We have performed continuous-wave electron paramagnetic resonance (CW-EPR) and electron spin echo electron nuclear double resonance (ESE-ENDOR) experiments on the multiline form of the S 2 -state of untreated, MeOH-treated, and ammonia-treated spinach photosystem II (PS II) centers. Through simultaneously constrained simulations of the CW-EPR and ESE-ENDOR data, we conclude that four effective 55 Mn hyperfine tensors (A X , A Y , A Z ) are required to properly simulate the experimental data [untreated and MeOH-treated PS

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...