| Popis: |
In the OH radical-induced oxidation of cyclo-Met-Met an OH adduct,cyclo-Met-Met(S∴OH), absorbing at γmax = 390 nm with e390cyclo-Met-Met(S∴OH) = 4500 dm3 mol−1 cm−1 is formed. This adduct reversibly eliminates hydroxide ion, pKb = 5.4 ± 0.1, forming corresponding (S ∴ S)-bonded radical cation, cyclo-Met-Met(S ∴ S)+, which absorbs at γmax = 510 nm with e510cyclo-Met-Met(S ∴ 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)×109dm3mol−1s−1 and k=(1.0±0.1)×104s−1 for the H+ catalysed and uncatalysed reactions, respectively. The rate constant for the reverse reaction is k[cyclo-Met-Met(S ∴ S)+ + OH-] = (2.6 ± 0.3) × 109 dm3 mol-1 s-1. A certain amount of cyclo-Met-Met(S |.: S)+, G ≈ 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 ∴ S)+ is inferred. |
