Proton-Coupled Electron Transfer from Tyrosine in a Tyrosine−Ruthenium−tris-Bipyridine Complex: Comparison with Tyrosine_Z Oxidation in Photosystem II

Abstract

The pH- and the temperature dependence of the rate constant for electron transfer from tyrosine to ruthenium in Ru(II)(bpy)2(4-Me-4‘CONH-l-tyrosine etyl ester-2,2‘-bpy) 2PF6 was investigated using flash photolysis. At a pH below the tyrosine pKa ≈ 10 the rate constant increased monotonically with pH. This increase was consistent with a concerted electron transfer/deprotonation mechanism. Also indicative of a concerted reaction was the unusually high reorganization energy, 2 eV, extracted from temperature-dependent measurements. Deprotonation of the tyrosine group, at pH > pKa, resulted in a 100-fold increase in rate constant due to a decreased reorganization energy, λ = 0.9 eV. Also, the rate constant became independent of pH. In Mn-depleted photosystem II a similar pH dependence has been found for electron transfer from tyrosineZ (TyrZ) to the oxidized primary donor P680+. On the basis of the kinetic similarities we propose that the mechanisms in the two systems are the same, that is, the electron transfer occurs as a concerted proton-coupled electron-transfer reaction, and at pH < 7 the TyrZ proton is released directly to the bulk water.