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The structures of kinetically distinct electron transfer complexes formed between the photosynthetic reaction center from Rhodobacter sphaeroides R‐26, and a water‐soluble cytochrome c2 were characterized using small angle neutron scattering, SANS. Reaction center‐cytochrome c2 complexes, RC‐C, exhibiting predominately single exponential electron transfer kinetics were found to be 1:1 molar complexes, consistent with a low resolution, co‐crystal, x‐ray structure (Adir et al., 1996), provided that the cofactor separation was adjusted to 14 ± 3 Å. Other RC‐C configurations are consistent 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.

Z)°(Cu2) = 2.01 ± 0.08 eV.4 These molecules are generally agreed to have (4sffg)2 bonds with weakly interacting 3d cores. Their diabatic dissociations lead to promotion energies of 0.06 and 0.0 eV, respectively, such that 2 eV is a reasonable estimate for the DDE of a (4sag)2 bond. Since the iron DDEs are ~40% stronger than this, the iron dimers appear to involve more than a simple (4$ 8)2 bond. They presumably must include extensive d-d bonding as well.Acknowledgment. This work is supported by the Army Research Office, DAAL03-87-2211, and a National Science Foundation Graduate Fellowship (D.A.H.). We thank Profs.Michael Morse and Jack Simons for several useful discussions.

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