A Structural Model for the Charge Separated State in Photosystem I from the Orientation of the Magnetic Interaction Tensors

Abstract

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