Modification of protein hydrogen bonds influences the efficiency of picosecond electron transfer in bacterial photosynthetic reaction centers

AbstractPicosecond absorption spectroscopy was used to monitor laser-induced oxidation-reductions of reaction center (RC) bacteriochlorophyll (P) and bacteriopheophytin (I) in Rhodopseudomonas sphaeroides RC preparations on exposure to different chemicals. The D2O isotope substitution of H2O or partial substitution of water by organic solvents (ethylene glycol, glycerol, propylene glycol, dimethyl sulfoxide) causes the appearance of a fast, nanosecond component of P+ reduction, the result of an increased probability of recombination of the primary ion-radical products P+I− → PI. The effect is accompanied by a noticeable slowing down of electron transfer from photoreduced bacteriopheophytin to the primary quinone acceptor QA. The effect of the organic solvents, known as cryoprotectors, is correlated with their degree of hydrophobicity, i.e. the ability to penetrate the RC protein and interact with bound water and protein hydrogen bonds. The conclusion drawn from the data is that the dielectric relaxation processes through which the intermediate energy levels of the carriers in the PIQA system are lowered to levels necessary for the stabilization of the photochemically separated charges proceed with the involvement of protons of the nearest water-protein surrounding of the RC pigments and electron transport cofactors