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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

1987; De Gruyter; Volume: 42; Issue: 1-2 Linguagem: Inglês

10.1515/znc-1987-1-222

1865-7125

Mohammed Shahid Akhlaq, Clemens von Sonntag,

Sulfur Compounds in Biology

Abstract Hydroxyl radicals were generated by pulse radiolysis of N 2 O-saturated aqueous solutions. The OH radicals react with thiols such as 1,4-dithiothreitol by abstracting S-bound H-atoms (k = 1.5 × 10 10 dm 3 mol -1 s -1 ). The 1,4-dithiothreitol radical HSCH 2 (CHOH) 2 CH 2 S (2) closes the ring (k - 1.5 × 10 6 s -1 ) thereby forming the three-electron-bonded species (3) in equilibrium (2, 3: ε(380 nm) = 450 dm 3 mol -1 cm -1 ) which has a reported pK a value of 5.2. At pH 7.2 3 deprotonates (k = 10 5 s -1 ) yielding the ring-closed radical anion 4 which is characterized by a strong absorption at 390 nm (ε = 5900 dm 3 mol -1 cm -1 ). The deprotonation of 3 is catalyzed by HPO 2 4 (k - 6 .8 × 10 7 dm 3 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 × 10 10 dm 3 mol -1 s -1 , with H 2 PO - 4 with k = 3.8 × 10 7 dm 3 mol -1 s -1 . In its reaction towards tetranitrom ethane 3 differs from its non-complexed analogue HOCH 2 -CH 2 S‘ (1) in that 3 is rapidly oxidized (k = 2 × 10 8 dm 3 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 × 10 7 dm 3 mol -1 s -1 . 1,4-Dithiothreitol has pK a 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.