Electron spin echo electron-nuclear double resonance (ESE-ENDOR) experiments performed on a broad radical electron paramagnetic resonance (EPR) signal observed in photosystem II particles depleted of Ca2+ indicate that this signal arises from the redox-active tyrosine Yz. The tyrosine EPR signal width is increased relative to that observed in a manganese-depleted preparation due to a magnetic interaction between the photosystem II manganese cluster and the tyrosine radical. The manganese cluster is located asymmetrically with respect to the symmetry-related tyrosines Yz and YD. The distance between the Yz tyrosine and the manganese cluster is estimated to be approximately 4.5 A. Due to this close proximity of the Mn cluster and the redox-active tyrosine Yz, we propose that this tyrosine abstracts protons from substrate water bound to the Mn cluster.The photosystem II (PS II) component of the plant photosynthetic apparatus oxidizes water at an oxygen-evolving complex (OEC) that consists of a tetranuclear cluster of manganese ions along with essential cofactors calcium and chloride (1). The overall architecture of PS II shows homologies to the reaction centers of the purple non-oxygen-evolving bacteria, including the C2 symmetry found in these reaction centers (2-4). In PS II this symmetry appears to persist to the sites of two important tyrosine residues, Yz (tyrosine-161 of the Dl polypeptide, with residues designated for Synechocystis sp. 6803) and YD (tyrosine-160 of the D2 polypeptide) (5-7). The Yz tyrosine serves as an electron transfer intermediate between the chargeseparating chlorophyll moiety P680 and the manganese cluster of the OEC. The symmetry-related tyrosine YD, which is typically present as a dark-stable neutral radical (YD-), is bypassed in the fast electron transfer between P680 and the OEC. Whether the Mn cluster is located on the C2 symmetry axis has been the subject of much debate. Instead, the PS II symmetry may be broken at the OEC level, locating the Mn cluster closer to the active electron transfer intermediate Yz.Calcium depletion of PS II particles by NaCl/EGTA washing (8-10) or low-pH citrate treatment (11-13) eliminates oxygen-evolving activity. In such Ca2+-depleted PS II particles, illumination at a temperature of 273 K leads to the formation of a broad (130-180 G full width at half maximum) g = 2 EPR signal. Other treatments that block oxygen evolution such as acetate or fluoride incubation lead to similar signals upon such illumination, though the signal widths vary appreciably, depending on the details of the treatment and the resulting extrinsic polypeptide composition (14-18). The broad g = 2 signal is thought to arise from a radical center, with the large linewidth caused by a magnetic interaction with the Mn cluster (18,19). UV (19) and IR (20) absorption changes in PS II that are observed concomitantly with the formation of the radical have been interpreted to favor an oxidized histidine as the origin of the broad radical signal. Alternatively, on the basis of EPR s...

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The pulsed EPR technique of electron spin echo-electron nuclear double resonance (ESE-ENDOR) is used to measure 55Mn nuclear spin transitions in a variety of orientationally disordered high-spin Mnz+ complexes.The large hyperfine interaction and the large net electronic spin of the S = 5/2 Mn2+ ion give rise to a number of interesting effects in the ENDOR spectrum. These are illustrated with ESE-ENDOR experiments on Mn2+ as a substitutional impurity in powdered SrO. The "powder pattern" 55Mn ENDOR spectrum of MnZ+ present as an impurity in CaC03 is more complicated due to the moderate zero field splitting interaction for the S = 5/2 ion in this matrix. The effects of substantial zero field splitting interactions are explored with 55Mn ENDOR of Mnz+ in the protein concanavalin A and Mn2+ in a frozen water glass.

The technique of 51V electron spin echo−electron nuclear double resonance (ESE-ENDOR) has been used to investigate a structurally diverse series of oxovanadium(IV) complexes. From ENDOR spectra, 51V nuclear quadrupole coupling constants, which reflect the electric field gradient established at the vanadium nucleus, are estimated using a perturbation theory approach. The magnitude of the nuclear quadrupole interaction is reduced by ligands binding trans to the oxygen of the oxovandium moiety due to a reduction of the electric field gradient that is established primarily by the oxo bond. Variation of ligands containing nitrogen and oxygen donor atoms oriented cis to the oxo bond gives rise to minimal changes in the quadrupole coupling constant. The nuclear quadrupole coupling constant is thus shown to be sensitive to ligands binding trans to the oxygen of the oxo bond. ENDOR can thusly serve as a new spectroscopic probe in the identification of weakly coordinating donors in the axial position such as may be found in vanadyl-substituted proteins.