Determination of absolute rate constants for the reversible hydrogen-atom transfer between thiyl radicals and alcohols or ethers

Abstract

Absolute rate constants have been determined for the reversible hydrogen-transfer process R˙+ RSH ⇄ RH + RS˙ by pulse radiolysis, mainly through direct observation of the RS˙ radical formation kinetics in water–RH (1 : 1, v/v) mixtures. The thiols investigated were penicillamine and glutathione; the RH hydrogen donors were methanol, ethanol, propan-1-ol, propan-2-ol, ethylene glycol, tetrahydrofuran and 1,4-dioxane with the abstracted hydrogen being located α to the hydroxy or alkoxy function. Rate constants for the forward reaction of the above equilibrium (in radiation biology referred to as ‘repair’ reaction) were typically of the order of 10⁷– 10⁸ dm³ mol^–1 s^–1 while hydrogen abstraction from RH by thiyl radicals (reverse process) occurred with rate constants of the order of 10³– 10⁴ dm³ mol^–1 s^–1. This yields equilibrium constants of the order of 10⁴. Based 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. Effective hydrogen-atom abstraction by RS˙ required activation by neighbouring groups of the C—H bond to be cleaved in RH. No such process was observed 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 their ability to disturb the (CH₃)₂C˙OH + RSH ⇄(CH₃)₂CHOH + RS˙ equilibrium through an irreversible electron-transfer reaction with the reducing α-hydroxyl radical, thereby drawing the equilibrium to the lefthand side. The respective efficiencies are found to be related to the electronegativities of the electron acceptors. The results are briefly discussed in terms of their biological relevance.