Investigation of the electron transfer reactions and redox characteristics of photoactive bacteriochlorophyll in Rhodobacter sphaeroides reaction centers modified by D2O and cryoprotectants.

ABSTRACT

The effects of D2O, glycerol and dimethyl sulfoxide (DMSO) on redox potential Em of bacteriochlorophyll of a special P2 or [P(M)P(L)] pair, the rate of energy migration from bacteriopheophytin H(M) to [P(M)P(L)], electron transfer from [P(M)P(L)] to bacteriopheophytin H(L) and then to quinone Q(A) in reaction centers (RC) of Rhodobacter sphaeroides were studied. The H2O --> D2O substitution did not change Em of the special pair, whereas addition of 70% glycerol or 35% DMSO (v/v) increased the values of Em by 30 and 45 mV, respectively. Rate constants of energy migration km(H(M)* (km)--> P2), charge separation ke([P(M)P(L)] *H(L) (ke)--> [P(M)P(L)] +H(L)-), electron transfer to quinone kQ did not change after the glycerol addition, whereas isotopic substitution and addition of DMSO caused a 2-3-fold increase in km, ke, and kQ values. Theoretical analysis of the redox center potential dependence on dielectric permeability epsilon, swelling of the protein globule in a solvent, and on changes in the charge distribution (charge shifts) in the protein interior near the redox center was carried out. It has been shown that the H2O replacement with DMSO can result in the Em increase by tens of mV. No correlation was found between the Em values and the rate of charge separation upon isotopic substitution and addition of cryoprotectants. The effect of epsilon of the medium on the rate of electron transport due to changes of energy of intermolecular interaction between the donor and acceptor molecules was estimated.