Mimicking photosystem II reactions in artificial photosynthesis

Abstract

In a Swedish collaboration project we make and study synthetic supramolecular systems that mimic the donor side reactions of Photosystem II (PSII). Thus, we have made the first compounds where manganese and tyrosine donors are linked to a Ru(II) photosensitiser, and regenerate the sensitiser after photooxidation. The long-lived, highly oxidising tyrosine radical generated in the light reaction could oxidise a Mn2(III,III) dimer, and the reaction sequence is the first mimic of a single electron transfer of the donor "triad" of PSII. A detailed study of the tyrosine oxidation in the model system has shown that the oxidation and deprotonation of the tyrosine is a concerted (not consecutive), one-step reaction. This dissociative, proton-coupled electron transfer reaction has an unusually large reorganisation energy, presumably due to the elongation of the tyrosine OH-bond. Because the kinetic data (isotope effects, pH- and temperature dependence) are very similar to data for Mn-depleted PSII, we suggest that the mechanisms are the same at pH<7.6, i.e. a concerted reaction with a proton release directly to the bulk. In another model complex where the tyrosine is hydrogen bonded, the reaction was much faster. This behaviour is analogous to PSII at pH>7.6, in which a histidine residue is proposed to form a hydrogen bond to TyrZ. We have also been able, for the first time, to oxidise a Ru(II)(bpy)3-linked manganese dimer three steps, from Mn2(II,II) to Mn2(III,IV) in a light-induced reaction. All oxidation steps are driven by the photo-oxidised Ru(III). The accumulation of three oxidising equivalents within a rather narrow potential range suggest that a charge compensating ligand exchange occurs during the oxidation process.