A Structural and Functional Mimic of P680.

One or multiple chlorophyll a molecules are employed in the reaction center of photosystem II's main electron donor (defined as P680). We have a poor understanding of how the reaction center facilitates water oxidation in photosystem II and the roles that mono- and/or multimeric chlorophyll groups play when P680 oxidizes a neighboring tyrosine in order to drive water oxidation at the oxygen evolving complex. We have prepared a dimeric Mg-porphyrin complex [Mg(BTPP)] (1, H-BTPP=1,2-bis-(10,15,20-triphenylporphyrin-5-yl)-benzene) as a structural and functional mimic of the dimeric core of P680. 1 was oxidized by one-electron to the corresponding π-cation radical complex 2. The radical cation was characterized by UV/Vis-NIR, FT-IR, and EPR spectroscopic techniques. 2 was shown to be reactive towards phenols to give the corresponding phenoxyl radicals, mimicking the reactivity of the P680 cation radical which oxidizes tyrosine to tyrosyl radical. Critically, the dimeric π-cation radical showed markedly higher rates of proton coupled electron transfer oxidation (PCET) of phenols when compared to its monomeric counterpart [Mg(TPP)] (TPP=5,10,15,20-tetraphenylporphyrin). Our findings demonstrate that Mg-porphyrin complexes are reliable mimics of photosynthetic PCET processes and suggest that photosynthetic reaction centers with multiple π-conjugated complexes likely lower the barrier to PCET oxidation by π-cation radical species.