DETERMINATION OF THE RATE-CONSTANT FOR THE REACTION OF HYDROXYL AND OXIDE RADICALS WITH CYSTEINE IN AQUEOUS-SOLUTION

The techniques of pulse radiolysis and absorption spectroscopy have be en used to determine absolute rate constants for the reaction of hydro xyl and oxide radicals with cysteine (CysS) species in aqueous solutio n. Using measured equilibrium constants for radical disulfide anion fo rmation, over the pH range 6.0-13.0, observed reaction rate constants were resolved into their constituent reactions (OH)-O-. + CysSH --> H2 O + CysS(.) and O-.(-) + CysS(-) (+ H2O) --> CysS(.) + 2OH(-) with cal culated limiting rate constants of (5.35 +/- 0.82) x 10(9) and (3.28 /- 0.49) x 10(8) dm(3) mol(-1) s(-1), respectively. The rate constant for hydroxyl radical reaction with CysS(-) was found to be the same as that for CysSH. (C) 1996 by Radiation Research Society