title: Weak temperature dependence of P (+) H A (-) recombination in mutant Rhodobacter sphaeroides reaction centers.
abstract: 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.
authors.affiliation: Department of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland. krzyszgi@amu.edu.pl.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Gibasiewicz
authors.affiliation: Department of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland.
authors.forename: Rafał
authors.initials: R
authors.lastname: Białek
authors.affiliation: Department of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland.
authors.forename: Maria
authors.initials: M
authors.lastname: Pajzderska
authors.affiliation: Department of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland.|Center for Ultrafast Laser Spectroscopy, A. Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland.
authors.forename: Jerzy
authors.initials: J
authors.lastname: Karolczak
authors.affiliation: Department of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614, Poznań, Poland.
authors.forename: Gotard
authors.initials: G
authors.lastname: Burdziński
authors.affiliation: School of Biochemistry, Medical Sciences Building, University of Bristol, University Walk, Bristol, BS8 1TD, UK.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Jones
authors.affiliation: Laboratoire Mécanismes Fondamentaux de la Bioénergétique, UMR 8221, CEA - iBiTec-S, CNRS, Université Paris Sud, 91191, Gif-Sur-Yvette, France.
authors.forename: Klaus
authors.initials: K
authors.lastname: Brettel
mesh_terms: D065966:Absorption, Physicochemical; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004579:Electron Transport; D007700:Kinetics; D008958:Models, Molecular; D009154:Mutation; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D045322:Photosynthetic Reaction Center Complex Proteins
keywords: Charge recombination; Electron transfer; Protein dynamics; Reaction centers; Rhodobacter sphaeroides; Transient absorption