SPIN-LATTICE RELAXATION OF THE PHEOPHYTIN, PHEO(-), RADICAL OF PHOTOSYSTEM-II

The spin-lattice relaxation times (T-1) of the pheophytin anion radica l, Pheo(-), of the PSII reaction center, were measured between 5 and 8 0 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 it s low-spin (S = 0) state, in CN-treated PSII, allowed the measurement of the intrinsic T-1 of the Pheo(-) radical. The temperature dependenc e of the intrinsic (T-1)(-1) was found to be similar to T-1.3+/-0.1. I n Mn-depleted PSII membranes the high-spin (S = 2) non-heme iron, enha nces the spin-lattice relaxation of Pheo(-). By analyzing the data wit h a dipolar model, the dipolar interaction (k(ld)) between the Pheo an d the Fe2+ (S = 2) is estimated over the temperature range 5-80 K. Com parison 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(+) dista nce by the ratio [k(ld)Pheo(-)]/[k(ld)Y(D)(+)], we estimate the Fe2+-P heo(-) distance in PSII to be 20 +/- 4.2 Angstrom. This distance is cl ose to the Fe2+-BPheo(-) distance in the bacterial reaction center, an d this result provides further evidence that the acceptor sides of the reaction centers in PSII and bacteria are homologous.