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, Q(A)(-), to the secondary quinone, Q(B), 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 after different regimes of RC photoexcitation (single laser pulse, double. ash, 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 Q(A)(-)-to-Q(B) electron transfer, involving two subpopulations of RCs. In one of these populations (i. e., active), the electron transfer to Q(B) is slowed but still successfully competing with P(+)Q(A)(-) recombination, even in the driest samples; in the other (i. e., inactive), electron transfer to Q(B) after a laser pulse is hindered, inasmuch as only recombination of the P(+)Q(A)(-) state is observed. Small residual water variations (-7 wt %) modulate fully the relative fraction of the two populations, with 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 that rate-limit electron transfer can be slowed by[ 4 orders of magnitude. The reported effects are consistent with conformational gating of the reaction and demonstrate that the conformational dynamics controlling electron transfer to Q(B) is strongly enslaved to the structure and dynamics of the surrounding medium. Comparing the effects of dehydration on P(+)Q(A)(-)-->PQ(A) recombination and Q(A)(-)Q(B)-->Q(A)Q(B)(-) electron transfer suggests that conformational changes gating the latter process are distinct from those stabilizing the primary charge-separated state.