The initial oxidized species in the photochemical charge separation in reaction centers from Rps. viridis is the primary donor, P+, a bacteriochlorophyll dimer. Bound c-type cytochromes, two high potential (Cyt c558) and two low potential (Cyt c553), act as secondary electron donors to P+. Flash induced absorption changes were measured at moderate redox potential, when the high potential cytochromes were chemically reduced. A fast absorption change was due to the initial oxidation of one of the Cyt c558 by P+ with a rate of 3.7×106s-1 (τ=270nsec). A slower absorption change was attributable to a transfer, or sharing, of the remaining electron from one high potential heme to the other, with a rate of 2.8×105s-1 (τ=3.5 μsec). The slow change was measured at a number of wavelengths throughout the visible and near infrared and revealed that the two high potential cytochromes have slightly different differential absorption spectra, with α-band maxima at 559 nm (Cyt c559) and 556.5 nm (Cyt c556), and dissimilar electrochromic effects on nearby pigments. The sequence of electron transfers, following a flash, is: Cyt c556→Cyt c559→P+. At lower redox potentials, a low midpoint potential cytochrome, Cyt c553, is preferentially oxidized by P+ with a rate of 7×106s-1 (τ=140 nsec). The assignment of the low and high potential cytochromes to the four, linearly arranged hemes of the reaction center is discussed. It is concluded that the closest heme to P must be the high potential Cyt c559, and it is suggested that a likely arrangement for the four hemes is: c553c556c553c559P.
reaction center cytochrome nanosecond kinetics Rps. viridis