Further studies on the composition and spectral properties of the photochemical reaction centers of bacteriochlorophyll b-containing bacteria

1. There are no basic differences in the biochemical composition of photochemical reaction centers between bacteriochlorophyll a- and bacteriochlorophyll b-containing purple bacteria. Both types have the same stoichiometric ratios of bacteriochlorophyll/bacteriopheophytin/special pair (i.e., 4 : 2 : 1) and of carotenoid/bacteriochlorophyll (i.e., 1 : 4), and the molecular weight equivalencies of the polypeptide subunits of the reaction center are basically similar. 2. The isolated reaction center of Rhodopseudomonas viridis contains polypeptide subunits of 38, 35, 28 and 24 kilodaltons. The latter two are aggregated by heat. Of the two c-type cytochromes present in the isolated material, c-553 (and possibly c-558) is coincident with the 38 kilodalton component. It is concluded that the 35, 28 and 24 kilodalton polypeptides correspond to the so-called H, M and L subunits of bacteriochlorophyll a-containing reaction centers. 3. The isolated reaction center contains carotenoid which can apparently be removed from the reaction center by 4% (w/v) lauryldimethylamine oxide. 1′,2′-dihydroneurosporene (50–70%) and 1′,2′-dihydrolycopene (30–40%) constitute the bulk of the reaction center carotenoid. There is no specificity of carotenoid type inserted into the reaction center. 4. The spectral forms of the chromophores in the 830-nm band of room and 77 K absorption spectra of reaction centers of Rps. viridis have been studied via fourth derivatives of the spectra. Four forms with maxima at 787, 812–813, 834–835 and ∼850 nm are present. We attribute the forms to bacteriopheophytin b (787 nm), two ‘voyeur’ bacteriochlorophyll b molecules (one at 812–813 nm and one at 834–835 nm), and a spectral component of P-960 (∼850 nm). Each spectral form in the 830-nm band occurs at the same wavelength maximum and in the same relative proportions in both intact photosynthetic membranes and isolated reaction centers. We thus conclude that each form, in particular the bacteriopheophytin and 812–813 nm bacteriochlorophyll components, is present in the intact photosynthetic membrane, and that they are not generated by treatment of the membranes with detergents; furthermore, we believe that the absorption bands between 780 and 860 nm in spectra of whole cells of Rps. viridis and Thiocapsa pfennigii are due to the absorbance of reaction center chromophores.