In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox "triad" consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium (II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn (III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn (III)/(IV) state. The redox potentials in our system are comparable to those in Photosystem II. Thus, our synthetic redox "triad" mimics important elements in the electron donor "triad" in Photosystem II, significantly advancing the development of systems for artificial photosynthesis based on ruthenium-manganese complexes.

ents as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru (II) in 4 to the electron acceptor took place, forming Ru (III) and the methylviologen radical MV+·or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru (III), regenerating Ru (II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant kET＞1×107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His 190 and tyrosineZ interaction in the donor side of PS 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 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, 2eV, 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＝0.9eV. 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 protoncoupled electron-transfer reaction, and at pH＜7 the Tyrz proton is released directly to the bulk water.