THE TETRANUCLEAR MANGANESE CLUSTER IN PHOTOSYSTEM .2. LOCATION AND MAGNETIC-PROPERTIES OF THE S-2 STATE AS DETERMINED BY SATURATION-RECOVERY EPR SPECTROSCOPY

The spin-lattice relaxation enhancement of the dark-stable tyrosine ra dical, Y-D(.), by the S-2 state of the O-2-evolving complex (OEC) of p hotosystem II (PSII) has been measured by using saturation-recovery EP R spectroscopy. Two forms of the S-2 state have been compared: the mul tiline EPR signal species in untreated PSII and the altered multiline EPR signal species in NH3-treated PSII. Previous work has shown that t he non-single-exponential spin-lattice relaxation kinetics of Y-D(.) i n S-2-state PSII result from a dipole-dipole interaction with the Mn-4 cluster of the OEC. By taking into account the temperature variation of the effective magnetic moment of the S-2-state multiline EPR signal form of the OEC, we provide a quantitative analysis of its temperatur e-dependent enhancement of the spin-lattice relaxation of Y-D(.). Diff erent spin states of the Mn-4 cluster in the S-2 state are responsible for the effect at different temperature regimes: for T less than or e qual to 10 K, it is the ground spin state (S = 1/2); for T greater tha n or equal to 30 KI it is the first excited spin state; and at interme diate temperatures, the contributions of the two spin states are compa rable. The relaxation enhancement of Y-D(.) is equivalent for both for ms of the S-2-state multiline EPR signal examined, indicating that the magnetic properties of the Mn-4 cluster are very similar in the S-2 s tate for both untreated and NH3-treated PSII. EPR progressive microwav e-power saturation has also been used to assess the spin-lattice relax ation properties of the Mn-4 cluster giving the altered S-2-state mult iline EPR signal in the NH3 derivative of PSII. The Orbach mechanism i s shown to provide the dominant relaxation pathway; the energy differe nce between the ground and first excited spin states is estimated to b e 30 +/- 2 cm(-1), which is very similar to the value found for the S- 2-state multiline EPR signal species in untreated PSII. Below 4 K, the effectiveness of the S-2-state multiline EPR signal species as a spin relaxation enhancer of Y-D(.) drops dramatically. This is interpreted to occur because of temperature-dependent Mn-55 nuclear spin-lattice relaxation which causes averaging of the effective Larmor frequency of the S-2-state multiline EPR signal species during the time scale for spin-lattice relaxation of Y-D(.) because the line shape of the S-2-st ate multiline EPR signal is dominated by isotropic Mn-55 nuclear hyper fine splittings, such nuclear relaxation processes allow frequencies i n near resonance with that of Y-D(.) to be accessed, thereby producing a greater relaxation enhancement. By using a dipolar model that inclu des the line shapes of both the Y-D(.) and S-2-state multiline EPR sig nals, the spin-lattice relaxation enhancement of Y-D(.) is analyzed to obtain a lower limit of 22 Angstrom for the distance between Y-D(.) a nd the OEC. Together with recent studies showing a close proximity of the Mn-4 cluster to Y-Z(.), these results provide further support for an asymmetric location of the Mn-4 cluster with respect to the two red ox-active tyrosines in PSII.