Ab Initio modeling of the effect of oxidation coupled with HnO deprotonation on carboxylate ligands in Mn/Ca clusters

On the comparison between differential vibrational spectroscopy spectra and theoretical data in the carboxyl region of photosystem II

Understanding the structural modification experienced by the Mn₄ CaO₅ oxygen-evolving complex of photosystem II along the Kok-Joliot's cycle has been a challenge for both theory and experiments since many decades. In particular, differential infrared spectroscopy was extensively used to probe the surroundings of the reaction center, to catch spectral changes between different S-states along the catalytic cycle. Because of the complexity of the signals, only a limited quantity of identified peaks have been assigned so far, also because of the difficulty of a direct comparison with theoretical calculations. In the present work, we critically reconsider the comparison between differential vibrational spectroscopy and theoretical calculations performed on the structural models of the photosystem II active site and an inorganic structural mimic. Several factors are currently limiting the reliability of a quantitative comparison, such as intrinsic errors associated to theoretical methods, and most of all, the uncertainty attributed to the lack of knowledge about the localization of the underlying structural changes. Critical points in this comparison are extensively discussed. Comparing several computational data of differential S₂ /S₁ infrared spectroscopy, we have identified weak and strong points in their interpretation when compared with experimental spectra.

Vibrational intensities in the mobile block Hessian approximation

Herein we present a proof of concept for the recovery of vibrational intensities from density functional theory vibrational calculations performed in the Mobile Block Hessian (MBH) approximation, which constrains the internal degrees of freedom of designated subsets of a molecule. We compare and contrast the behaviour of this methodology with respect to conventional vibrational calculations, and characterise the performance and accuracy of our method with respect to the size of MBH constrained regions within a variety of species. We demonstrate the viability of this method as a means by which to obtain vibrational intensities for regions of interest within a molecule whilst potentially dramatically reducing computational expense with respect to conventional all-atom vibrational calculations, and discuss caveats for application.

Deprotonation of Water Ligands in V, Cr, Mn, Fe, and Co Complexes Reduces Oxidation-Driven Carboxylate Ligand Frequency Shifts

In Mn complexes, it has been shown that oxidation-driven changes in carboxylate ligand vibrations are suppressed, if a water or hydroxo ligand is simultaneously deprotonated. Deprotonation with oxidation has also been shown to greatly reduce the dependence of Mn complex redox energies on the oxidation state of the metal. We have here investigated the effect of oxidation with deprotonation on the carboxylate ligand frequencies of V, Cr, Mn, Fe, and Co complexes. The effects of anionic ligand substitution (instead of deprotonation) and solvent dielectric were also investigated to determine the mechanism that drives carboxylate frequency shifts. It is shown that the effect of deprotonation was similar for all of the metals tested in this study. C-O bond lengths and O-C-O angle changes in the carboxylate ligand were also reduced by deprotonation. Furthermore, the effect of anionic ligand substitution was similar to deprotonation in the suppression of carboxylate frequency shifts. These shifts were also reduced by increases in the solvent dielectric, in the absence of charge conservation through deprotonation. Therefore, we conclude that carboxylate frequency shifts are largely driven by electrostatic effects.

Deprotonation of Water/Hydroxo Ligands in Clusters Mimicking the Water Oxidizing Complex of PSII and Its Effect on the Vibrational Frequencies of Ligated Carboxylate Groups

The IR absorptions of several first-shell carboxylate ligands of the water oxidizing complex (WOC) have been experimentally shown to be unaffected by oxidation state changes in the WOC during its catalytic cycle. Several model clusters that mimic the Mn4O5Ca core of the WOC in the S1 state, with electronic configurations that correspond to both the so-called "high" and "low" oxidation paradigms, were investigated. Deprotonation at W2, W1, or O3 sites was found to strongly reduce carboxylate ligand frequency shifts on oxidation of the metal cluster. The frequency shifts were smallest in neutrally charged clusters where the initial mean Mn oxidation state was +3, with W2 as an hydroxide and O5 a water. Deprotonation also reduced and balanced the oxidation energy of all clusters in successive oxidations.

Effect of concomitant oxidation and deprotonation of hydrated Mn centres in rationalising the FTIR difference silence of D1-Asp170 in Photosystem II

The observation of negligible FTIR differences in carboxylate vibrational modes for the D1-Asp170 residue of Photosystem II (PSII) on successive one-electron oxidations of the Mn4CaO5 oxygen-evolving complex (OEC) is counterintuitive in light of the apparent ligation of D1-Asp170 to an oxidisable Mn ion in the X-ray crystallographic structures of PSII. Here, we show computational support for the hypothesis that suppression of the FTIR difference spectrum in the 1100cm(-1) to 1700cm(-1) region of D1-Asp170 occurs by concomitant Mn oxidation and deprotonation of water ligands bound to the ligated metal centre. Density functional theory calculations on the model species [Mn(II)Ca(COOH)(OH)2(H2O)2](+) over two successive oxidations of the Mn ion are performed, where those oxidations are accompanied by deprotonation of water and μ-hydroxo ligands coordinated to the Mn ion. In contrast, dramatically increased FTIR difference activity is observed where these oxidations are unaccompanied by proton loss.