Magnetic interactions between the reduced bacteriopheophytin and quinone electron acceptors in reaction centers of the photosynthetic purple bacterium rhodopseudomonas viridis: An X-band and Q-band electron paramagnetic resonance study
The electron paramagnetic resonance (EPR) signal associated with the photo-accumulated radical anion of the primary electron acceptor (I⋅-, a bacteriopheophytin radical) in reaction centers (RCs) of the photosynthetic purple bacterium rhodopseudomonas viridis shows a characteristic splitting of about 14.0 mT. This splitting has been attributed to an exchange interaction between I⋅- and the reduced complex of the second electron acceptor, a quinone molecule, and a divalent, high-spin (S=2) Fe-ion, [Q⋅-AFe++]. The magnetic structure of the three-spin complex, Fe++ (S=2), Q⋅-A (S=1/2), and I⋅- (S=1/2), is assessed by Q-band (34.8 GHz) and X-band (9.2 GHz) EPR spectroscopy. The EPR spectrum of [I⋅-Q⋅-AFe++] is simulated accurately for the first time, using the magnetic parameters for the quinone-iron complex [W. F. Butler, R. Calvo, D. R. Fredkin, M. Y. Okamura, and G. Feher [Biophys. J. 45, 947 (1984)]. A largely isotropic interaction between I⋅- and Q⋅-A is required (JIQ=-7.5 mT), together with an anisotropic interaction between I⋅- and the Fe++-ion, whose y-component, CIFe,y, is -3.5 mT. The simulations were insensitive to the magnitude of the x,z-components, CIFe,x/z. The experimental magnetic interactions correspond very well with values calculated from the distances in the RC crystal structure. Thus, the interaction between I⋅- and Q⋅-A is largely isotropic (exchange), whereas the interaction between I⋅- and Fe++ has a purely dipolar character. This result is used to determine the principal directions of the magnetic interaction tensors of the Fe++-ion.