Large hydrated electron yields in the gamma radiolysis of acidic, oxygen-containing cysteine solutions

Int. J. Radiat. Phys. Chem. 1975, Vol. 7, pp. 559-563. Pergamon Press. Printed in Great Britain

LARGE HYDRATED ELECTRON YIELDS IN THE GAMMA RADIOLYSIS OF ACIDIC, OXYGEN-CONTAINING CYSTEINE SOLUTIONS A. AL THANNON,* A. EL SAMAHY~"and C. N. TRUMBORE Department of Chemistry, University of Delaware, Newark, Delaware 19711, U.S.A.

(Received 29 August 1974) Abstract--The dependence of H~ yields on cysteine concentration has been studied for aqueous, airsaturated, acidic solutions of cysteine. Up to cysteine concentrations of about 5 x 10 -3 mol dm -a, kinetic parameters and yields obtained previously for air-free, acidic solutions of cysteine may be used to predict quantitatively the hydrogen yields observed. However, above these cysteine concentrations experimental yields are larger than predicted. This difference is interpreted in terms of an increase of hydrated electron yields with increasing cysteine concentration via a spur scavenging mechanism. Calculated hydrated electron yields as high as 4.5 are found at cysteine concentrations of about 0'1 tool dm -a and correspond with initial hydrated electron yields observed in picosecond pulse radiolysis studies of aqueous solutions. The concentration dependence of the hydrated electron yields also agrees with that predicted from these pulse radiolysis studies. An unusual effect of oxygen remains unexplained and apparently cannot be attributed to chain reactions occurring in the system. INTRODUCTION

THE RADIOLYSmof neutral and acidic cysteine (RSH) in oxygen-free solutions has been investigated extensively tt-a~with general agreement regarding the mechanism of cysteine decomposition by the radiolysis products of water in the RSH concentration region of 10-3-10 -~ mol dm -a. We have proposed tl~ that in acid solutions the radiation yields of hydrated electrons scavenged by cysteine are a very sensitive function of cysteine above 10 -2 moldm -3. However, in the presence of oxygen, the radiolytic decomposition of aqueous cysteine is much more complex and probably involves a chain mechanism, t4~ RESULTS A N D E X I - E R I M E N T A L Using techniques previously described ~1~, we examined the radiolytic hydrogen yields, G(H2), in air-saturated, acidic cysteine solutions as a function of cysteine concentration over the range 10-4-10 -1 tool dm -a with results shown in the lower portions of Figs 1 and 2 where data from airsaturated solutions are contrasted with some previously published results from air-free solutions. ~1~ DISCUSSION

Qualitatively the data for air-saturated solutions may be explained in terms of the set of equations listed below, of which reactions (3), (4), (6) and (7) have been postulated for air-free solutions ~1). (1)

H20

(2)

H" + 02

(3)

H" + RSH

,~ ) e~q-, H', "OH, H2, H202, > HO2", ) H~ + RS',

* Present address: Faculty of Science, Chemistry Department, Riyadh, Saudi Arabia. t Present address: Faculty of Education, University of Tripoli, Tripoli, Libya. 559

560

A. AL THANNON, A. EL SAMAHY and C. N. TRUMBORE

(4) (5) (6) (7)

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M Cysl-eine Flo. ]. Measured and calculated H 2 and eaq- radiations yields in p H = ] (HC10 0 aqueous solutions containing varying cysteine (RSH) concentrations in the presence and absence of air. L o w e r part o f figure: & , measured H 2 yields for air-free R S H solutions; O, measured H 2 yields for air-saturated solutions (dotted line connecting experimental points). Solid lines are theoretical predictions o f G(H2) based u p o n equation (8) using the values: k4/k a = 0.33, k~/k3 = 5-0, GH = 0-6, Ge~- = 3.3 and GtI , = 0.45. U p p e r part o f f g u r e : x , Gea~- calculated for air-saturated solutions on the basis o f the above kinetic parameters except for Ge~-- a n d Gn,, which is function o f ( R S H ) according to the c o n c e n t r a t i o n dependence o f Fig. 1 o f the data collected by H. Schwarz, tS~ assuming a scavenging efficiency in keeping with ke~q-+ R S H = 3 x 10 10 d m 3 mo1-1 s -1 taken f r o m ref. (1); + , Ge~- calculated for air-free solutions from the difference between anticipated and calculated G(HzS) values ta) .

