a negative

p h o s p h a t e g r o u p o n the 5 ' p o s i t i o n is m u c h closer to the site of e~

m

attack

t h a n w h e n it is o n the 2 ' or 3 ' p o s i t i o n , a n d s h o u l d h a v e a greater i n f l u


ence. T h e 2', 3'-eyclic U M P ( F i g u r e 3 ) does not h a v e a s e c o n d i o n i z a b l e g r o u p o n the p h o s p h a t e as this is t a k e n u p f o r m i n g the 2', 3 ' b o n d . A s expected, the decrease i n r e a c t i v i t y at p H 5.9 is absent f o r 2', 3'-eyehc UMP.

T h e three m o n o n u c l e o t i d e s , l i k e u r a c i l a n d u r i d i n e , a l l s h o w a

large decrease i n r e a c t i v i t y at the pK

a

for the i o n i z a t i o n of the c a r b o n y l

group. F o r u r i d y l i c a c i d d i n u c l e o t i d e s ( U p U ) a n d short c h a i n p o l y n u c l e o tides ( o l i g o U ) , a m a r k e d p r o t e c t i v e effect against e~

attack is o b s e r v e d .

m

T h e r e a c t i v i t y per n u c l e o t i d e has b e e n m e a s u r e d as a f u n c t i o n of p H for U p U a n d for a n o l i g o U m o l e c u l e w h i c h contains a b o u t 20 nucleotides. T h e results are c o m p a r e d w i t h those f o r u r a c i l i n F i g u r e 5. I n the absence _

,

l

1

4

1

, 1 , 1 Polynucleotides

,

i

.

«

6

i

.

8

i

10

,

1

i

.

12

,

i

:

I

14

pH Figure 5. The effect of pH on the rate constants for the reactions of e~ with the dinucleotide (UpU) and oligonucleotide (oligo U, about 20 bases) of uracil. Note that these are plotted as the absolute rate constant per nucleotide. The reactivity of uracil (u) is shown for comparison aa


410

RADIATION CHEMISTRY

1

of a n y s h i e l d i n g effect, the rate constant per n u c l e o t i d e w i l l be u n c h a n g e d . H o w e v e r , i n these results, the r e a c t i v i t y p e r n u c l e o t i d e decreases as the n u m b e r of nucleotides i n the c h a i n increases. H y d r o x y l Radical Reaction Rates. of &.OH, the

COMPETITION METHOD.

Values

- O H r e a c t i o n rate w i t h u r a c i l , h a v e b e e n m e a s u r e d as a

f u n c t i o n of p H b y u t i l i z i n g c o m p e t i t i o n w i t h C N S " ( E q u a t i o n s 1 a n d 2 ) , and

also m o r e d i r e c t l y b y f o l l o w i n g the rate of d i s a p p e a r a n c e

of

the

c h r o m o p h o r e for the 5,6 d o u b l e b o n d . I n the c o m p e t i t i o n t e c h n i q u e , the v a l u e of fc.oii depends u p o n u s i n g the correct v a l u e of k ,

the • O H reac


2

t i o n rate w i t h C N S . B e c a u s e this rate is still s o m e w h a t i n d o u b t n e w measurements

species i n 1 0 " M solutions of K C N S ( F i g u r e 6 ) . 4

If one assumes that the

rate l i m i t i n g step i n the f o r m a t i o n of the a b s o r b i n g species is k

2

k

s

(9)

w e r e m a d e of the rate of b u i l d u p of the a b s o r b i n g

( R e a c t i o n s 5 a n d 6 ) , t h e n a v a l u e for k

2

and not

of 7.5 db 0.5 X 1 0 M 9

sec.

_ 1

1

is o b t a i n e d w h i c h is i n d e p e n d e n t of p H w i t h i n o u r e x p e r i m e n t a l errors (±7%).

T h i s v a l u e is s l i g h t l y h i g h e r t h a n the v a l u e of 6.6 X

sec." r e p o r t e d b y A d a m s et al. 1

(3,

10 M 9

Since there is e v i d e n c e

4, 5).

_ 1

that

the a b s o r b i n g species is not C N S - b u t instead a n i o n - r a d i c a l c o m p l e x ( C N S ) " ( 9 ) , the f o r m a t i o n of this c o m p l e x b y R e a c t i o n 6 m a y a c t u a l l y 2

b e the rate l i m i t i n g step i n this process.

