Spatio-temporal resolution of primary processes of photosynthesis

Abstract

Technical progress in laser-sources and detectors has allowed the temporal and spatial resolution of chemical reactions down to femtoseconds and Å-units. In photon-excitable systems the key to chemical kinetics, trajectories across the vibrational saddle landscape, are experimentally accessible. Simple and thus well-defined chemical compounds are preferred objects for calibrating new methodologies and carving out paradigms of chemical dynamics, as shown in several contributions to this Faraday Discussion. Aerobic life on earth is powered by solar energy, which is captured by microorganisms and plants. Oxygenic photosynthesis relies on a three billion year old molecular machinery which is as well defined as simpler chemical constructs. It has been analysed to a very high precision. The transfer of excitation between pigments in antennae proteins, of electrons between redox-cofactors in reaction centres, and the oxidation of water by a Mn4Ca-cluster are solid state reactions. ATP, the general energy currency of the cell, is synthesized by a most agile, rotary molecular machine. While the efficiency of photosynthesis competes well with photovoltaics at the time scale of nanoseconds, it is lower by an order of magnitude for crops and again lower for bio-fuels. The enormous energy demand of mankind calls for engineered (bio-mimetic or bio-inspired) solar-electric and solar-fuel devices.

Artificial Mn4Ca Clusters with Exchangeable Solvent Molecules Mimicking the Oxygen‐Evolving Center in Photosynthesis

“…The oxygen-evolving center (OEC) of photosystem II (PSII) is a unique biological catalyst that catalyzes the watersplitting reaction at ambient temperatures and pressures, to release protons, electrons, and dioxygen molecules. [1] Crystallographic studies of PSII [2] have revealed that OEC is composed of one Mn 3 CaO 4 cubane attached by a "dangler" manganese via two bridging oxides, forming an asymmetric Mn 4 CaO 5 cluster (Figure 1 A,B). Its peripheral ligands are provided by four water molecules, one imidazole and six carboxylate groups of amino acid residues in PSII.…”