Vera Krewald

Theoretical evaluation of structural models of the S2 state in the oxygen evolving complex of Photosystem II: protonation states and magnetic interactions.

Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates S(n) (n = 0-4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S(2) state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn(4)O(5)Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (Mn(A)) of the cluster is deprotonated in the S(2) state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal-metal distances. Deprotonation of Ca(2+)-bound water molecules is strongly disfavored in the S(2) state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to Mn(III). The present results impose limits for the total charge and the proton configuration of the OEC in the S(2) state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.

Theoretical magnetochemistry of dinuclear manganese complexes: broken symmetry density functional theory investigation on the influence of bridging motifs on structure and magnetism

A systematic study of the magnetic coupling in homovalent (III-III and IV-IV) and heterovalent (III-IV) manganese dimers as a function of the chemical identity and coordination mode of the bridging ligands is carried out with the aim of establishing a reference library of magnetostructural correlations. Emphasis is placed on rationalising the results through analysis of the superexchange pathways in terms of corresponding orbitals. Additionally, the influence of specific structural distortions on magnetic properties is explored. Consistent with chemical intuition and decades of experience, oxo bridges are shown to be efficient mediators of superexchange, primarily through pi-type pathways, whereas the introduction of bridging carboxylates inhibits the magnetic coupling of the metal centres by introducing structural distortions in the core and by reducing the antiferromagnetic contribution to the exchange. Protonation of oxo bridges is shown to induce a predictably systematic reduction in the magnitude of antiferromagnetic coupling by switching off the dominant antiferromagnetic exchange pathways. In the case of weakly coupled dimers, this can even induce a reversal of the coupling from antiferromagnetic to ferromagnetic.

Structure, ligands and substrate coordination of the oxygen-evolving complex of photosystem II in the S2 state: a combined EPR and DFT study

The S2 state of the oxygen-evolving complex of photosystem II, which consists of a Mn4O5Ca cofactor, is EPR-active, typically displaying a multiline signal, which arises from a ground spin state of total spin ST = 1/2. The precise appearance of the signal varies amongst different photosynthetic species, preparation and solvent conditions/compositions. Over the past five years, using the model species Thermosynechococcus elongatus, we have examined modifications that induce changes in the multiline signal, i.e. Ca(2+)/Sr(2+)-substitution and the binding of ammonia, to ascertain how structural perturbations of the cluster are reflected in its magnetic/electronic properties. This refined analysis, which now includes high-field (W-band) data, demonstrates that the electronic structure of the S2 state is essentially invariant to these modifications. This assessment is based on spectroscopies that examine the metal centres themselves (EPR, (55)Mn-ENDOR) and their first coordination sphere ligands ((14)N/(15)N- and (17)O-ESEEM, -HYSCORE and -EDNMR). In addition, extended quantum mechanical models from broken-symmetry DFT now reproduce all EPR, (55)Mn and (14)N experimental magnetic observables, with the inclusion of second coordination sphere ligands being crucial for accurately describing the interaction of NH3 with the Mn tetramer. These results support a mechanism of multiline heterogeneity reported for species differences and the effect of methanol [Biochim. Biophys. Acta, Bioenerg., 2011, 1807, 829], involving small changes in the magnetic connectivity of the solvent accessible outer MnA4 to the cuboidal unit Mn3O3Ca, resulting in predictable changes of the measured effective (55)Mn hyperfine tensors. Sr(2+) and NH3 replacement both affect the observed (17)O-EDNMR signal envelope supporting the assignment of O5 as the exchangeable μ-oxo bridge and it acting as the first site of substrate inclusion.

