Natural and artificial photosynthesis

Photosynthetic organisms capture the energy of sunlight and store it in the chemical bonds of carbohydrates. Photosynthesis, the most important biochemical process on Earth, defines life as we know it by providing the building blocks of most living organisms and producing atmospheric oxygen. The main engine of the photosynthetic apparatus is the enzyme Photosystem II, where solar energy splits water into molecular oxygen, protons and electrons, providing the reducing equivalents subsequently employed in carbon fixation. At the heart of PS-II is the oxygen-evolving complex (OEC), the Mn4O5Ca functional unit that catalyzes the oxidation of water. Elucidating the chemistry that takes place at the OEC is central for efforts toward artificial photosynthesis, potentially enabling sustainable solar-driven hydrogen production from water.

Our research addresses foremost the problem of establishing firm correlations between structural features and spectroscopic properties. Accurate calculations of spectroscopic properties for potential structural models allow meaningful comparisons with data obtained, for example, from EPR/ENDOR and XAS experiments. To this end we develop methods for the calculation of spectroscopic observables and apply them in the study of the properties and reactivity of catalytic models that range in scale from small clusters to enzymes to surfaces, using a whole range of methods from ab initio and DFT to QM/MM and MD. The safe understanding of structure-property relationships forms the basis for deciphering (and subsequently controlling) catalytic reactivity. For these reasons a topic of special focus is to understand in detail how the various levels of description and interpretation correlate and give rise to a certain phenomenology.

Among the theoretical developments achieved so far are methodologies relevant to the calculation of EPR parameters such as hyperfine coupling constants (HFCs) for oligonuclear metal clusters of arbitrary shape and nuclearity. We have employed these methods in evaluations of models of the S2 state of the OEC, considering structural variations such as the bonding and connectivity within the inorganic Mn4O5Ca part of the OEC, the ligation modes of coordinating aminoacids, and the protonation states of Mn- and Ca-coordinated water molecules and oxo bridges. Given the wealth of state-selective information obtained on the OEC by modern spectroscopic methods, this approach has led to the definition of the topology of the OEC with unprecedented accuracy that is not achievable by methods such as protein crystallography.

Another example of how the interplay of computational and experimental spectroscopy can lead to firm conclusions regarding the electronic structure and chemical reactivity of an unknown species is the elucidation of the nature of a Mn complex resulting from activation of water. In that case the reference experimental information was the Mn pre-edge X-ray absorption spectrum (XAS) that contains information about the valence electronic structure of the metal. Calculations of the XAS pre-edge spectra enabled us to distinguish between protonated and unprotonated adducts and electromeric forms such as oxo and oxyl. These calculations are possible thanks to a recently calibrated TD-DFT protocol, while more recent theoretical advances include an RO-CIS approach.

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