Dehydration affects the electronic structure of the primary electron donor in bacterial photosynthetic reaction centers: evidence from visible-NIR and light-induced difference FTIR spectroscopy

The photosynthetic reaction center (RC) is a membrane pigment-protein complex that catalyzes the initial charge separation reactions of photosynthesis. Following photoexcitation, the RC undergoes conformational relaxations which stabilize the charge-separated state. Dehydration of the complex inhibits its conformational dynamics, providing a useful tool to gain insights into the relaxational processes. We analyzed the effects of dehydration on the electronic structure of the primary electron donor P, as probed by visible-NIR and light-induced FTIR difference spectroscopy, in RC films equilibrated at different relative humidities r . Previous FTIR and ENDOR spectroscopic studies revealed that P, an excitonically coupled dimer of bacteriochlorophylls, can be switched between two conformations, P 866 and P 850 , which differ in the extent of delocalization of the unpaired electron between the two bacteriochlorophyll moieties (P L and P M ) of the photo-oxidized radical P + . We found that dehydration (at r = 11%) shifts the optical Q y band of P from 866 to 850-845 nm, a large part of the effect occurring already at r = 76%. Such a dehydration weakens light-induced difference FTIR marker bands, which probe the delocalization of charge distribution within the P + dimer (the electronic band of P + at 2700 cm −1 , and the associated phase-phonon vibrational modes at around 1300, 1480, and 1550 cm −1 ). From the analysis of the P + keto C&z.dbd;O bands at 1703 and 1713-15 cm −1 , we inferred that dehydration induces a stronger localization of the unpaired electron on P L + . The observed charge redistribution is discussed in relation to the dielectric relaxation of the photoexcited RC on a long (10 2 s) time scale. Dehydration of bacterial photosynthetic reaction centers alters the electronic structure of the primary electron donor. Implications for photocatalytic activity are discussed.