DETERMINATION OF ABSOLUTE RATE CONSTANTS FOR THE REVERSIBLE HYDROGEN-ATOM TRANSFER BETWEEN THIYL RADICALS AND ALCOHOLS OR ETHERS

Absolute rate constants have been determined for the reversible hydrog en-transfer process R' + RSH reversible arrow RH + RS' by pulse radiol ysis, mainly through direct observation of the RS' radical formation k inetics in water-RH (1:1, v/v) mixtures. The thiols investigated were penicillamine and glutathione; the RH hydrogen donors were methanol, e thanol, propan-1-ol, propan-2-ol, ethylene glycol, tetrahydrofuran and 1,4-dioxane with the abstracted hydrogen being located ct to the hydr oxy or alkoxy function. Rate constants for the forward reaction of the above equilibrium (in radiation biology referred to as 'repair' react ion) were typically of the order of 10(7) - 10(8) dm(3) mol(-1) s(-1) while hydrogen abstraction from RH by thiyl radicals (reverse process) occurred with rate constants of the order of 10(3)-10(4) dm(3) mol(-1 ) s(-1). This yields equilibrium constants of the order of 10(4). Base d on these data, standard reduction potentials could be evaluated for the R'R '' C'OH/H+//R'R '' CHOH, R'R '' CO/H+//R'R '' C'(OH) and R'R ' ' CO//R'R '' C'O- couples from methanol, ethanol and propan-2-ol. Effe ctive hydrogen-atom abstraction by RS' required activation by neighbou ring groups of the C-H bond to be cleaved in RH. No such process was o bserved for the RS' reaction with -CH3 groups, e.g. in 2-methylpropan- 2-ol. Several halogenated hydrocarbons, including some anaesthetics (e .g. halothane) and Fe(CN)(6)(3-) have been tested with respect to thei r ability to disturb the (CH3)(2)C'OH + RSH reversible arrow (CH3)(2)C HOH + RS' equilibrium through an irreversible electron-transfer reacti on with the reducing alpha-hydroxyl radical, thereby drawing the equil ibrium to the left-hand side. The respective efficiencies are found to be related to the electronegativities of the electron accepters. The results are briefly discussed in terms of their biological relevance.