We do not believe that OH radicals play a role in the production of hydrogen and will, therefore, ignore reactions involving them for the purposes of this discussion, which will be concerned with the mechanism of H 2 production. Equation (8) may be derived from reactions (1)-(7), excluding (5) since reaction (7) will be dominant at all RSH concentrations at the low pH's employed in this study. (8)

G(H2) = Gn{1 +k~+ ka[RSH]jk2[O2] 1-x k.[RSH]~ "-1 (-

+ ao o-

k-t-ffff '1

k4

k~[O2] t-1

+%

Hydrated electron yields in gamma radiolysis of cysteine solutions

561

In our previous work with air-free solutions (z), we showed excellent theoretical fits for H 2 yields, shown in Figs 1 and 2 by solid lines through the air-free data points, using equation (8) and the following kinetic parameters and radiation yields: k4/k s = 0.33; k J k 7 = 1.36; G , = 0-6; Ge,~- = 3"3 ( p H = 1), 3"4 ( p H = 0"5); Gri= = 0.45. When these parameters were used to predict H2S yields in air-free solutions, a good fit was found with experimentally determined yields up to RSH concentrations of about 3 x 10 -2 m o l d m -s. With increasing RSH concentration, experimental G(H=S) values increased in their deviation above calculated values, until at [RSH] = 10 -1 moldm -3 the difference was unity, as seen in the upper part of Fig. 1. This discrepancy was attributed to "spur scavenging ''(z). .......

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FIG. 2. Measured and calculated Hz and ¢aq- radiation yields in pH 0'5 (HCIO4) and 0"4 mol d m - 8 HsSO~ aqueous solutions containing varying cysteine (RSH) concentrations in the presence and absence of air. Lower part: Measured yields of H~; A, air-free p H = 0"5 (HC104); V, air-free 0.4 mol d m -3 H=SO4; O, air-saturated pH = 0"5 (HCIO~) solutions. Solid lines are theoretical predictions of G(H=) based upon equation (8) using the parameters indicated in the legend to Fig. 1 except for Ge~- = 3"4. Upper part: x , values of Ge~a- calculated as indicated in Fig. 1 for pH = 0.5, air-saturated solutions.

Oxygen effects In order to calculate hydrogen yields in aerated solutions with equation (8), only one additional ratio of rate constants, k2/k 3, is needed in addition to the collection of kinetic and yield parameters listed above. The value of this ratio is fixed by the average values taken from the literature (8) at 5.0 but should only be critical at low RSH concentrations. Usingthe above listed parameters in equation (8), good agreement between predicted and experimental hydrogen yields is obtained only at RSH concentrations up

562

A. AL THANNON,A. EL SAMAHYand C. N. TRUMBORE

to 5 × 10 -3 m o l d m -3 for pH = 1 and 10 --2 m o l d m -z for solutions at p H = 0"5. At higher R S H concentrations, calculated G(H2) values are too low, as seen in Figs l and 2. An important point to note is the crossing of the air-saturated and air-free experimental yield curves at a given pH. The above mechanism would predict that, provided the yield of hydrated electrons is the same in air-free and air-saturated solutions, any hydrogen yields for air-saturated solutions would of necessity be equal to or lower than those for air-free solutions of the same RSH concentration. The observed differences between experimental and calculated yields are not large but, since all measurements were performed during the same time period using the same experimental equipment and technique, we believe the differences are outside the experimental error of the measurements. In these acidic solutions, the net charge on the eysteine will be + 1. Thus, as the cysteine concentration increases, the ionic strength also increases and at sufficiently high ionic strength should alter the rate constant for reaction (6). However, since both (6) and (7) involve reaction between oppositely charged species, increasing ionic strength will have the same effect on the rate constants for both reactions.

Chahr mechanisms In view of the large amount of evidence for a chain mechanism occurring in these oxygen-containing solutions, it is necessary to examine the possibility that increased hydrogen yields could arise from such a mechanism. In order to produce an increased hydrogen yield in a chain reaction, one would probably have to increase the yield of hydrogen atoms produced. The reaction of H atoms is already known from previous workC~, z) to produce RS" and R H (alanine), which are not likely to produce H., in a further chain reaction. Of all the yields studied thus far in oxygenated solutions of cysteine, H~ yields have the lowest sensitivity with respect to 02 and RSH concentrations ~7~. Therefore, we feel that the larger than calculated hydrogen yields are again a reflection of "spur scavenging".

