SPIN-LATTICE RELAXATION OF THE PHYLLOSEMIQUINONE RADICAL OF PHOTOSYSTEM-I

The spin-lattice relaxation time (T-1) of the phyllosemiquinone anion radical, A(1)(-), of the photosystem I (PSI) reaction center, were mea sured between 4.5 and 85 K by electron spin-echo spectroscopy. The sel ective removal of the iron-sulfur centers, F-A, F-B, and F-X, from PSI allowed the measurement of the intrinsic T-1 of the A(1)(-) radical. The temperature dependence of the intrinsic (T-1)(-1) for A(1)(-) was found to be similar to T1.3+/-0.1. The spin-lattice relaxation of the reduced form of iron-sulfur center F-X was also measured at low temper atures, in F-A/F-B-depleted PSI membranes. It was found that the fast- relaxing F-X center enhances the spin-lattice relaxation of the phyllo semiquinone due to dipolar coupling. The effect of the reduced forms o f F-A/F-B on the T-1 of the phyllosemiquinone was minor compared to th e effect of F-X. By analyzing the data with a dipolar model in the lig ht of limitations imposed by other information present in the literatu re, the distance between the phyllosemiquinone and F-X in PSI is estim ated to be 14.8 +/- 4 Angstrom.