The protonation states of oxo-bridged MnIV dimers resolved by experimental and computational Mn K pre-edge X-ray absorption spectroscopy

In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-μ-oxo-bridge protonation in a Mn dimer is represented by the system [Mn(IV)2(salpn)2(μ-OHn)2](n+). Although the three species [Mn(IV)2(salpn)2(μ-O)2], [Mn(IV)2(salpn)2(μ-O)(μ-OH)](+) and [Mn(IV)2(salpn)2(μ-OH)2](2+) differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretically calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high spin (HS) and broken symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn 1s core electrons into the unoccupied orbitals of local e(g) character (d(z)(2) and d(xy) based in the chosen coordinate system). The lowest energy experimental feature is dominated by excitations of 1s-α electrons, and the second observed feature is primarily attributed to 1s-β electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn-O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in photosystem II.

Principles of Natural Photosynthesis

Nature relies on a unique and intricate biochemical setup to achieve sunlight-driven water splitting. Combined experimental and computational efforts have produced significant insights into the structural and functional principles governing the operation of the water-oxidizing enzyme Photosystem II in general, and of the oxygen-evolving manganese-calcium cluster at its active site in particular. Here we review the most important aspects of biological water oxidation, emphasizing current knowledge on the organization of the enzyme, the geometric and electronic structure of the catalyst, and the role of calcium and chloride cofactors. The combination of recent experimental work on the identification of possible substrate sites with computational modeling have considerably limited the possible mechanistic pathways for the critical O-O bond formation step. Taken together, the key features and principles of natural photosynthesis may serve as inspiration for the design, development, and implementation of artificial systems.

Ca K-Edge XAS as a Probe of Calcium Centers in Complex Systems

Herein, Ca K-edge X-ray absorption spectroscopy (XAS) is developed as a means to characterize the local environment of calcium centers. The spectra for six, seven, and eight coordinate inorganic and molecular calcium complexes were analyzed and determined to be primarily influenced by the coordination environment and site symmetry at the calcium center. The experimental results are closely correlated to time-dependent density functional theory (TD-DFT) calculations of the XAS spectra. The applicability of this methodology to complex systems was investigated using structural mimics of the oxygen-evolving complex (OEC) of PSII. It was found that Ca K-edge XAS is a sensitive probe for structural changes occurring in the cubane heterometallic cluster due to Mn oxidation. Future applications to the OEC are discussed.

Water dynamics in silica nanopores: The self-intermediate scattering functions

The dynamics of water molecules confined in approximately cylindrical silica nanopores is investigated using molecular simulation. The model systems are pores of diameter varying between 20 and 40 Å containing water at room temperature and at full hydration, prepared using grand canonical Monte Carlo simulation. Water dynamics in these systems is studied via molecular dynamics simulation. The results of the basic characterization of these systems have been reported in A. A. Milischuk and B. M. Ladanyi [J. Chem. Phys. 135, 174709 (2011)]. The main focus of the present study is the self-intermediate scattering function (ISF), F(S)(Q, t), of water hydrogens, the observable in quasi-elastic neutron scattering experiments. We investigate how F(S)(Q, t) depends on the pore diameter, the direction and magnitude of the momentum transfer Q, and the proximity of water molecules to the silica surface. We also study the contributions to F(S)(Q, t) from rotational and translational motions of water molecules and the extent of rotation-translation coupling present in F(S)(Q, t). We find that F(S)(Q, t) depends strongly on the pore diameter and that this dependence is due mainly to the contributions to the ISF from water translational motion and can be attributed to the decreased mobility of water molecules near the silica surface. The relaxation rate depends on the direction of Q and is faster for Q in the axial than in the radial direction. As the magnitude of Q increases, this difference diminishes but does not disappear. We find that its source is mainly the anisotropy in translational diffusion at low Q and in molecular reorientation at higher Q values.

How Accurately Can Extended X-ray Absorption Spectra Be Predicted from First Principles? Implications for Modeling the Oxygen-Evolving Complex in Photosystem II.

