Monomeric spin density distribution in the primary donor of photosystem I as determined by electron magnetic resonance: Functional and thermodynamic implications

The primary electron donor (P-700) in Photosystem I (PSI) has been shown to be a dimeric chlorophyll a species. Electron magnetic resonance studies of the cation radical have clearly established that the unpaired electron is delocalized asymmetrically over this dimer; however, the extent to which th is asymmetry exists remains ambiguous. Comprehensive electron nuclear doubl e resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) expe riments combined with isotopic substitution and numerical simulations have been used to determine the electronic structure of P-700+ This approach uti lizes the strengths of each spectroscopy to elucidate the electron nuclear hyperfine and nuclear quadrupole coupling constants for the nitrogen nuclei in P-700+ These assignments are then confirmed by performing numerical sim ulations of the ESEEM data. Further confirmation of these values is obtaine d by performing the spin-echo experiments at multiple microwave frequencies . The same set of hyperfine and quadrupole coupling constants is used to si mulate all of the ESEEM data for P-700+ containing either natural abundance N-14 or isotopically enriched with N-15. These simulations indicate that t he unpaired spin is localized over only one of the chlorophylls that make u p the special pair. The ramifications of this monomeric spin density on the function and thermodynamics of electron transfer in PSI are discussed.