Spin-lattice relaxation of the pheophytin, Pheo(-), radical of photosystem II

The spin-lattice relaxation times (T-1) of the pheophytin anion radical, Pheo(-), of the PSII reaction center, were measured between 5 and 80 K by electron spin-echo spectroscopy. The Pheo(-) was studied in Mn-depleted PSII reaction centers in which the primary quinone, Q(A), was doubly reduced, The selective conversion of the non-heme Fe2+ into its low-spin (S = 0) state, in CN-treated PSII, allowed the measurement of the intrinsic T-1 of the Pheo(-) radical. The temperature dependence of the intrinsic (T-1)(-1) was found to be similar to T-1.3+/-0.1. In Mn-depleted PSII membranes the high-spin (S = 2) non-heme iron, enhances the spin-lattice relaxation of Pheo(-). By analyzing the data with a dipolar model, the dipolar interaction (k(ld)) between the Pheo and the Fe2+ (S = 2) is estimated over the temperature range 5-80 K. Comparison with the dipolar coupling between the iron and the tyrosine, Y-D(+), shows that the Pheo is much closer to the iron than the Y-D(+) in the PSII reaction center. By scaling the reported Fe2+-Y-D(+) distance by the ratio [k(ld)Pheo(-)]/[k(ld)Y(D)(+)], we estimate the Fe2+-Pheo(-) distance in PSII to be 20 +/- 4.2 Angstrom. This distance is close to the Fe2+-BPheo(-) distance in the bacterial reaction center, and this result provides further evidence that the acceptor sides of the reaction centers in PSII and bacteria are homologous.