Electronic interactions between iron and the bound semiquinones in bacterial photosynthesis. EPR spectroscopy of oriented cells of Rhodopseudomonas viridis

Electron paramagnetic resonance (EPR) spectroscopy of the iron-semiquinone complex in photosynthetic bacterial cells and chromatophores of Rhodopseudomonas viridis is reported. Magnetic fields are used to orient the prolate ellipsoidal-shaped cells which possess a highly ordered internal structure, consisting of concentric, nearly cylindrical membranes. The field-oriented suspension of cells exhibits a highly dichroic EPR signal for the iron-semiquinone complex, showing that the iron possesses a low-symmetry ligand field and exists in a preferred orientation within the native reaction-center membrane complex. The EPR spectrum is analyzed utilizing a spin hamiltonian formalism to extract physical information describing the electronic structure of the iron and the nature of its interaction with the semiquinones. Exact numerical solutions and analytical expressions for the transition frequencies and intensities derived from a perturbation theory expansion are presented, and a computer-simulated spectrum is given. It has been found that, for a model which assumes no preferred orientation within the plane of the membranes, the orientation of the Fe²⁺ ligand axis of largest zero-field splitting (Z, the principal magnetic axis) is titled 64±6° from the membrane normal. The ligand field for Fe²⁺ has low symmetry, with zero-field splitting parameters of |D₁|=7.0±1.3 cm^-1 and |E₁|=1.7±0.5 cm^-1 and | E₁ D₁|=0.26 for the redox state Q₁^-Fe²⁺Q^2-. The rhombic character of the ligand field is increased in the redox state Q₁Fe²⁺Q^-₂, where 0.33>| E₂ D₂|>0.26. This indicates that the redox state of the quinones can influence the ligand field symmetry and splitting of the Fe²⁺. There exists an electron-spin exchange interaction between Fe²⁺ and Q^-₁ and Q^-₂, having magnitudes |J₁|=0.12±0.03 cm^-1 and |J₂|{reversed tilde equals}0.06 cm^-1, respectively. Such weak interactions indicate that a proper electronic picture of the complex is as a pair of immobilized semiquinone radicals having very little orbital overlap (probably fostered by superexchange) with the Fe²⁺ orbitals. The exchange interaction is analyzed by comparison with model systems of paramagnetic metals and free radicals to indicate an absence of direct coordination between Fe²⁺ and Q^-₁ and Q^-₂. Selective line-broadening of some of the EPR transitions, involving Q^- coupling to the magnetic sublevels of the Fe²⁺ ground state, is interpreted as arising from an electron-electron dipolar interaction. Analysis of this line-broadening indicates a distance of 6.2-7.8 Ȧ between Fe²⁺ and Q^-₁, thus placing Q₁ outside the immediate coordination shell of Fe²⁺.