Two Distinct Conformations of the Primary Electron Donor in Reaction Centers from Rhodobacter sphaeroides Revealed by ENDOR/TRIPLE-Spectroscopy

Abstract

The effect of solubilization of photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides with different detergents on the electronic structure of the oxidized primary donor, P.+, is investigated. Electron paramagnetic resonance spectroscopy and related multiple resonance techniques (ENDOR/Special TRIPLE) show that two distinct conformations of P.+ can be obtained, depending on the detergent properties, the detergent/RC ratio, and the temperature. The two states correspond to different positions of the long-wavelength Qy-band of the neutral state, P (lambda1 = 866 nm and lambda2 = 850 nm at room temperature) and therefore are called P866.+ and P850.+, respectively. P866.+ is found in chromatophores and in RCs solubilized with nonionic detergents and bile salts. P850.+ is induced by zwitterionic and ionic detergents with aliphatic hydrophobic chains. The TRIPLE resonance spectra reveal that both states coexist in the range lambda2 < lambda(max) < lambda1. The main property of the detergent that determines the ability to induce P850.+ is the polarity of the head group. A simple phenomenological model is presented that relates the standard Gibbs free energy difference between the two conformations to the detergent/RC ratio and the temperature. Of special interest is the observation that the widely used detergent LDAO can induce P850.+ upon freezing the RCs without cryoprotectants. The spectroscopic properties of the two states are compared and their possible roles in RC function are discussed.