Intermolecular H-Abstraction of Thiyl Radicals from Thiols and the Intramolecular Complexing of the Thiyl Radical with the Thiol Group in 1,4-Dithiothreitol. A Pulse Radiolysis Study

Hydroxyl radicals were generated by pulse radiolysis of N20-saturated aqueous solutions. The O H radicals react with thiols such as 1,4-dithiothreitol by abstracting S-bound H-atoms (k = 1.5 x 101() dm3 m ol“ 1 s_l). The 1,4-dithiothreitol radical HSCH2(C H O H )2CH2S' (2) closes the ring (k — 1.5 x 106 s“1) thereby forming the three-electron-bonded species Q H 2(C H O H )2C H 2S— $ H (3) in equilibrium (2, 3: e(380 nm) = 450 dm3 mol-1 cm-1) which has a reported pKa value of 5.2. A t pH 7.2 3 deprotonates (k = 105 s_1) yielding the ring-closed radical anion 4 which is charac­ terized by a strong absorption at 390 nm (e = 5900 dm3 mol“1 cm-1). The deprotonation of 3 is catalyzed by HPO^~ (k 6 .8 x l0 7 dm3 m oP 1 s ' 1). The ring-closed radical anion 4 has also been generated by reacting trans-4,5-dihydroxy1,2-dithian with the solvated electron and the H atom in A r-saturated solution. Its protonation by protons yielding 3 occurs with k — 1.8 x 10KI dm3 m o P 1 s-1, with H2PC>4 with k = 3.8 x lO 7 dm3 mol-1 s_1. In its reaction towards tetranitrom ethane 3 differs from its non-complexed analogue H O C H 2—CH 2S‘ (1) in that 3 is rapidly oxidized (k = 2 x l0 8 dm3 mol s_1) while 1 is not. Isoenergetic H transfer from 1,4-dithiothreitol to 1 occurs with a rate constant of k = 1.7 x 107 dm3 m ol-1 s“ 1. 1,4-Dithiothreitol has pKa values at 9.12 and 10.15. At pH 9.1 thiolate ions compete with ringclosure by complexing with 2 thereby forming an open-chain disulphide radical anion 6 which has very similar spectral properties as its ring-closed analogue 4.