If this w e r e true, the values of

10 M

CNS"

• I0 M

H 0

2

2

2

A N2O saturated 10

pK£ll.9 •0H"T"0"

10"

12

10

14

PH Figure 6. The rate constant for the reaction of OH with the thiocyanate ion (CNS~) as a function of pH, using 10~ M H 0 or N 0 as the e~ scavenger. Note the sharp drop in reactivity above the pK for dissociation of OH to O'. Observations made in a I0~*M KCNS solution at a wavelength of 500 m^ 2

2

2

ao


2

a

28.


Nucleic

Acid

411

Derivatives

k r e p o r t e d here w o u l d b e l o w , a n d a l l values f o r k. n 2

0

w o u l d have to b e

corrected accordingly. A s a check o n the v a l i d i t y of t h e c o m p e t i t i o n k i n e t i c m e t h o d u n d e r the c o n d i t i o n s of these experiments, the rate of - O H r e a c t i o n w i t h isop r o p y l a l c o h o l w a s d e t e r m i n e d u s i n g t h e a b o v e v a l u e of k

2

and compared

w i t h t h e values r e p o r t e d b y other w o r k e r s . T a b l e I V shows the results


o b t a i n e d u s i n g the different m e t h o d s m e n t i o n e d . T h e v a l u e o b t a i n e d f o r

Table I V .

Comparison of O H Reaction Rates with Isopropyl Alcohol by Different Methods

k. y

Competitive

0J

1.25 X 10° 1.2 X 1 0

9

1.7 X 1 0 1.74 X 1 0

Solute

Reference

2 X 10~ M C N S " i n 1 0 " M H O o (aerated) 8-20 X 10~ M thymine ( p H 2 , aerated) 5 X 10" M P N D A (pH9) 3-20 X 1 0 " M K I 3

2

2

5

9

5

9

5

(fc.

0H

This work = 7.5 X l O ^ M ' s e e r ) 23 1

20 27

Figure 7, Competition for OH between uracil and thiocyanate. The ratio of the optical density at 500 mim without solute (ODj, to that with solute (OD) has been plotted as a function of the ratio of solute (S) to CNS~ concentration. The lower curve is corrected for the theoretical decrease in absorbance arising from the failure of H 0 to completely scavenge e~ 2

2

aq


1

412

RADIATION CHEMISTRY

1

&.OH falls w i t h i n the s p r e a d of values r e p o r t e d i n the other experiments, b u t b o t h the C N S " a n d t h y m i n e c o m p e t i t i o n m e t h o d s give values

50%

l o w e r t h a n o b t a i n e d b y u s i n g I" or p - n i t r o s o d i m e t h y l a n i l i n e ( P N D A )

as

the c o m p e t i t i v e solute. F i g u r e 7 shows the result of a t y p i c a l c o m p e t i t i o n p l o t f o r u r a c i l at p H 6.5.

F r o m the slope of the u p p e r straight l i n e (k. n/k ), 0

the - O H r e a c t i o n rate w i t h u r a c i l of 6.4 ± 0.8 X 1 0 M 9

u s i n g the m e a s u r e d v a l u e of k .

1

to u r a c i l ( l o w e r l i n e i n F i g u r e 7)

m


sec.

is o b t a i n e d

W h e n the slope is c o r r e c t e d for the

2

t h e o r e t i c a l loss of e~

a v a l u e for

2

_ 1

as d e s c r i b e d

earlier, a l o w e r v a l u e ( & . H c o r r . ) f o r the • O H r e a c t i o n rate w i t h u r a c i l 0

of 5.0 ± 0.6 X 1 0 M 9

sec." is o b t a i n e d . T h i s c o r r e c t e d v a l u e f o r the - O H

_ 1

1

r e a c t i o n rate w i t h u r a c i l at n e u t r a l p H has b e e n v e r i f i e d u s i n g a h i g h e r c o n c e n t r a t i o n of H 0 2

(5 X

2

10" M).

I n this s o l u t i o n the c o r r e c t i o n is

2

less t h a n 5 % a n d gives a v a l u e of 5.7 ± 1

i6°

I

0.6 X 1 0 M 9

_ 1

sec." . 1

I

i

/

i *

\

'\

• %

!

A

I

\

\

\

!

