Pulse radiolysis studies of intramolecular electron transfer in model peptides and proteins. IV. Met/S.^..Br->Tyr/O^. radical transformation in aqueous solution of H-Tyr-(Pro)_n-Met-OH peptides

The intramolecular radical transformation Met/S∴Br→Tyr/O· in aqueous peptides H-Tyr-(Pro)_n-Met-OH, n = 0–3, was investigated in the temperature range of 283–328 K by pulse radiolysis. Corresponding first-order rate constants and thermodynamic parameters of activation of electron transfer, E_a and ΔS^‡, were determined from kinetic data. The rate constants of the reaction were found to decrease exponentially with the number of Pro units and the distance between CR atoms of the terminal amino acids, with a correlation coefficient α = 3·2 ± 0·5 nm⁻¹ at 298 K. Its value appeared to be temperature dependent suggesting the occurrence of thermally induced conformational changes in the peptides. Analysis of experimental data in terms of known conformational properties of the peptides indicates that apparent values of α, E_a and ΔS^‡ are probably complicated functions of conformation and thermodynamic stability of the oligoproline bridge, varying with the number of Pro residues, and of intramolecular hydrophobic interactions between side chains of tyrosine and methionine. Estimation of the relative efficiency of electron transfer pathways through the peptide backbone and through direct and/or water mediated contact between groups bearing radical sites led to the conclusion that partitioning of electron transfer along these pathways is likely to occur.