Mimicking Electron Transfer Reactions in Photosystem II:  Synthesis and Photochemical Characterization of a Ruthenium(II) Tris(bipyridyl) Complex with a Covalently Linked Tyrosine

In the natural photosynthetic reaction center photosystem II, absorption of a photon leads to photooxidation of the primary electron donor P 680, which subsequently retrieves electrons from a tyrosyl residue, functioning as an interface to the oxygen-evolving manganese complex. In a first step toward mimicking these reactions, we have made a Ru(II)−polypyridine complex with an attached tyrosyl moiety. The photoexcited ruthenium complex played the role of P 680 and was first oxidized by external acceptors. Combined transient absorbance and EPR studies provided evidence that the Ru(III) formed was reduced by intramolecular electron transfer from the attached tyrosine, with a rate constant of 5 × 10 4 s - 1 . Thus we show that a tyrosine radical could be formed by light-induced electron transfer reactions, and we indicate future directions for developing a closer analogy with the photosystem II reactions.