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

Changes of redox midpoint potential E-m of bacteriochlorophyll of the speci al pair {PMPL}, the rates of energy migration from bacteriopheophytin H-M t o {PMPL}, electron transfer from {PMPL} to H-L and from HL to quinone QA we re investigated in reaction centers (RC) of Rhodobacter sphaeroides under t he action of deuteration and addition of glycerol and DMSO. It was shown th at H2O --> D2O Substitution did not change E-m of the special pair, while t he addition of 70% glycerol and 35% DMSO (v/v) increases the value of E-m b y 30 and 45 mV, correspondingly. Rate constants of energy migration k(m) (H -M* ? P-2), charge separation k(c) ({PMPL}*H-L ? {PMPL}+HL-), electron tran sfer to Q(A) k(Q) remained stable upon addition of glycerol. On the other h and, isotopic substitution of water and addition of DMSO lead to a 2-3-fold increase of k(m), k(c) and k(Q) values. Theoretical analysis of the dependence of the redox-center potential on the dielectric constant epsilon, swelling of the protein globule in solvent, a nd changing in the distribution of charges in the protein interior in the v icinity of the redox center was performed. It was shown that replacement of H2O by DMSO resulted in the increase of E-m by tens of mV. There was no co rrelation between changes of E-m and k(m), k(c) and k(Q) under deuteration and DMSO addition in the RC's. Direct influence of changes in epsilon of th e medium on the rate constants of electron transfer as a result of decrease of energy of intermolecular interaction between donor and acceptor molecul es was analysed.