V

pKohl.9 •0H-|-0" 10 —

~i

i

i

6

8

10

14

PH Figure 8, The effect of pH on the rate constant for the reactions of OH with uracil obtained using the CNS~ competition technique T h e r e appears to b e a g r a d u a l increase i n • O H r e a c t i v i t y w i t h u r a c i l as the p H is v a r i e d f r o m 5 to 10.5 ( F i g u r e 8 ) .

T h e r e is no a b r u p t c h a n g e

i n r e a c t i v i t y at the p K i n d i c a t i n g that the changes i n charge a n d tauto 0

m e r i c f o r m w h i c h o c c u r at the pK

a

h a v e no effect o n the - O H r e a c t i o n .

T h e l o w e r values f o r the - O H r e a c t i v i t y o b t a i n e d at p H 2 a n d 13.5 both considerably i n doubt.

are

T h e v a l u e at p H 2 is d o u b t f u l because of

the p o s s i b i l i t y that H - m a y b e e n t e r i n g i n t o the r e a c t i o n i n some w a y , a n d the p H 13.5 v a l u e because this is above the p K f o r the d i s s o c i a t i o n a


28.

of

Nucleic


Acid

413

Derivatives

- O H to O " . T h e n o n - e q u i l i b r i u m c o n d i t i o n s existing f o l l o w i n g

the

e l e c t r o n p u l s e m a k e it u n c e r t a i n w h e t h e r the • O H or 0 ~ r a d i c a l is react ing.

The

- O H r e a c t i v i t y w i t h d i h y d r o u r a e i l , i n w h i c h the 5,6 b o n d is

saturated, is less t h a n one-fifth of that of u r a c i l

1 X

10 M 9

- 1

sec. ). - 1

T h i s is e v i d e n c e that - O H attacks the 5,6 d o u b l e b o n d as suggested

by

the r a d i o l y s i s p r o d u c t analysis studies i n w h i c h the t o t a l loss i n p y r i m i d i n e


is the same as the y i e l d c a l c u l a t e d f r o m the loss of 5,6 d o u b l e b o n d

(24).

Figure 9. The disappearance of the 5,6 double bond absorption in an aerated 0.25 X 10~ M uracil solution containing 0.5 X 10~ M H 0 as an e~ scavenger. The wavelength of observation was 270 mjn and dose about 3 Krads. The two upper traces correspond to the signal levels with analyzing light off (upper) and on (central trace) at a gain of 0.05 volts/cm. The lower traces were obtained at a gain of 0.005 volts/cm., the increase in light transmission corresponding to loss of 5,6 double bond absorption for approximately 12% of the uracil. The sweep rate was 5fx sec./cm. graticule division 4

3

2

2

DIRECT OBSERVATION OF

* O H REACTION

RATES.

The

reactivity

of

the 5,6 d o u b l e b o n d of p y r i m i d i n e s w i t h • O H r a d i c a l s suggested that the disappearance measurement

of this c h r o m o p h o r e m i g h t b e u s e d f o r a m o r e of the absolute rate constant

for t h e i r reactivities.

direct The

increase i n t r a n s m i s s i o n at 2 7 0 m/x because of the • O H attack o n u r a c i l is s h o w n i n F i g u r e 9. T h e • O H r e a c t i o n rates w i t h u r a c i l o b t a i n e d u s i n g this t e c h n i q u e are c o m p a r e d w i t h values o b t a i n e d u s i n g c o m p e t i t i o n m e t h o d s a n d different values of k

2

(see

Table V ) .

T h e rate constants m e a s u r e d


414

RADIATION CHEMISTRY

Table V .

Comparison of Rate Constants Measured k ( M " seer )

Method 1.

1

2

J

[C.S.] (M)

1

Loss of 5,6 double bond

Dose rate reduced 50% 2.

CNS" Competition


3. 4.

(corrected)

7.5 ± 0.5 X 10

9

2 X 10"

7.5 ± 0.5 X 10

9

2 X lO" 3

2 X 10 (9)

CNS" (corrected)

2 X 10-s

10

6.6 X 10

5. CNS" 6.

3

10"

4

9

5 X 10"*

P N D A competition

using three different uracil concentrations

agree within

experimental

error, the value of 7.4 ± 1.0 X 10 being quite close to the uncorrected 9

value obtained by C N S " competition. A reduction in the dose by a factor of two did not change the decay rate, indicating that radical-radical reactions were not entering into the reaction scheme.

