Orientation of chromophores in reaction centers of Rhodopseudomonas sphaeroides. Evidence for two absorption bands of the dimeric primary electron donor.

Chromatophores from Rhodopseudomonas sphaeroides were oriented by allowing aqueous suspensions to dry on glass plates. Orientation of reaction center pigments was investigated by studying the linear dichroism of chromatophores in which the absorption by antenna bacteriochlorophyll had been attenuated through selective oxidation. Alternatively the light-induced absorbance changes, in the ranges 550-650 and 700-950nm, were studied in untreated chromatophores. The long wave transition moment of reaction center bacteriochlorophyll (P-870) was found to be nearly parallel to the plane of the membrane, whereas the long wave transition moments of bacteriopheophytin are polarized out of this plane. For light-induced changes the linear dichroic ratios, defined as deltaav/deltaah, are nearly the same for untreated and for oxidized chromatophores. Typical values are 1.60 at 870 nm, 0.80 at 810nm, 1.20 at 790 nm, 0.70 at 765 nm, 0.30 at 745 nm , and 0.50 at 600 nm. The different values for the absorbance decrease at 810 nm (0.80) and the increase at 790 nm (1.20) are incompatible with the hypothesis that these changes are due to the blue-shift of a single band. We propose that the decreases at 870 and 810 nm reflect bleaching of the two components of a bacteriochlorophyll dimer, the "special pair" that shares in the photochemical donation of a single electron. The increase at 790 nm then represents the appearance of a monomer band in place of the dimer spectrum, as a result of electron donation. This hypothesis is consistent with available data on circular dichroism. It is confirmed by the presence of a shoulder at 810 nm in the absorption spectrum of reaction centers at low temperature; this band disappears upon photooxidation of the reaction centers. For the changes near 760 nm, associated with bacteriopheophytin, the polarization and the shape of the "light-dark" difference spectrum (identical to the first derivative of the absorption spectrum) show that the 760 nm band undergoes a light-induced shift to greater wavelengths.