Reversible H-atom abstraction from alcohols by thiyl radicals: determination of absolute rate constants by pulse radiolysis.

Penicillamine thiyl radicals, PenS., are shown to abstract hydrogen atoms from 2-propanol and to establish an equilibrium PenS. + (CH3)2 CHOH in equilibrium PenSH + (CH3)2 COH. The rate constants for the forward and back reaction have been determined to (1.4 +/- 0.3) x 10(4) and (1.2 +/- 0.3) x 10(8) M-1 s-1, respectively, by means of pulse radiolysis. The data have been obtained from various independent methods which include direct measurements and competitive schemes involving irreversible interception of the alcohol radical by electron acceptors (e.g. CCl4, PNAP) and/or the thiyl radical by antioxidants (e.g. alpha-tocopherol). The results demonstrate that the reaction of carbon-centered radicals with thiols, in radiation biology commonly known as "repair" reaction, may be reversed and thus imply the possibility of thiyl radical induced biological damage.