Solution structure of a biological bimolecular electron transfer complex: characterization of the photosynthetic reaction center-cytochrome c(2) protein complex by small angle neutron scattering

The structures of kinetically distinct electron transfer complexes formed b etween the photosynthetic reaction center from Rhodobacter sphaeroides R-26 , and a water-soluble cytochrome c(2) were characterized using small angle neutron scattering, SANS. Reaction center-cytochrome c(2) complexes, RC-C, exhibiting predominately single exponential electron transfer kinetics were found to be 1:1 molar complexes, consistent with a low resolution, co-crys tal, x-ray structure (Adir et al., 1996), provided that the cofactor separa tion was adjusted to 14 +/- 3 Angstrom. Other RC-C configurations are consi stent with SANS data, but are distinguishable by cofactor separation. RC-C preparations exhibiting more complex kinetics were found to have a particle volume markedly greater than that of a 1: 1 complex. These results suggest that RC aggregation is associated with the variation in kinetics reported in the literature, and provide evidence that the model for the 1: 1 complex in co-crystals is relevant to the solution environment.