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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11120-016-0239-9) contains supplementary material, which is available to authorized users.