Since the direct

measurement of 5,6 double bond disappearance does not suffer from the uncertainties of the C N S " measurement, it should yield a reliable value. It should be noted, however, that if the value of k reported by Baxendale 2

et al. (9) is used in these calculations (Table V , No. 4), a much higher value of 1.3 X 10 is obtained for k. u corr. Using a competition reaction 10

0

with P N D A , Kraljic and Trumbore (20) have also calculated a value of 5.4 X 1 0 M 9

_ 1

s e c . for fc. H, at p H 9 which is in reasonable 1

0

with the values reported in this paper.

agreement

From the results in Table V it

appears that the C N S " competition technique of Adams et al. (3, 4, 5) gives a slightly lower value for the - O H reaction rate with uracil but a much larger rate constant for the reaction, C N S " -\- • O H —> C N S • + • O H " (9) would not be in keeping with the direct observations. A true value for k of 1.0 X 1 0 M 2

1 0

1

sec." is suggested by these measurements: 1

R A T E S WITH O T H E R N U C L E I C ACID DERIVATIVES.

T h e - O H reaction

rates with uridine, U M P , and adenine are compared in Table V I with those for uracil obtained using the same C N S " competition method. U r i dine and 2',3'-UMP (mixed) at neutral p H show lower corrected - O H reaction rates of 2.9 X 1 0 M 9

_ 1

sec. and 3.5 X 1 0 M 1

9

_ 1

sec." respectively, 1

indicating that the ribose sugar and phosphate groups are not highly reactive sites for - O H attack. Adenine, the complementary purine base to uracil in R N A has a much lower - O H reactivity than uracil of 1.9 X


28.


for the Reaction of [fi] (M) 0.25 0.5 1.0 0.25

X X X X

Nucleic

k. 7.9 6.9 8.6 6.2 7.4

4

7.0

4

4 4

M e a n value


^ 4 X 10~

0 H

(/LQH) Reference

( M - ' sec. *)

± ± ± ± ±

1.1 0.9 1.2 0.6 1.0

X X X X X

415

Derivatives

O H Radicals with U r a c i l

pH

10" 10~ 10" 10"

Acid

10 10 10 10 10

This This This This

9 9 9 9

work work work work

9

3

6.5

6.4 ± 0.8 X 1 0

9

This work

^ 4 X 10-s

6.5

5.0 ± 0.6 X 1 0

9

This work

< 4 X lO-a

6.5

^ 3 X 10"

5-7

4

9

1

9 and this work

3.1 ± 0.4 X 1 0

9

23

5.4 X 1 0

9

20

(1.3 ± 0.2 X 1 0 )

9 10 M

10

sec." . T h i s w o u l d b e i n agreement w i t h t h e l o w e r G - v a l u e f o r 1

loss of t h e a d e n i n e base i n i r r a d i a t e d D N A solutions

(24).

Conclusion, Summary and Discussion T h e r e a c t i o n rates of u r a c i l a n d its d e r i v a t i v e s w i t h solvated electrons h a v e b e e n s t u d i e d i n d e t a i l . O n t h e basis of v a r i a t i o n s of t h e e~

m

reaction

rate as a f u n c t i o n of p H , a n d also w i t h the t a u t o m e r i c f o r m of t h e m o l e c u l e , i t is c o n c l u d e d that t h e changes i n r e a c t i v i t y are too great to b e a c c o u n t e d f o r b y charge alone, a n d that t h e k e t o - e n o l p r o t o t r o p h y w h i c h occurs d u r i n g i o n i z a t i o n is r e s p o n s i b l e f o r t h e large f r a c t i o n of t h e c h a n g e i n t h e r e a c t i o n rate. T h e s e results also s h o w that t h e c a r b o n y l groups of u r a c i l are r e s p o n s i b l e f o r t h e h i g h r e a c t i v i t y of these m o l e c u l e s . Table V I .

