The charge separated state P700̇+A1̇+ (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 P700̇+A1̇+ 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 A1̇+ and P700̇+ with respect to the axis connecting both spins. So far incomplete information on the orientation of A1̇+ 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 P700̇+A1̇+ is compared with the presently available X-ray structural model. The g-tensor of P700̇+ has also been determined from cw-EPR experiments at W-band on single crystals, independent of the orientational data of the P700̇+ g-tensor from the time-resolved EPR experiments. The direction of the principal axes of g(P700̇+) 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 A1̇+ and P700̇+ molecular magnetic interaction tensors in the active charge separated state P700̇+A1̇+ in PS I are discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry