Monomeric Spin Density Distribution in the Primary Donor of Photosystem I as Determined by Electron Magnetic Resonance: Functional and Thermodynamic Implications

Abstract

The primary electron donor (P700) 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 this asymmetry exists remains ambiguous. Comprehensive electron nuclear double resonance (ENDOR) and electron spin−echo envelope modulation (ESEEM) experiments combined with isotopic substitution and numerical simulations have been used to determine the electronic structure of P700+. This approach utilizes the strengths of each spectroscopy to elucidate the electron nuclear hyperfine and nuclear quadrupole coupling constants for the nitrogen nuclei in P700+. These assignments are then confirmed by performing numerical simulations of the ESEEM data. Further confirmation of these values is obtained by performing the spin−echo experiments at multiple microwave frequencies. The same set of hyperfine and quadrupole cou...