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

Abstract

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.

Electron transfer from A−1to an iron‐sulfur center witht= 200 ns at room temperature in photosystem I Characterization by flash absorption spectroscopy

“…The half-time of 200 ns determined for this reaction in the present work suggests that the striking change in the polarized EPR spectrum of PS I at room temperature during the first few hundred nanoseconds after excitation (review [26]), reflects the same reaction. This would be consistent with the interpretation that the spin-polarized signals at early times arise from P700+ and a quinone-type AT [13,27,28].…”

Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

“…The X-band ESP EPR spectrum from FeZ+-depleted RCs shows the same features (emission/absorption/emission and g-factors) as those studied using more direct time resolved EPR methods [31,32]. The X-band ESP EPR spectrum observed from the biochemically Zn2+-reconstituted RCs has similar spectral features as that obta]ned from Fe2+-depleted RCs (comparison in Fig.2a).…”

Characterization of free radicals from vitamin K1 and menadione by 2 mm-band EPR, ENDOR and ESEEM

“…The anisotropic g-factor of the reduced A 1 in deuterated PSI preparation reportedly differs from that expected for a benzoquinone [6] although experiments at Q-band find g-factors for reduced A 1 and the vitamin K 1 anion radical that are consistent with each other [7,8]. To add to the confusion, vitamin K 1 is rather easily altered and can rearrange under mild conditions into a number of related forms (two ortho-quinone methides, a chromenol and a chromanol, Fig.1) which, at one time or another, have been suggested as photosynthetic intermediates [9--12].…”