Multifrequency Pulsed Electron Paramagnetic Resonance Study of the S₂ State of the Photosystem II Manganese Cluster

Abstract

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.

Proton Matrix ENDOR Studies on Ca2+-depleted and Sr2+-substituted Manganese Cluster in Photosystem II

“…The spin density distribution on the manganese cluster has also been obtained by QM/MM (quantum mechanics/molecular mechanics) calculations (40,41). These studies showed that the largest spin density is ϳ2 and distributed on Mn1, and the spin densities for other manganese are ϳ1, which are in agreement with those obtained from the ESEEM measurements (28). On the other hand, the sign of the spin densities was inconsistent between the pulsed ELDOR results (30) and theoretical studies, because the former showed a spin topology of ␣␤␣␤, whereas the latter showed a ␣␤␤␣ topology.…”

“…EPR spectroscopy has been used to characterize the manganese cluster (23)(24)(25)(26) extensively, and a typical, S 2 state multiline signal centered at g ϭ 2 has been well characterized. The multiline signal showed 19 -21 peaks in a range over ϳ180 mT width, and its properties have been studied by several advanced EPR techniques such as 55 Mn-ENDOR (27), ESEEM (28,29), and pulsed ELDOR (30). The ENDOR results indicated that the S 2 state is composed of one Mn(III) and three Mn(IV), where the Mn(III) ion is assigned to the Mn1 ion in the crystal structure ( Fig.…”

Altered Structure of the Mn4Ca Cluster in the Oxygen-evolving Complex of Photosystem II by a Histidine Ligand Mutation

“…In the recent x-ray crystallographic structural models of PSII, the Mn 4 Ca cluster is ligated by a single histidine residue, His 332 of the D1 polypeptide (8 -11). This assignment is supported by recent 31-34 GHz ESEEM studies of the Mn 4 Ca cluster in its S 2 state (15,16). These studies show that the nitrogen couplings observed in earlier ESEEM studies conducted at lower frequencies (9 -12 GHz) (17)(18)(19)(20) originate from a single 14 N nucleus.…”

“…These studies show that the nitrogen couplings observed in earlier ESEEM studies conducted at lower frequencies (9 -12 GHz) (17)(18)(19)(20) originate from a single 14 N nucleus. The higher frequency studies permit the quantitative determination of the hyperfine and nuclear quadrupolar couplings of the coupled nucleus; the higher frequency studies are consistent with direct coordination of the 14 N atom to a manganese ion in the S 2 state (15,16). On the basis of earlier 9-GHz ESEEM studies conducted with unlabeled and […”

Room temperature femtosecond X-ray diffraction of photosystem II microcrystals

“…Extensive X-ray diffraction (XRD) studies using synchrotron radiation (SR) (16)(17)(18)(19)(20)(21) and various spectroscopic techniques (22)(23)(24)(25)(26)(27)(28)(29) have been used over the last several decades to elucidate the structure and mechanism of the OEC in PS II. Among the various methods, XRD is clearly the technique of choice for obtaining an overall detailed structure of PS II and the Mn 4 CaO 5 cluster.…”