Hybrid density functional studies of the oxidation of phenol-imidazole hydrogen-bonded complexes: A model for tyrosine oxidation in oxygenic photosynthesis

Hybrid density functional calculations (B3LYP) show that one-electron oxidation of a phenol-imidazole hydrogen-bonded complex leads to spontaneous transfer of the phenolic proton to the imidazole, resulting in the formation of a phenoxyl radical-imidazolium ion complex. On comparison of the spin density distribution and hyperfine couplings, hydrogen bonding is shown principally to lead to a redistribution of spin density from the phenoxyl carbonyl oxygen atom to the carbonyl carbon atom. Loss of a proton from the phenoxyl-imidazolium ion results in a more loosely bound phenoxyl-imidazole complex, where a smaller spin redistribution is shown to occur on hydrogen bond formation. Comparisons between predicted hyperfine couplings for both hydrogen-bonded models and those reported for tyrosyl-histidine radical complexes involved in photosynthetic oxygen evolution indicate good agreement between experiment and theory.