N-14 electron spin echo envelope modulation (ESEEM) spectroscopy of the cation radical P840(+center dot), the primary electron donor of the Chlorobium limicola reaction center

The electronic structure of the oxidized primary chlorophyll electron donor, P840(+.), of the green sulfur bacterium Chlorobium limicola has been investigated using electron spin echo envelope modulation (ESEEM) spectroscopy. This ESEEM investigation of the electron spin density distribution in the radical cation P840(+.) in membranes isolated from C. limicola confirms that the electron spin is shared equally between the two bacteriochlorophyll a molecules. Observation of the small hyperfine couplings to the ring nitrogens by ESEEM gives results that are in agreement with those obtained from ENDOR measurements (S. E. J. Rigby, R. Thapar, M. C. W. Evans and P. Heathcote, FEES Lett, 350, 24-28, 1994) of the large hyperfine couplings to the methyl group protons. These results in combination with the Raman spectroscopy of P840 (U. Feiler, D. Albouy, B. Robert and T. A. Mattioli, Biochemistry 34, 11099-11105, 1995) all indicate that the reaction center of green sulfur photosynthetic bacteria is functionally a protein homodimer providing a symmetrical protein environment for the primary electron donor.