(photosynthesis/primary process/bacteriochlorophyll/special pair gec

The light-induced cation radical of the pri- mary electron donor, P80o, in photosynthetic reaction centers from Rhodospirillum rubrum G-9, has been investigated by electron-nuclear double resonance (ENDOR) in liquid aque- ous solution. The measured hyperfine coupling constants are assigned to specific molecular positions by partial deuteration. Comparison with the bacteriochlorophyll a cation radical shows different reduction factors of the individual coupling constants deviating from the value 2.0 reported in earlier in- vestigations in frozen solutions. The average of the coupling constants is, however, reduced by a factor very close to 2.0. EPR simulations using the ENDOR coupling constants support a dimer model for P8so with C2 symmetry, where the two mac- rocycles are close enough to form a supermolecular orbital re- sulting in a different distribution of the unpaired electron, compared with the monomeric bacteriochlorophyll a cation radical. Molecular orbital calculations were used'to obtain structural information about this dimer.