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:P₆₈₀, 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 P₆₈₀ and was first oxidized by external accepters. 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 x 10⁴ 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.