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

Abstract

Hydroxyl radicals were generated by pulse radiolysis of N20-saturated aqueous solutions. The OH radicals react with thiols such as 1,4-dithiothreitol by abstracting S-bound H-atom s (k = 1.5 × 1010) dm 3 mol-1s-1). The 1,4-dithiothreitol radical HSCH2(CHOH)2CH 2S' (2) closes the ring (k - 1.5 x10) thereby forming the three-electron-bonded species QH2(CHOH)2CH2S-SH (3) in equilibrium (2,3: e(380 nm) = 450 dm3 mol-1 cm -1) which has a reported pKa value of 5.2. At pH 7.2 3 deprotonates (k = 105s-1) yielding the ring-closed radical anion 4 which is characterized 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 × 107dm3 mo1-1 s-1). The ring-closed radical anion 4 has also been generated by reacting trans-4,5-dihydroxy- 1,2-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 × 10KI dm3mol-1s-1, with H2PC4- with k = 3.8 × lO7 dm3 mol-1s-1. In its reaction towards tetranitrom ethane 3 differs from its non-complexed analogue HOCH2-CH 2S- (1) in that 3 is rapidly oxidized (k = 2 × l0 8 dm3 mol -1s-1) while 1 is not. Isoenergetic H transfer from 1,4-dithiothreitol to 1 occurs with a rate constant of k = 1.7 × 107 dm3 mol-1s-1. 1,4-Dithiothreitol has pKa values at 9.12 and 10.15. At pH 9.1 thiolate ions com pete 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. © 1987, Walter de Gruyter. All rights reserved.