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

Abstract

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, P₈₆₆ and P₈₅₀, 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 CO 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 (10² s) time scale.