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

Abstract

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(A)(.-)Fe(++)]. 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(A)(.-)Fe(++)] 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 (J(IQ)=-7.5 mT), together with an anisotropic interaction between I-.- and the F++-ion, whose y-component, C-IFe,C-y, is -3.5 mT. The simulations were insensitive to the magnitude of the x,z-components, C-IFe,C-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. (C) 1996 American Institute of Physics.