O H Reaction Rates with Other Nucleic A c i d Derivatives Molecule

k. (corrected)

Uracil Uridine 3 ' , 5 ' - U M P (mixed) Adenine

5.0 2.9 3.5 1.9

Studies of the e~

0H

m

± ± ± ±

0.6 0.4 0.5 0.3

(M~* sec.' ) 1

X X X X

10 10 10 10

9 9 9 9

r e a c t i o n rates w i t h u r i d i n e , U M P , a n d o l i g o - U

s h o w that t h e sugar a n d p h o s p h a t e groups are u n r e a c t i v e , a n d as o n e w o u l d p r e d i c t , s l i g h t l y r e d u c e d changes o c c u r i n t h e r e a c t i o n rates at t h e p H values w h e r e i o n i z a t i o n takes p l a c e i n the p h o s p h a t e a n d sugar g r o u p s . I n t h e larger c h a i n o l i g o U m o l e c u l e s s t u d i e d ( a b o u t 20 b a s e s ) , t h e r e a c t i v i t y p e r n u c l e o t i d e is c o n s i d e r a b l y r e d u c e d , suggesting that steric effects are i m p o r t a n t .


416

RADIATION CHEMISTRY

1

T h e values here f o r - O H r a d i c a l r e a c t i o n rates w i t h u r a c i l m e a s u r e d u s i n g t h e C N S " c o m p e t i t i o n m e t h o d , w h e t h e r c o r r e c t e d or not, are a p p r e c i a b l y h i g h e r t h a n those o b t a i n e d b y Scholes et al. (23) m e t h o d (6.4 or 5.0 X 1 0 M 9

5-7).

- 1

using a similar

sec." , as against 3.1 X 1 0 M 1

9

1

sec." at p H 1

T h i s c a n o n l y p a r t i a l l y b e e x p l a i n e d b y the s l i g h t l y h i g h e r v a l u e f o r

k . o H + cNs- o b t a i n e d i n o u r measurements.

H o w e v e r , t h e values o b t a i n e d

b y Scholes w e r e m a d e u s i n g s u c h a l o w c o n c e n t r a t i o n of C N S " that these ions w o u l d have b e e n u n a b l e c o m p l e t e l y to scavenge the - O H r a d i c a l s


(1,2)

a n d m i g h t l e a d to this d i s c r e p a n c y . T h e - O H r e a c t i v i t y w i t h u r a c i l

m e a s u r e d d i r e c t l y b y the d i s a p p e a r a n c e of the u r a c i l 5,6 d o u b l e b o n d is s l i g h t l y h i g h e r t h a n that o b t a i n e d b y C N S " c o m p e t i t i o n . T h i s m a y i n d i cate that the v a l u e of k

2

of 7.5 X l O - W "

m a x i m u m of about 5 0 % . A t present, h i g h e r values of k

2

(9)

1

sec." s h o u l d b e i n c r e a s e d a 1

there is n o evidence that

w o u l d b e c o m p a t i b l e w i t h the m o r e

even direct

observations. T h e l o w e r r e a c t i v i t y of t h e h y d r o x y l r a d i c a l w i t h t h e p a r t i a l l y satu r a t e d u r a c i l d e r i v a t i v e , d i h y d r o u r a c i l , is i n agreement w i t h t h e r e a c t i o n schemes p r o p o s e d o n the basis of r a d i o l y s i s measurements w h i c h i n d i c a t e that d a m a g e to u r a c i l is a l w a y s a c c o m p a n i e d b y saturation of t h e 5,6 d o u b l e b o n d . H o w e v e r , w h e n one compares the r a d i o l y s i s y i e l d s i n t h e absence of t h e electron scavenger

o x y g e n , m a n y pieces

a p p e a r to b e

m i s s i n g i n t h e p u z z l e . A l t h o u g h s o l v a t e d electrons react m o r e r a p i d l y w i t h u r a c i l t h a n d o - O H r a d i c a l s , the p r o d u c t y i e l d u n d e r c o n d i t i o n s w h e r e b o t h species react is greatly r e d u c e d — i . e . , the y i e l d f o r loss of u r a c i l i n o x y g e n is 2.1 as against 0.4 i n a d e o x y g e n a t e d s o l u t i o n i n s t e a d of b e i n g d o u b l e d b y the attack of b o t h • O H a n d e~ .

(29)

T h i s sug

aq

gests some t y p e of a b a c k reaction, a n d experiments are b e i n g c a r r i e d out at present i n a n attempt to s t u d y this.

Acknowledgments W e are p l e a s e d to a c k n o w l e d g e t h e use of the l i n e a r

accelerator

facilities i n t h e P h y s i c s D e p a r t m e n t of the U n i v e r s i t y of T o r o n t o .

