Weak temperature dependence of P ⁺ H _A ⁻ recombination in mutant Rhodobacter sphaeroides reaction centers

In contrast with findings on the wild-type Rhodobacter sphaeroides reaction center, biexponential P⁺H_A ⁻ → PH_A charge recombination is shown to be weakly dependent on temperature between 78 and 298 K in three variants with single amino acids exchanged in the vicinity of primary electron acceptors. These mutated reaction centers have diverse overall kinetics of charge recombination, spanning an average lifetime from ~2 to ~20 ns. Despite these differences a protein relaxation model applied previously to wild-type reaction centers was successfully used to relate the observed kinetics to the temporal evolution of the free energy level of the state P⁺H_A ⁻ relative to P⁺B_A ⁻. We conclude that the observed variety in the kinetics of charge recombination, together with their weak temperature dependence, is caused by a combination of factors that are each affected to a different extent by the point mutations in a particular mutant complex. These are as follows: (1) the initial free energy gap between the states P⁺B_A ⁻ and P⁺H_A ⁻, (2) the intrinsic rate of P⁺B_A ⁻ → PB_A charge recombination, and (3) the rate of protein relaxation in response to the appearance of the charge separated states. In the case of a mutant which displays rapid P⁺H_A ⁻ recombination (ELL), most of this recombination occurs in an unrelaxed protein in which P⁺B_A ⁻ and P⁺H_A ⁻ are almost isoenergetic. In contrast, in a mutant in which P⁺H_A ⁻ recombination is relatively slow (GML), most of the recombination occurs in a relaxed protein in which P⁺H_A ⁻ is much lower in energy than P⁺H_A ⁻. The weak temperature dependence in the ELL reaction center and a YLH mutant was modeled in two ways: (1) by assuming that the initial P⁺B_A ⁻ and P⁺H_A ⁻ states in an unrelaxed protein are isoenergetic, whereas the final free energy gap between these states following the protein relaxation is large (~250 meV or more), independent of temperature and (2) by assuming that the initial and final free energy gaps between P⁺B_A ⁻ and P⁺H_A ⁻ are moderate and temperature dependent. In the case of the GML mutant, it was concluded that the free energy gap between P⁺B_A ⁻ and P⁺H_A ⁻ is large at all times.