Orientation and electronic structure of the primary donor radical cation P₇₀₀^+̇ in photosystem I: A single crystals EPR and ENDOR study

The primary donor in single crystals of photosystem I (PS I) obtained from the thermophilic cyanobacterium Synechococcits elongatus is investigated by EPR and ENDOR techniques. The cation radical P₇₀₀^+̇, was generated chemically or photolytically in the single crystals, and angular dependent spectra were obtained at a temperature of 100 K in the frozen state. The principal values and corresponding axes orientations were determined for three different proton hyperfine coupling (hfc) tensors. On the basis of the tensor magnitudes, their symmetry, and the relative orientations of their axes, they were assigned to the protons of three methyl groups of a single chlorophyll (Chl) a molecule. From X-ray crystallography of PS I single crystals, it is known that P₇₀₀ is structurally a chlorophyll dimer (Klukas et al., J. Biol. Chein. 1999, 274, 7361). The EPR/ENDOR experiments show that the second chlorophyll half carries no or very little (≤15%) spin density. The molecular plane of the spin-carrying Chl a molecule in P₇₀₀^+̇ is parallel to the crystallographic c axis. From the orientation of the hf tensors of the methyl groups with respect to this axis, the orientation of the Chl a molecule could be determined. The orientation of this Chl a is very similar to that of the two symmetrically placed bacteriochlorophyll molecules forming the primary donors P₈₆₅ and P₉₆₀ in the RC of the purple bacteria Rhodobacter sphaeroides and Rhodopseiidomonas viridis, respectively. These results indicate a structural similarity of the electron donors in purple bacteria and photosystem I and point to a common evolutionary origin of both types of reaction centers.