Proton-coupled electron transfer from tyrosine in a tyrosine-ruthenium- tris-bipyridine complex: Comparison with tyrosinez oxidation in photosystem II

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 pK(a) ≃ 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 > pK(a), 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 tyrosine(z) (Tyr(z)) 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 protoncoupled electron-transfer reaction, and at pH < 7 the Tyr(z) proton is released directly to the bulk water.