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, absorpti on of a photon leads to photooxidation of the primary electron donor P -680, which subsequently retrieves electrons from a tyrosyl residue, f unctioning as an interface to the oxygen-evolving manganese complex. I n a first step toward mimicking these reactions, we have made a Ru(II) -polypyridine complex with an attached tyrosyl moiety. The photoexcite d ruthenium complex played the role of P-680 and was first oxidized by external accepters. Combined transient absorbance and EPR studies pro vided evidence that the Ru(III) formed was reduced by intramolecular e lectron transfer from the attached tyrosine, with a rate constant of 5 x 10(4) s(-1). Thus we show that a tyrosine radical could be formed b y light-induced electron transfer reactions, and we indicate future di rections for developing a closer analogy with the photosystem II react ions.