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

The charge separated state P(700)(.+)A(1)(.-) (P-700 = primary electron donor, A(1) = 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 P(700)(.+)A(1)(.-) 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 A(1)(.-) and P-700(.+), with respect to the axis connecting both spins. So far incomplete information on the orientation of A(1)(.-) 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 P(700)(.+)A(1)(.-) is compared with the presently available X-ray structural model. The g-tensor of P-700(.+) has also been determined from cw-EPR experiments at W-band on single crystals, independent of the orientational data of the P-700(.+), g-tensor from the time-resolved EPR experiments. The direction of the principal axes of g(P-700(.+)) differ from the molecular axes system of the chlorophylls comprising P700 as found previously in the case of P-865(.+) in bacterial reaction centers. The implications of the complete structural model from the A(1)(.-) and P-700(.+) molecular magnetic interaction tensors in the active charge separated state P(700)(.+)A(1)(.-) in PS I are discussed.