THE REACTION OF SUPEROXIDE RADICAL-ANION WITH DITHIOTHREITOL - A CHAIN PROCESS

The radical-induced oxidation of dithiothreitol (DTT) by O2 to 4,5-dihydroxy-1,2-dithiane (ox-DTT) in aqueous solutions of pH 8.7 proceeds via a chain reaction. Radicals such as OH generated by the radiolysis of N2O/O2-saturated aqueous solutions abstract an H atom from DTT. The resulting DTT(-H). radicals cyclize and deprotonate, yielding the disulfide radical anion ox-DTT.-. The latter reacts with O2 (kappa = 7.1 x 10(8) dm3 mol-1 s-1) giving rise to ox-DTT and O2.-. The G value of ox-DTT formation increases with decreasing dose rate, displaying a linear yield vs (dose rate)-1/2 relationship indicative of a chain reaction. The chain is inhibited by superoxide dismutase, showing that O2.- is a chain carrier. Hydrogen peroxide is not a chain product, from which it is concluded that O2.- does not abstract hydrogen from DTT but rather forms a short-lived complex with the DTT anion which then predominantly decomposes into an RSO. radical and an OH-ion, (and to a lesser extent into an OH radical and an RSO-ion). In a second step, the RSO. radical reacts with DTT (probably the anionic form DTT-), thus regenerating DTT(-H).. The resulting sulfenic acid eliminates water and yields the chain product ox-DTT. An overall propagation rate constant of 35 dm3 mol-1 s-1 has been estimated.