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

Abstract Hydroxyl radicals were generated by pulse radiolysis of N2O-saturated aqueous solutions. The OH radicals react with thiols such as 1,4-dithiothreitol by abstracting S-bound H-atoms (k = 1.5 × 1010 dm3 mol-1 s-1). The 1,4-dithiothreitol radical HSCH2(CHOH)2CH2S (2) closes the ring (k - 1.5 × 106 s-1) thereby forming the three-electron-bonded species (3) in equilibrium (2, 3: ε(380 nm) = 450 dm3 mol-1 cm-1) which has a reported pKa value of 5.2. At pH 7.2 3 deprotonates (k = 105 s-1) yielding the ring-closed radical anion 4 which is characterized by a strong absorption at 390 nm (ε = 5900 dm3 mol-1 cm-1). The deprotonation of 3 is catalyzed by HPO24 (k - 6 .8 × 107 dm3 mol-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 Ar-saturated solution. Its protonation by protons yielding 3 occurs with k - 1.8 × 1010 dm3 mol-1 s-1, with H2PO-4 with k = 3.8 × 107 dm3 mol-1 s-1. In its reaction towards tetranitrom ethane 3 differs from its non-complexed analogue HOCH2-CH2S‘ (1) in that 3 is rapidly oxidized (k = 2 × 108 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 × 107 dm3 mol-1 s-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.