Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III):  A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II

As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P 680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosine Z and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1 H NMR, COSY, NOESY, 13 C NMR, IR, and mass spectrometry. 1 H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4 . The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV 2+, or cobalt pentaminechloride, Co 3+ ) 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 Co 2+ . 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 k ET > 1 × 10 7 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 His190 and tyrosine Z interaction in the donor side of PS II.