Residual water modulates QA- to QB electron transfer in bacterial reaction centers embedded in trehalose glasses

The role of protein dynamics in the electron transfer from the reduced primary quinone, QA-, to the secondary quinone, QB, was studied at room temperature in isolated reaction centers (RC) from the photosynthetic bacterium Rhodobacter sphaeroides by incorporating the protein in trehalose water systems of different trehalose/water ratios. The effects of dehydration on the reaction kinetics were examined by analyzing charge recombination following different regimes of RC photoexcitation (single laser pulse, double flash and continuous light) as well as by monitoring flash-induced electrochromic effects in the near infrared spectral region. Independent approaches show that dehydration of RC-containing matrices causes reversible, inhomogeneous inhibition of QA--to-QB electron transfer, involving two subpopulations of RCs. In one of these populations ("active") the electron transfer to QB is slowed but still successfully competing with P+QA- recombination, even in the driest samples; in the other one ("inactive") electron transfer to QB after a laser pulse is hindered since only recombination of the P+QA- state is observed. Small residual water variations (ƒî7 wt %) modulate fully the relative fraction of the two populations, the "active" one decreasing to zero in the driest samples. Analysis of charge recombination after continuous illumination indicates that in the "inactive" subpopulation the conformational changes which rate limit electron transfer can be slowed by more than four orders of magnitude. The reported effects are consistent with conformational gating of the reaction and demonstrate that the conformational dynamics controlling electron transfer to QB is strongly slaved to the structure and dynamics of the surrounding medium. Comparing the effects of dehydration on P+QA-„_PQA recombination and QA-QB„_QAQB- electron transfer suggests that conformational changes gating the latter process are distinct from the ones stabilizing the primary charge separated state.