A structural model for the charge separated state P₇₀₀^̇+A₁^̇+ photosystem I from the orientation of the magnetic interaction tensors

The charge separated state P₇₀₀^̇+A₁^̇+ (P₇₀₀ = primary electron donor, A₁ = phylloquinone electron acceptor) in photosystem I of oxygenic photosynthesis has been investigated by EPR spectroscopy in frozen solution and single crystals. The transient EPR spectra of P₇₀₀^̇+A₁^̇+ recorded in frozen solution of fully deuterated samples at X-, Q-, and W-band frequencies are shown to contain sufficient information to yield the orientation of the g-tensors of both A₁^̇+ and P₇₀₀^̇+ with respect to the axis connecting both spins. So far incomplete information on the orientation of A₁^̇+ relative to the membrane plane has been complemented by data from time-resolved EPR on single crystals measured at Q-band. The phylloquinone headgroup orientation evaluated from the EPR data in the charge-separated state P₇₀₀^̇+A₁^̇+ is compared with the presently available X-ray structural model. The g-tensor of P₇₀₀^̇+ has also been determined from cw-EPR experiments at W-band on single crystals, independent of the orientational data of the P₇₀₀^̇+ g-tensor from the time-resolved EPR experiments. The direction of the principal axes of g(P₇₀₀^̇+) differ from the molecular axes system of the chlorophylls comprising P₇₀₀ as found previously in the case of P₈₆₅^+̇ in bacterial reaction centers. The implications of the complete structural model from the A₁^̇+ and P₇₀₀^̇+ molecular magnetic interaction tensors in the active charge separated state P₇₀₀^̇+A₁^̇+ in PS I are discussed.