OH-induced oxidation of cyclo-Met-Met. A search for a complexed OH-radical

In the OH radical-induced oxidation of cyclo-Met-Met an OH adduct, cyclo-Met-Met(S therefore OH), absorbing at lambda-max = 390 nm with epsilon-390 cyclo-Met-Met(S therefore OH) = 4500 dm3 mol-1 cm-1 is formed. This adduct reversibly eliminates hydroxide ion, pK(b) = 5.4 +/- 0.1, forming corresponding (S therefore S)-bonded radical cation, cyclo-Met-Met(S therefore S)+, which absorbs at lambda-max = 510 nm with epsilon-510 cyclo-Met-Met(S therefore S)+ = 6200 +/- 400 dm3 mol-1 cm-1. The OH- elimination is catalysed by H+ and buffers. The rate constants are k = (1.4 +/- 0.1) x 10(9) dm3 mol-1 s-1 and k = (1.0 +/- 0.1) x 10(4) s-1 for the H+ catalysed and uncatalysed reactions, respectively. The rate constant for the reverse reaction is k[cyclo-Met-Met(S therefore S)+ + OH-] = (2.6 +/- 0.3) x 10(9) dm3 mol-1 s-1. A certain amount of cyclo-Met-Met(S therefore S)+, G almost-equal-to 1, is formed immediately after the pulse, independently of the actual equilibrium position. Therefore, the existence of at least two different precursors of the cyclo-Met-Met(S therefore S)+ is inferred.