Electron transfer kinetics in photosynthetic reaction centers embedded in polyvinyl alcohol films

The coupling between electron transfer and protein dynamics has been studied at room temperature in isolated reaction centers (RCs) from the photosynthetic bacterium Rhodobacter sphaeroides by incorporating the protein in polyvinyl alcohol (PVA) films of different water/RC ratios. The kinetic analysis of charge recombination shows that dehydration of RC-containing PVA films causes reversible, inhomogeneous inhibition of electron transfer from the reduced primary quinone acceptor (QA) to the secondary quinone QB. A more extensive dehydration of solid PVA matrices accelerates electron transfer from QA to the primary photooxidized electron donor P+. These effects indicate that incorporation of RCs into dehydrated PVA films hinders the conformational dynamics gating QA to QB electron transfer at room temperature and slows down protein relaxation which stabilizes the primary charge-separated state P+QA. A comparison with analogous effects observed in trehalose-coated RCs suggests that protein motions are less severely reduced in PVA films than in trehalose matrices at comparable water/RC ratios.