Abstract
In contrast with findings on the wild-type Rhodobacter sphaeroides reaction center, biexponential P+HA− → PHA 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+HA− relative to P+BA−. 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+BA− and P+HA−, (2) the intrinsic rate of P+BA− → PBA 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+HA− recombination (ELL), most of this recombination occurs in an unrelaxed protein in which P+BA− and P+HA− are almost isoenergetic. In contrast, in a mutant in which P+HA− recombination is relatively slow (GML), most of the recombination occurs in a relaxed protein in which P+HA− is much lower in energy than P+HA−. 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+BA− and P+HA− 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+BA− and P+HA− are moderate and temperature dependent. In the case of the GML mutant, it was concluded that the free energy gap between P+BA− and P+HA− is large at all times.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-016-0239-9) contains supplementary material, which is available to authorized users.
Highlights
The contributions of protein dynamics to electron transfer occurring inside proteins are poorly understood
Experimental data relating to the influence of protein dynamics on light-induced electron transfer in reaction centers (RCs) have been interpreted either in terms of an active contribution of the protein, the spontaneous diffusion of which in the conformational space is a factor that largely determines the rate of electron transfer (Wang et al 2007, 2009, 2012; Torchała and Kurzynski 2008; Pieper and Renger 2009; Kurzynski and Chełminiak 2013) or in terms of passive conformational reorganization of the protein in response to the appearance of a strong electrical field caused by the light-induced charge separated states (Woodbury and Parson 1984; Peloquin et al 1994; Gibasiewicz et al 2013a)
We indicate minor differences between the decay-associated difference spectra (DADS) for the WT and mutant RCs and we focus on the spectral features that are most important for the further interpretation
Summary
The contributions of protein dynamics to electron transfer occurring inside proteins are poorly understood. Experimental data relating to the influence of protein dynamics on light-induced electron transfer in RCs have been interpreted either in terms of an active contribution of the protein, the spontaneous diffusion of which in the conformational space is a factor that largely determines the rate of electron transfer (Wang et al 2007, 2009, 2012; Torchała and Kurzynski 2008; Pieper and Renger 2009; Kurzynski and Chełminiak 2013) or in terms of passive conformational reorganization of the protein in response to the appearance of a strong electrical field caused by the light-induced charge separated states (Woodbury and Parson 1984; Peloquin et al 1994; Gibasiewicz et al 2013a). Lightinduced charge separation inside this protein occurs along a chain of electron transfer cofactors that engage in noncovalent interactions with the neighboring amino acids. Within *3–7 ps, the electron is transferred from the first excited singlet state of
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