Structure of the Primary Electron Donor in Photosystem I: A Resonance Raman Study

Low-temperature resonance Raman (RR) spectra have been obtained at resonance with the Soret transition of chlorophyll a in photosystem I particles containing large amounts either of the triplet state of P₇₀₀ or of its radical cation state. Subtracting these spectra from those of resting reaction centers yielded RR spectra of P₇₀o in its neutral, ground state. These spectra arise from two distinct chlorophyll a molecules differing by the strengths of the bonding interactions assumed by their keto carbonyl groups, the stretching frequencies of which are found at 1655 and 1675 cm⁻¹. The present results rule out previous hypotheses that P₇₀₀ might have consisted of a single, chemically modified chlorophyll a molecule. Neither of the bonding interactions assumed by the keto carbonyls of the P₇₀o chlorophylls most probably involves chlorophyll-chlorophyll bridging through water molecules, as surmised in the so-called special pair models, but likely consists of H bonds with distinct protein sites. The magnesium atoms of the two P₇₀₀ chlorophylls are 5-coordinated. Hence, the structural model of P₇₀o provided by the present data is qualitatively the same, in terms of bonding interactions, as that currently accepted for the bacterial primary donor.