Bacterial photosynthetic reaction centers in trehalose glasses: coupling between protein conformational dynamics and electron-transfer kinetics as studied by laser-flash and high-field EPR spectroscopies

The coupling between electron transfer (ET) and the conformational dynamics of the cofactor-protein complex in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides in water-glycerol solutions or embedded in dehydrated polyvinyl alcohol (PVA) films or trehalose glasses is reported. Matrix effects have been studied by time-resolved 95 GHz high-field EPR spectroscopy at room (290 K) and low (150 K) temperature. ET from the photo-reduced quinone acceptor (QA-) to the photo-oxidized donor (P+) is strongly matrix dependent at room temperature: In the trehalose glasses the recombination kinetics of P+QA- , probed by EPR and optical spectroscopy, are faster and broadly distributed as compared to RCs in solution, reflecting the inhibition of the RC relaxation from the dark- to the light-adapted conformational substate and the hindrance of substate interconversion. Similarly accelerated kinetics were observed also in PVA at a water-to-RC molar ratio 10-fold lower than in trehalose. In spite of the matrix dependence of the ET kinetics, cw EPR and electron spin echo (ESE) analysis of the photo-generated P+ and QA- radical ions and P+QA- radical pairs do not reveal significant matrix effects, neither at 290 K nor at 150 K, indicating no change in the molecular radical-pair configuration of the P+ and QA- cofactors. Furthermore, the field dependence of the transverse relaxation times T2 of QA- essentially coincides in trehalose and PVA at 290 K. T2 is similar in these two matrices and in the glycerol-water system at 150 K, implying that the librational dynamics of QA- are also unaffected by the matrix. We infer that the relative geometry of the primary donor and acceptor, as well as the local dynamics and hydrogen bonding of QA in its binding pocket, are not involved in the stabilization of P+QA-. We suggest that the RC relaxation occurs rather by changes throughout the protein-solvent system. The control of the RC dynamics and ET by the environment is discussed, particularly with respect to the extraordinary efficacy of trehalose matrices in restricting the RC motional degrees of freedom at elevated temperatures.