Comparative study of reaction centers from purple photosynthetic bacteria: Isolation and optical spectroscopy

Reaction centers from two species of purple bacteria, Rhodospirillum rubrum and Rhodospirillum centenum, have been characterized and compared to reaction centers from Rhodobacter sphaeroides and Rhodobacter capsulatus. The reaction centers purified from these four species can be divided into two classes according to the spectral characteristics of the primary donor. Reaction centers from one class have a donor optical band at a longer wavelength, 865 nm compared to 850 nm, and an optical absorption band associated with the oxidized donor at 1250 nm that has a larger oscillator strength than reaction centers from the second class. Under normal buffering conditions, reaction centers isolated from Rb. sphaeroides and Rs. rubrum exhibit characteristics of the first class while those from Rb. capsulatus and Rs. centenum exhibit characteristics of the second class. However, the reaction centers can be converted between the two groups by the addition of charged detergents. Thus, the observed spectral differences are not due to intrinsic differences between reaction centers but represent changes in the electronic structure of the donor due to interactions with the detergents as has been confirmed by recent ENDOR measurements (Rautter J, Lendzian F, Lubitz W, Wang S and Allen JP (1994) Biochemistry 33: 12077-12084). The oxidation midpoint potential for the donor has values of 445 mV, 475 mV, 480 mV and 495 mV for Rs. rubrum, Rs. centenum, Rb. capsulatus, and Rb. sphaeroides, respectively. Despite this range of values for the midpoint potential, the decay rates of the stimulated emission are all fast with values of 4.1 ps, 4.5 ps. 5.5 ps and 6.1 ps for quinone-reduced RCs from Rs. rubrum, Rb. capsulatus, Rs. centenum, and Rb. sphaeroides, respectively. The general spectral features of the initial charge separated state are essentially the same for the four species, except for differences in the wavelengths of the absorption changes due to the different donor band positions. The pH dependence of the charge recombination rates from the primary and secondary quinones differ for reaction centers from the four species indicating different interactions between the quinones and ionizable residues. A different mechanism for charge recombination from the secondary quinone, that probably is direct recombination, is proposed for RCs from Rs. centenum.