First principle calculations of extended X-ray absorption fine structure (EXAFS) data have seen widespread use in bioinorganic chemistry, perhaps most notably for modeling the Mn4Ca site in the oxygen evolving complex (OEC) of photosystem II (PSII). The logic implied by the calculations rests on the assumption that it is possible to a priori predict an accurate EXAFS spectrum provided that the underlying geometric structure is correct. The present study investigates the extent to which this is possible using state of the art EXAFS theory. The FEFF program is used to evaluate the ability of a multiple scattering-based approach to directly calculate the EXAFS spectrum of crystallographically defined model complexes. The results of these parameter free predictions are compared with the more traditional approach of fitting FEFF calculated spectra to experimental data. A series of seven crystallographically characterized Mn monomers and dimers is used as a test set. The largest deviations between the FEFF calculated EXAFS spectra and the experimental EXAFS spectra arise from the amplitudes. The amplitude errors result from a combination of errors in calculated S0(2) and Debye-Waller values as well as uncertainties in background subtraction. Additional errors may be attributed to structural parameters, particularly in cases where reliable high-resolution crystal structures are not available. Based on these investigations, the strengths and weaknesses of using first-principle EXAFS calculations as a predictive tool are discussed. We demonstrate that a range of DFT optimized structures of the OEC may all be considered consistent with experimental EXAFS data and that caution must be exercised when using EXAFS data to obtain topological arrangements of complex clusters.

The First State in the Catalytic Cycle of the Water-Oxidizing Enzyme: Identification of a Water-Derived μ-Hydroxo Bridge

Natures water-splitting catalyst, an oxygen-bridged tetramanganese calcium (Mn4O5Ca) complex, sequentially activates two substrate water molecules generating molecular O2. Its reaction cycle is composed of five intermediate (Si) states, where the index i indicates the number of oxidizing equivalents stored by the cofactor. After formation of the S4 state, the product dioxygen is released and the cofactor returns to its lowest oxidation state, S0. Membrane-inlet mass spectrometry measurements suggest that at least one substrate is bound throughout the catalytic cycle, as the rate of 18O-labeled water incorporation into the product O2 is slow, on a millisecond to second time scale depending on the S state. Here, we demonstrate that the Mn4O5Ca complex poised in the S0 state contains an exchangeable hydroxo bridge. On the basis of a combination of magnetic multiresonance (EPR) spectroscopies, comparison to biochemical models and theoretical calculations we assign this bridge to O5, the same bridge identified in the S2 state as an exchangeable fully deprotonated oxo bridge [Pérez Navarro, M.; et al. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 15561]. This oxygen species is the most probable candidate for the slowly exchanging substrate water in the S0 state. Additional measurements provide new information on the Mn ions that constitute the catalyst. A structural model for the S0 state is proposed that is consistent with available experimental data and explains the observed evolution of water exchange kinetics in the first three states of the catalytic cycle.

Understanding and tuning the properties of redox-accumulating manganese helicates

The pentanuclear Mn complex [Mn5(μ3-O)(bpp)6]n+ can access six consecutive total oxidation levels from Mn(ii)5 to Mn(iii)5. The electronic structure and redox behavior of this cluster are studied computationally and the results are compared with experimental data. The tunability of the redox potential span and of the absolute potential position is explored through systematic modifications of the 3,5-bis(pyridin-2-yl)-pyrazole (Hbpp) ligand. By substitutions with electron-donating and electron-withdrawing groups, the position of the redox events can be shifted by ca. 1 eV, however no ligand modification is predicted to alter the total redox span. By changing the pyridyl groups to benzimidazole groups, yielding the 3,5-bis(benzimidazol-2-yl)pyrazole (H3bbp) ligand, it is predicted that higher oxidation states can be stabilized, from Mn(iii)4Mn(iv) up to Mn(iv)5. In this system, the redox span for the same number of redox equivalents accumulated is slightly smaller than that in the original system. The manganese system is compared with its pentanuclear iron analogue that was recently reported to be catalytically active in oxygen evolution (Okamura et al., Nature, 2016, 530, 465). The electronic and structural requirements for utilization of the stored oxidizing equivalents in water oxidation are discussed.