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.

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The kinetics of electron transfer of cytochrome c2 from Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum centenum to reaction centers from Rb. sphaeroides and Rb. capsulatus have been measured. Observed in the kinetics of decay of the oxidized donor are a rapid first-order rate and one or more slower rates that are due to diffusion-limited complex formation. For reaction centers from Rb. sphaeroides, the fast component had time constants of 1.0 and 0.5 microsecond for cytochrome c2 from Rb. sphaeroides and Rb. capsulatus, respectively, but only a slow component was observed for cytochrome c2 from Rs. centenum. For reaction centers from Rb. capsulatus, the kinetics from all three cytochromes had a fast component with time constants of 1.0, 0.7, and 1.9 microseconds for cytochrome c2 from Rb. sphaeroides, Rb. capsulatus, and Rs. centenum, respectively, although the dissociation constant for cytochrome c2 from Rs. centenum was approximately 20 times larger than that of the other cytochromes. The observation of the fast component for cytochrome c2 from Rs. centenum in reaction centers from Rb. capsulatus but not Rb. sphaeroides demonstrates that the binding interactions for the two reaction centers differ, and the involvement of amino acid residues in the binding is discussed. The kinetics of electron transfer from cytochrome c2 to reaction centers of Rb. sphaeroides from wild type and three mutant strains that have altered carboxyl-terminal regions of the M subunit of the reaction center have also been measured. For cytochrome c2 from Rb. sphaeroides, the kinetics are very similar between the mutants and wild type.(ABSTRACT TRUNCATED AT 250 WORDS)

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