Two distinct conformations of the primary electron donor in reaction centers from Rhodobacter sphaeroides revealed by ENDOR/TRIPLE-spectroscopy

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 Q(y)-band of the neutral state, P (lambda(1) = 866 nm and lambda(2) = 850 nm at room temperature) and therefore are called P-866(.+) and P-850(.+), respectively. P-866(.+) is found in chromatophores and in RCs solubilized with nonionic detergents and bile salts. P-850(.+) is induced by zwitterionic and ionic detergents with aliphatic hydrophobic chains. The TRIPLE resonance spectra reveal that both states coexist in the range lambda(2) < lambda(max) < lambda(1). The main property of the detergent that determines the ability to induce P-850(.+) 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 P-850(.+) upon freezing the RCs without cryoprotectants. The spectroscopic properties of the two states are compared and their possible roles in RC function are discussed.