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

The spin-lattice relaxation times (T1) 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, QA, was doubly reduced. The selective conversion of the non-heme Fe2+ into its low-spin (S = O) state, in CN-treated PSII, allowed the measurement of the intrinsic T1 of the Pheo- radical. The temperature dependence of the intrinsic (T1)-1 was found to be approximately T1.3 +/- 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 (k1d) 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, YD+, shows that the Pheo is much closer to the iron than the YD+ in the PSII reaction center. By scaling the reported Fe(2+)-YD+ distance by the ratio [k1dPheo-]/[k1dYD+], we estimate the Fe(2+)-Pheo- distance in PSII to be 20 +/- 4.2 A. This distance is close to the Fe(2+)-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.