Ligated chlorophyll cation radicals: Their function in photosystem II of plant photosynthesis

Abstract

Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl)a and its cation radicals. Oxidation potentials for Chl .fwdarw. Chl+. as high as + 0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl+. ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of 2 nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparisons of the properties of chlorophyll in vitro with the optical, redox and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates O evolution in plant photosynthesis, leads to the suggestion that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.