Highly resolved proton matrix ENDOR of oriented photosystem II membranes in the S2 state

Hyperfine Decoupling of ESR Spectra Using Wavelet Transform

The objective of spectral analysis is to resolve and extract relevant features from experimental data in an optimal fashion. In continuous-wave (cw) electron spin resonance (ESR) spectroscopy, both g values of a paramagnetic center and hyperfine splitting (A) caused by its interaction with neighboring magnetic nuclei in a molecule provide important structural and electronic information. However, in the presence of g- and/or A-anisotropy and/or large number of resonance lines, spectral analysis becomes highly challenging. Either high-resolution experimental techniques are employed to resolve the spectra in those cases or a range of suitable ESR frequencies are used in combination with simulations to identify the corresponding g and A values. In this work, we present a wavelet transform technique in resolving both simulated and experimental cw-ESR spectra by separating the hyperfine and super-hyperfine components. We exploit the multiresolution property of wavelet transforms that allow the separation of distinct features of a spectrum based on simultaneous analysis of spectrum and its varying frequency. We retain the wavelet components that stored the hyperfine and/or super-hyperfine features, while eliminating the wavelet components representing the remaining spectrum. We tested the method on simulated cases of metal–ligand adducts at L-, S-, and X-band frequencies, and showed that extracted g values, hyperfine and super-hyperfine coupling constants from simulated spectra, were in excellent agreement with the values of those parameters used in the simulations. For the experimental case of a copper(II) complex with distorted octahedral geometry, the method was able to extract g and hyperfine coupling constant values, and revealed features that were buried in the overlapped spectra.

Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.

Magnetic structure of manganese cluster in photosystem II investigated by electron paramagnetic resonance

The electronic structure of manganese (Mn) cluster in photosystem II was investigated by electron paramagnetic resonance (EPR) spectroscopy. In order to determine the spin density distribution in magnetically coupled Mn in the S₂ state Mn cluster, pulsed electron–electron double resonance (PELDOR) measurement was performed. The local environment of the Mn cluster was investigated by electron-nuclear double resonance (ENDOR). Using spin projections determined by PELDOR, ENDOR signals were assigned to the water molecules ligated to the Mn cluster. The location of a high-affinity Mn²⁺ site in apo-photosystem II, which is the initial site of photoactivation of the Mn cluster, was determined by PELDOR.

Regulated Electron Tunneling of Photoinduced Primary Charge-Separated State in the Photosystem II Reaction Center

In initial events of the photosynthesis by higher plants, the photosystem II (PSII) generates photoinduced primary charge-separated (CS) state composed of reduced pheophytin (Pheo_D1^-•) and oxidized special pair (P^+•) in chlorophyll a (Chla) P_D1/P_D2 in the D1/D2 heterodimer, ultimately leading to the water oxidation at the oxygen-evolving Mn₄CaO₅ cluster by P^+•. To understand the mechanism of the efficient generation of initially localized CS state (P_D1^+• Pheo_D1^-•), we have characterized cofactor geometries and electronic coupling of the photoinduced primary CS state in quinone prereduced membrane of PSII from spinach using the time-resolved electron paramagnetic resonance method. It has been revealed that the electronic coupling between the charges is significantly weak in the CS state separated by 1.5 nm, showing the importance of regulated cofactor-cofactor electronic interaction between a vinyl substituent in Pheo_D1 and an accessory chlorophyll to inhibit the energy-wasting charge recombination after the primary electron-transfer processes.

Location of Methanol on the S2 State Mn Cluster in Photosystem II Studied by Proton Matrix Electron Nuclear Double Resonance

Proton matrix electron nuclear double resonance (ENDOR) spectroscopy was performed to specify the location of the methanol molecule near the manganese cluster in photosystem II. Comparison of the ENDOR spectra in the presence of CH₃OH and CD₃OH revealed two pairs of hyperfine couplings, 1.2 MHz for A_⊥ and 2.5 MHz for A_//, arising from the methyl group in methanol. On the basis of the crystal structure, the possible location of methanol close to the manganese cluster was discussed.

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

Proton matrix ENDOR spectra were measured for Ca(2+)-depleted and Sr(2+)-substituted photosystem II (PSII) membrane samples from spinach and core complexes from Thermosynechococcus vulcanus in the S2 state. The ENDOR spectra obtained were similar for untreated PSII from T. vulcanus and spinach, as well as for Ca(2+)-containing and Sr(2+)-substituted PSII, indicating that the proton arrangements around the manganese cluster in cyanobacterial and higher plant PSII and Ca(2+)-containing and Sr(2+)-substituted PSII are similar in the S2 state, in agreement with the similarity of the crystal structure of both Ca(2+)-containing and Sr(2+)-substituted PSII in the S1 state. Nevertheless, slightly different hyperfine separations were found between Ca(2+)-containing and Sr(2+)-substituted PSII because of modifications of the water protons ligating to the Sr(2+) ion. Importantly, Ca(2+) depletion caused the loss of ENDOR signals with a 1.36-MHz separation because of the loss of the water proton W4 connecting Ca(2+) and YZ directly. With respect to the crystal structure and the functions of Ca(2+) in oxygen evolution, it was concluded that the roles of Ca(2+) and Sr(2+) involve the maintenance of the hydrogen bond network near the Ca(2+) site and electron transfer pathway to the manganese cluster.