ENDOR Studies of the Primary Donor in Bacterial Reaction Centers

The structure of the primary donor P in bacterial photosynthesis is of considerable interest due to the unique role this species plays in the very first events of the charge-separation process in the reaction center (RC). It could be shown that P is a specialized bacteriochlorophyll (BChl) [l] which was proposed by NORRIS et al. [2] to be a BChl-dimer. This “special pair” model was a matter of considerable controversy in recent years. Different dimer models [3, 4, 5] and perturbed monomer models [6, 7] have been proposed for the primary donor. The existence of a dimer has recently been demonstrated, at least for Rhodopseudomonas (Rp.) viridis RC’s by an X-ray structure analysis of RC single crystals [8]. An alternative approach to the characterization of the primary donor in its cationic state is provided by EPR and electron nuclear doubleresonance (ENDOR) spectroscopy [4, 5, 9–12]. In this paper we summarize the most recent contributions of EPR and ENDOR in elucidating the electronic structure details of the primary donor cation radicals. In particular P+.865in RC’s of Rp. sphaeroides and Rhodospirillum (R.) rubrum, and P+.960 in RC’s of Rp. viridis are studied. The isotropic hyperfine coupling constants (hfc’s) extracted by ENDOR and TRIPLE resonance [13, 14] are compared with values from advanced MO calculations, thereby relating the experimental values to structural details of the dimers.