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 spont aneous transfer of the phenolic proton to the imidazole, resulting in the formation of a phenoxyl radical-imidazolium ion complex. On compar ison of the spin density distribution and hyperfine couplings, hydroge n bonding is shown principally to lead to a redistribution of spin den sity from the phenoxyl carbonyl oxygen atom to the carbonyl carbon ato m. Loss of a proton from the phenoxyl-imidazolium ion results in a mor e loosely bound phenoxyl-imidazole complex, where a smaller spin redis tribution is shown to occur on hydrogen bond formation. Comparisons be tween predicted hyperfine couplings for both hydrogen-bonded models an d those reported for tyrosyl-histidine radical complexes involved in p hotosynthetic oxygen evolution indicate good agreement between experim ent and theory.