In

p a r t i c u l a r w e w i s h to t h a n k K . G . M c N e i l , E . H o r r i g a n , T . E l d e r , a n d t h e l i n a c operators

f o r their assistance i n g e t t i n g the l i n a c o p e r a t i n g f o r

p u l s e r a d i o l y s i s w o r k . T h e experiments w e r e c a r r i e d o u t w i t h

financial

assistance f r o m t h e M e d i c a l R e s e a r c h C o u n c i l of C a n a d a , the N a t i o n a l C a n c e r Institute, a n d the O n t a r i o C a n c e r Institute.

T w o of us ( C . L . C ,

a n d M . N . ) w i s h to a c k n o w l e d g e t h e financial s u p p o r t of t h e N a t i o n a l C a n c e r Institute.

T h e authors w i s h to t h a n k their f e l l o w colleagues K i m

U m e m o t o , M . J . B r o n s k i l l , a n d M . D u k e f o r their assistance i n this w o r k a n d to H . E . Johns f o r h i s k e e n interest i n r e a d i n g t h e m a n u s c r i p t a n d o f f e r i n g v a l u a b l e suggestions.


28.


Nucleic

Acid Derivatives

417


Literature Cited (1) Adams, G. E., Boag, J. W., Currant, J., Michael, B. D., "Pulse Radiolysis," p. 117, Academic Press, New York, N. Y., 1965. (2) Ibid., p. 131. (3) Adams, G. E., Boag, J. W., Michael, B. D., Proc. Chem. Soc. 1964, 411. (4) Adams, G. E., Boag, J. W., Michael, B. D., Trans. Faraday Soc. 61, 1417 (1965). (5) Ibid., 61, 1674 (1965). (6) Anbar, M., Hart, E. J., J. Am. Chem. Soc. 86, 5633 (1964). (7) Anbar, M., Meyerstein, D., Neta, P.,J.Phys. Chem. 70, 2660 (1966). (8) Anbar, M., Alfassi, Ζ. B., Reissler, H. (unpublished results). (9) Baxendale, T. H., Stott, D. Α., Chem. Comm. 1967, 699. (10) Braams, R., Rad. Res. 27, 319 (1966). (11) Ibid., 31, 8 (1967). (12) Dainton, F. S., Watt, W. S, Proc. Roy. Soc. 275A, 447 (1963). (13) Debye, P., Trans. Electrochem. Soc. 82, 265 (1942). (14) Gordon, S., Hart, E. J., Matheson,M.S.,Rabani, J., Thomas, J. K.,J.Am. Chem. Soc. 85, 1375 (1963). (15) Greenstock, C. L., Bronskill, M. J., Hunt, J. W. (in preparation). (16)

Hart, E. J., Fielden, E. M., ADVAN. C H E M . SER. 50, 253 (1965).

(17) (18) (19) (20) (21) (22) (23)

Hart, E. J., Gordon,S.,Thomas, J. K., J. Phys. Chem. 68, 127 (1964) Hart, E. J., Thomas, J. K., Gordon, S., Radiation Res. Suppl. 4, 74 (1964). Karzmarck, J., Rev. Sci. Inst. 35, 1646 (1964). Kraljic,I.,Trumbore, C. N.,J.Am. Chem. Soc. 87, 2547 (1965). Rabani, J. Matheson, M. S.,J.Am. Chem. Soc. 86, 3175 (964) Schmidt, K., personal communication (1966). Scholes,G.,Shaw, P., Willson, R. L., Ebert, M., "Puise Radiolysis," p. 151, Academic Press, New York, N. Y., 1965. Scholes, G., Ward, J. F., Weiss, J.,J.Mol.Biol.2, 379 (1960). Szutka, Α., Thomas, J. K., Gordon, S., Hart, E. J.,J.Phys. Chem. 69, 289 (1965). Taimuty, S.I.,Deaver, B. S., Jr., Rev. Sci. Inst. 32, 1098 (1961). Thomas, J. K., Trans. Faraday Soc. 61, 702 (1965). Thomas, J. K., Rabani, J., Matheson, M. S., Hart, E. J., Gordon, S., J. Phys. Chem. 70, 2409 (1966). Umemoto, K.(unpublishedresults).

(24) (25) (26) (27) (28) (29)

RECEIVED February 2,

1968.


Pulse Radiolysis Studies of Reactions of Primary Species in Water

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