Comparison of the electron spin polarized spectrum found in plant photosystem I and in iron-depleted bacterial reaction centers with time-resolved K-band EPR; evidence that the photosystem I acceptor A1 is a quinone.

The suggestion that the electron acceptor A1 in plant photosystem I (PSI) is a quinone molecule is tested by comparisons with the bacterial photosystem. The electron spin polarized (ESP) EPR signal due to the oxidized donor and reduced quinone acceptor (P 870 (+) Q(-)) in iron-depleted bacterial reaction centers has similar spectral characteristics as the ESP EPR signal in PSI which is believed to be due to P 700 (+) A 1 (-) , the oxidized PSI donor and reduced A1. This is also true for better resolved spectra obtained at K-band (∼24 GHz). These same spectral characteristics can be simulated using a powder spectrum based on the known g-anisotropy of reduced quinones and with the same parameter set for Q(-) and A1 (-). The best resolution of the ESP EPR signal has been obtained for deuterated PSI particles at K-band. Simulation of the A1 (-) contribution based on g-anisotropy yields the same parameters as for bacterial Q(-) (except for an overall shift in the anisotropic g-factors, which have previously been determined for Q(-)). These results provide evidence that A1 is a quinone molecule. The electron spin polarized signal of P700 (+) is part of the better resolved spectrum from the deuterated PSI particles. The nature of the P700 (+) ESP is not clear; however, it appears that it does not exhibit the polarization pattern required by mechanisms which have been used so far to explain the ESP in PSI.