Orientation of chromophores in reaction centers of Rhodopseudomonas sphaeroides: A photoselection study

Abstract

The relative orientation of the pigments of reaction centers from Rhodopseudomonas sphaeroides has been studied by the photoselection technique. A high value (+0.45) of p = (ΔA V − ΔA H ) (ΔA V + ΔA H ) is obtained when exciting and observing within the 870 nm band which is contradictory to the results of Mar and Gingras (Mar, T. and Gingras, G. (1976) Biochim. Biophys. Acta 440, 609–621) and Shuvalov et al. (Shuvalov, V.A., Asadov, A.A. and Krakhmaleva, I.N. (1977) FEBS Lett. 76, 240–245). It is shown that the low values of p obtained by both groups were erroneous due to excitation conditions. Analysis of the polarization of light-induced changes when exciting with polarized light in single transitions (spheroiden band and bacteriopheophytin Q x bands) enable us to propose a possible arrangement of the pigments within the reaction center. It is concluded that the 870 nm band corresponds to a single transition and is one of the two bands of the primary electron donor ( P-870). The second band of the bacteriochlorophyll dimer is centred at 805 nm. The Q y transitions of the molecules constituting the bacteriochlorophyll dimer are nearly parallel (angle less than 25°). The two bacteriopheophytin molecules present slightly different absorption spectra in the near infra-red. Both bacteriopheophytin absorption bands are subject to a small shift under illumination. The angle between the Q y bacteriopheophytin transitions is 55° or 125°. Both Q y transitions are nearly perpendicular to the 870 nm absorption band. Finally, the carotenoid molecules makes an angle greater than 70° with the 870 nm band and the other bacteriochlorophyll molecules.