Energetics and mechanisms of high efficiency of charge separation and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes due to D2O/H2O substitution and cryosolvent addition on the energetics of the special pair and the rate constants of forward and back electron transfer reactions in the picosecond–nanosecond time domain have been studied in isolated reaction centers (RC) of the anaxogenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (i) replacement of H2O by D2O or addition of either 70% (v/v) glycerol or 35% (v/v) DMSO do not influence the absorption spectra; (ii) in marked contrast to this invariance of absorption, the maxima of fluorescence spectra are red shifted relative to control by 3.5, 6.8 and 14.5 nm for RCs suspended in glycerol, D2O or DMSO, respectively; (iii) D2O/H2O substitution or DMSO addition give rise to an increase of the time constants of charge separation (τe), and QA− formation (τQ) by a factors of 2.5–3.1 and 1.7–2.5, respectively; (iv) addition of 70% glycerol is virtually without effect on the values of τe and τQ; (v) the midpoint potential Em of P/P+ is shifted by about 30 and 45 mV towards higher values by addition of 70% glycerol and 35% DMSO, respectively, but remains invariant to D2O/H2O exchange; and (vi) an additional fast component with τ1=0.5–0.8 ns in the kinetics of charge recombination P+HA−→P*(P)HA emerges in RC suspensions modified either by D2O/H2O substitution or by DMSO treatment. sults have been analysed with special emphasis on the role of deformations of hydrogen bonds for the solvation mechanism of nonequilibrium states of cofactors. Reorientation of hydrogen bonds provides the major contribution of the very fast environmental response to excitation of the special pair P. The Gibbs standard free energy gap between 1P* and P+BA− due to solvation is estimated to be ∼70, 59 and 48 meV for control, D2O- and DMSO-treated RC samples, respectively.