GENERATION AND REACTIONS OF THE DISULFIDE RADICAL-ANION DERIVED FROM METALLOTHIONEIN - A PULSE RADIOLYTIC-STUDY

OH-radicals were generated by pulse radiolysis of aqueous solutions of rabbit (Zn,Cd)-metallothionein (MT). They react with MT mainly by for ming a thiyl radical with a rate constant of 1.7 X 10(12) dm(3) mol(-1 ) s(-1). The thiyl radical reacts rapidly but reversibly with a thiola te function to form RSSR(.-): RS(.) + RS(-) GRAPHICS RSSR(.-). The k inetics of the formation and decay of this radical anion have been stu died pulse radiolytically by monitoring the evolution of the optical a bsorption of RSSR(.-) at 450 nm. This process is mostly intermolecular , i.e. bimolecular in MT. In the absence of O-2, RSSR(.-) decays bimol ecularly: RSSR(.-) + RS(.) GRAPHICS RSSR + RS(-). In the presence of O-2, RS(.) may be scavenged by O-2 and thus the yield of RSSR(.-) dec reases: RS(.) + O-2 GRAPHICS RSOO(.). Under these conditions RSSR(.- ) decays by first-order kinetics: RSSR(.-) + O-2 GRAPHICS RSSR + O-2 (.-). The rate constants of these reactions have been determined at ro om temperature: k(4) = 1.8 X 10(9) dm(3) mol(-1) s(-1), k(5) = 7 X 10( 4) s(-1), k(10) = 9.2 X 10(8) dm(3) mol(-1) s(-1), and k(18) about 3 X 10(7) dm(3) mol(-1) s(-1). From the dependence of the maximal absorba nce at 450 nm the thiolate concentration in the absence. of oxygen, ep silon(RSSR(.-))= 9 X 10(3) dm(3) mol(-1) cm(-1) and the stability cons tant (K-4/5) of 2.3 X 10(4) dm(3) ml(-1) was determined. K-4/5 is in g ood agreement with that determined kinetically, k(4)/k(5) = 2.6 X 10(4 ) dm(3) mol(-1). The stability constant K-15/16 of the thiylperoxyl ra dical, RSOO(.), was determined to be 5.5 X 10(3) dm(3) mol(-1).