Hydrated electron yields In order to gain further information regarding this process, we have retained the above set of kinetic and yield parameters, except for G,,~,_, and have used equation (8) and the experimental G(Hz) yields to calculate Ge~q as a function of RSH concentration. We have also assumed that increasing cysteine concentration reduces the molecular hydrogen yield, Gil ~, according to the concentration dependences noted by Schwarz Is), assuming a scavenger efficiency based upon our reported hydrated electron reaction rate constant for cysteine in acidic solutions "~. Without the latter assumption calculated G , , values go through a maximum with increasing cysteine concentration. The values of the resulting hydrated electron yields calculated in this manner are shown in the upper portions of Figs 1 and 2. If the values of Ge,q- calculated are valid, they are among the largest hydrated electron yields found for any solute at comparable concentrations in g a m m a radiolysis studies. They are also of the same general magnitude as those reported in pulse radiolysis at very early times following the pulse ~9-1l), i.e. between 4 and 5. If cysteine acts as an effective scavenger of spur processes, part of the reason would be the large constants for scavenging of both oxidizing and reducing radicals reported in the

Hydrated electron yields in gamma radiolysis of cysteine solutions

563

literature tl-a). A possible reason for cysteine being such an effective scavenger is its ability to convert reaction intermediates into thermodynamicallystable compounds so that back reactions between oxidized and reduced intermediates cannot negate the original scavenging reaction. Unusual role o f oxygen

The role of oxygen, especially at RSH concentrations around 10-2 tool dm -s, is puzzling. If, indeed, oxygen causes an increase in the hydrated electron yield when the oxygen itself is not able to compete kinetically for the electron, according to established rate constants, then the influence of oxygen in these solutions is quite unexpected. We can conceive of no role for oxygen in secondary reactions which would, in this system, increase the hydrogen yield. The RSH concentration dependence of hydrated electron yields calculated for air-free cysteine solutions shown in the upper portion of Fig. 1 and the dependence of Ge,~_ shown in the same pH and RSH concentration regions for aerated solutions are in disagreement. If the electron yields calculated in this paper for air-saturated solutions are even qualitatively correct, cysteine is much too efficient a scavenger according to the predictions of the spur theory as outlined by Czapski and Peled cl~). However, if Ge~~_ values are adjusted for the low pH, there is a very good fit for the concentration dependence of electron scavenging with the predictions of Jonah et al3 iS) based upon hydrated electron decay data from picosecond and nanosecond pulse radiolysis studies. Acknowledgement--This work was supported in part by The U.S. Atomic Energy Commission. REFERENCES 1. A. AL THANNON, R. M. PETERSON and C. N. TRUMBORE, J. phys. Chem. 1968, 72, 2395. 2. n . A. ARMSTRONG and V. G. WILKENING, Can. J. Chem. 1964, 42, 2631. 3. G. E. ADAMS, Advances in Radiation Chemistry, edited by M. BURTON and J. L. MAGEE, Wiley-Interscience, 1972, Vol. 3, p. 138. 4. A. AL THANNON,J. P. BARTON, J. E. PACKER, R. J. SIMS,C. N. TRUMBORE, R. V. WINCHESTER, lnt. J. Radiat. Phys. Chem. 1974, 6, 223. 5. I. DRAGANIt~ and Z. DRAGANIt~, The Radiation Chemistry o f Water, Academic Press, New York, 1971, p. 76. 6. M. ANBAR and P. NETA, Int. J. appl. Radiat. Isot. 1967, 18, 493. 7. Unpublished work of A. A1 T h a n n o n and C. N. Trumbore. 8. H. SCHWARZ, J. Am. chem. Soc. 1955, 77, 4962. 9. J. W. HUNT, R. K. WOLFF, i . J. BRONSKILL, C. n . JONAH, E. J. HART and M. S. MATHESON, J. Am. chem. Soc. 1973, 77, 425. 10. G. V. BUXTON, Proc. R. Soc. 1972, A328, 9. 11. J. E. ALDRICH, i . J. BRONSKILL, R. K. WOLFF and J. W. HUNT, J. chem. Phys. 1971, 55, 530. 12. G. CZAPSKI and E. PELED, J. phys. Chem. 1973, 77, 893. 13. C. D. JONAH, E. J. HART and M. S. MATHESON, J. phys. Chem. 1973, 77, 1838.