Probing the electronic structure of tyrosine radical YD in photosystem II by EPR spectroscopy using site specific isotope labelling in Spirodela oligorrhiza

Microsolvation of the Redox-Active Tyrosine-D in Photosystem II: Correlation of Energetics with EPR Spectroscopy and Oxidation-Induced Proton Transfer

Photosystem II (PSII) of oxygenic photosynthesis captures sunlight to drive the catalytic oxidation of water and the reduction of plastoquinone. Among the several redox-active cofactors that participate in intricate electron transfer pathways there are two tyrosine residues, YZ and YD. They are situated in symmetry-related electron transfer branches but have different environments and play distinct roles. YZ is the immediate oxidant of the oxygen-evolving Mn4CaO5 cluster, whereas YD serves regulatory and protective functions. The protonation states and hydrogen-bond network in the environment of YD remain debated, while the role of microsolvation in stabilizing different redox states of YD and facilitating oxidation or mediating deprotonation, as well the fate of the phenolic proton, is unclear. Here we present detailed structural models of YD and its environment using large-scale quantum mechanical models and all-atom molecular dynamics of a complete PSII monomer. The energetics of water distribution within a hydrophobic cavity adjacent to YD are shown to correlate directly with electron paramagnetic resonance (EPR) parameters such as the tyrosyl g-tensor, allowing us to map the correspondence between specific structural models and available experimental observations. EPR spectra obtained under different conditions are explained with respect to the mode of interaction of the proximal water with the tyrosyl radical and the position of the phenolic proton within the cavity. Our results revise previous models of the energetics and build a detailed view of the role of confined water in the oxidation and deprotonation of YD. Finally, the model of microsolvation developed in the present work rationalizes in a straightforward way the biphasic oxidation kinetics of YD, offering new structural insights regarding the function of the radical in biological photosynthesis.

Photochemically induced dynamic nuclear polarization NMR on photosystem II: donor cofactor observed in entire plant

The solid-state photo-CIDNP (photochemically induced dynamic nuclear polarization) effect allows for increase of signal and sensitivity in magic-angle spinning (MAS) NMR experiments. The effect occurs in photosynthetic reaction centers (RC) proteins upon illumination and induction of cyclic electron transfer. Here we show that the strength of the effect allows for observation of the cofactors forming the spin-correlated radical pair (SCRP) in isolated proteins, in natural photosynthetic membranes as well as in entire plants. To this end, we measured entire selectively 13C isotope enriched duckweed plants (Spirodela oligorrhiza) directly in the MAS rotor. Comparison of 13C photo-CIDNP MAS NMR spectra of photosystem II (PS2) obtained from different levels of RC isolation, from entire plant to isolated RC complex, demonstrates the intactness of the photochemical machinery upon isolation. The SCRP in PS2 is structurally and functionally very similar in duckweed and spinach (Spinacia oleracea). The analysis of the photo-CIDNP MAS NMR spectra reveals a monomeric Chl a donor. There is an experimental evidence for matrix involvement, most likely due to the axial donor histidine, in the formation of the SCRP. Data do not suggest a chemical modification of C-131 carbonyl position of the donor cofactor.

Analysis of electron donors in photosystems in oxygenic photosynthesis by photo-CIDNP MAS NMR

Both photosystem I and photosystem II are considerably similar in molecular architecture but they operate at very different electrochemical potentials. The origin of the different redox properties of these RCs is not yet clear. In recent years, insight was gained into the electronic structure of photosynthetic cofactors through the application of photochemically induced dynamic nuclear polarization (photo-CIDNP) with magic-angle spinning NMR (MAS NMR). Non-Boltzmann populated nuclear spin states of the radical pair lead to strongly enhanced signal intensities that allow one to observe the solid-state photo-CIDNP effect from both photosystem I and II from isolated reaction center of spinach (Spinacia oleracea) and duckweed (Spirodela oligorrhiza) and from the intact cells of the cyanobacterium Synechocystis by (13)C and (15)N MAS NMR. This review provides an overview on the photo-CIDNP MAS NMR studies performed on PSI and PSII that provide important ingredients toward reconstruction of the electronic structures of the donors in PSI and PSII.

N Photo-CIDNP MAS NMR To Reveal Functional Heterogeneity in Electron Donor of Different Plant Organisms

In plants and cyanobacteria, two light-driven electron pumps, photosystems I and II (PSI, PSII), facilitate electron transfer from water to carbon dioxide with quantum efficiency close to unity. While similar in structure and function, the reaction centers of PSI and PSII operate at widely different potentials with PSI being the strongest reducing agent known in living nature. Photochemically induced dynamic nuclear polarization (photo-CIDNP) in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements provides direct excess to the heart of large photosynthetic complexes (A. Diller, Alia, E. Roy, P. Gast, H.J. van Gorkom, J. Zaanen, H.J.M. de Groot, C. Glaubitz, J. Matysik, Photosynth. Res. 84, 303-308, 2005; Alia, E. Roy, P. Gast, H.J. van Gorkom, H.J.M. de Groot, G. Jeschke, J. Matysik, J. Am. Chem. Soc. 126, 12819-12826, 2004). By combining the dramatic signal increase obtained from the solid-state photo-CIDNP effect with (15)N isotope labeling of PSI, we were able to map the electron spin density in the active cofactors of PSI and study primary charge separation at atomic level. We compare data obtained from two different PSI proteins, one from spinach (Spinacia oleracea) and other from the aquatic plant duckweed (Spirodella oligorrhiza). Results demonstrate a large flexibility of the PSI in terms of its electronic architecture while their electronic ground states are strictly conserved.

Photoemission and Near-Edge X-Ray Absorption Fine Structure Studies of the Bacterial Surface Protein Layer ofBacillussphaericusNCTC 9602

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.

Electronic structure of regular bacterial surface layers

We report photoemission and near-edge x-ray absorption fine structure measurements of the occupied and unoccupied valence electronic states of the regular surface layer of Bacillus sphaericus, which is widely used as the protein template for the fabrication of metallic nanostructures. The two-dimensional protein crystal shows a semiconductorlike behavior with a gap value of approximately 3.0 eV and the Fermi energy close to the bottom of the lowest unoccupied molecular orbital. We anticipate that these results will open up new possibilities for the electric addressability of biotemplated low-dimensional hybrid structures.