The stable tyrosine radical YD• (tyrosine 160 in the D2 polypeptide) in photosystem II (PSII) exhibits nonexponential electron spin-lattice relaxation transients at low temperature. As previously reported, the tetranuclear Mn complex in PSII significantly enhances the spin-lattice relaxation of YD•. However, in-depleted PSII membranes, the spin-lattice relaxation transients of YD• are also nonexponential, and progressive power saturation (P1/2) experiments show that it does not behave like an isolated tyrosine radical. A model is developed to treat the interaction of two paramagnets in a rigid lattice at a fixed distance apart but with a random orientation in a magnetic field. This model describes the spin-lattice relaxation of a radical in proximity to another paramagnetic site in terms of three relaxation rate constants: the “intrinsic” relaxation rate, the relaxation rate due to scalar exchange, and the relaxation rate due to dipole-dipole interactions. The intrinsic and the scalar exchange relaxation rates are isotropic and together contribute a single rate constant to the spin-lattice relaxation transients. However, the dipolar relaxation rate is orientation dependent. Each orientation contributes a different dipolar relaxation rate constant to the net spin-lattice relaxation rate constant. The result is a superposition of single-exponential recoveries, each with a different net rate constant, causing the observed saturation-recovery transients to be non-(single)-exponential. Saturation-recovery relaxation transients of YD• are compared with those of a model tyrosine radical, generated by UV photolysis of L-tyrosine in a borate glass. From this comparison, we conclude that scalar exchange does not make a significant contribution to the spin-lattice relaxation of YD• in-depleted PSII. We account for the nonexponential relaxation transients obtained from YD• in Mn-depleted PSII membranes in terms of dipolar-induced relaxation enhancement from the non-heme Fe(II). From simulations of the spin-lattice relaxation transients, we obtain the magnitude of the magnetic dipolar interaction between YD• and the non-heme Fe(II), which can be used to calculate the distance between them. Using data on the non-heme Fe(II) in the reaction center of Rhodobacter sphaeroides to model the non-heme Fe(II) in PSII, we calculate a YD•-Fe(II) distance of ≥38 Å in PSII. This agrees well with the distance predicted from the structure of the bacterial reaction center.
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