Comparative Study of Reaction Centers from Photosynthetic Purple Bacteria: Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Spectroscopy

Reaction centers (RCs) from four species of purple bacteria, Rhodobacter sphaeroides, Rhodobacter capsulatus, Rhodospirillum rubrum, and the recently discovered bacterium Rhodospirillum centenum, have been characterized by optical spectroscopy [Wang, S., Lin, X., Woodbury, N. W., & Allen, J. P. (1994) Photosynth. Res. (submitted for publication)] and magnetic resonance spectroscopy. All RCs contain a bacteriochlorophyll (BChl) a dimer as the primary donor. For Rb. sphaeroides and Rs. rubrum the donor Q_y optical band is at~ 865 nm, compared to ~850 nm for Rb. capsulatus and Rs. centenum. The primary donor in the RCs can be converted between these two forms by the addition or removal of charged detergents. The electronic structure of the cation radical of the primary electron donor P^{+ ·} was investigated in these species using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and electron nuclear triple resonance (TRIPLE) spectroscopy. The EPR line widths of P^{+ ·} vary significantly and the ENDOR and Special TRIPLE spectra reveal drastic differences in the spin density distribution of the dimer for the different species. Reaction centers from Rb. sphaeroides and Rs. rubrum have a slightly asymmetric spin density distribution over the two halves of the dimer. The respective ratios are 2:1 and 1.6:1 in favor of the L-half of the BChl a dimer. In contrast, the spectra of P^{+ ·} in reaction centers from Rb. capsulatus and Rs. centenum show an almost complete localization of the unpaired electron on the L-half of the dimer (ratio ~5:1). The different spin density distributions in the four species are discussed in the framework of a simple theoretical model for the dimer [Plato, M., Lendzian, F., Lubitz, W., & Möbius, K. (1992) in The Photosynthetic Bacterial Reaction Center II: Structure, Spectroscopy and Dynamics (Breton, J., & Vermeglio, A., Eds.) pp 99-108, Plenum Press, New York]. In this model the observed asymmetries are attributed to different orbital energies of the two BChl a halves of the dimer. Possible reasons for the energetic inequivalencies of the dimer halves are discussed. The observed orbital asymmetries in the primary donor cation radicals correlate with the different Q_y absorption bands of P and also with the different charge recombination rates from the primary quinone in these species. In contrast to the results obtained for reaction centers, the electron spin density distribution of P^{+ ·} is identical in chromatophores of all four investigated bacterial species and corresponds to a ratio of 2:1 in favor of the L half of the dimer. These results indicate that the environment of the reaction centers influences both the electronic structure of the cofactors and the electron transfer rates.