Contacts:

Photosynthesis one electron at a time

One effort focuses on the understanding of the efficient energy and electron transfer processes that constitute the primary events of bacterial and plant photosynthesis. New femtosecond nonlinear spectroscopies based on phase control of optical pulses and photon echo techniques are being developed along with theories describing the methods. These techniques are employed to gain a quantitative understanding of the mechanism by which the components of photosynthetic pigment-protein complexes interact with each other and their environments. Experiments probe the extent of electronic mixing between coupled chromophores of the light harvesting complex and the role of energetic and coupling disorder. The spectroscopic work is complemented by electronic structure calculations, and by theoretical modeling. In parallel studies, the mechanisms of the control of energy migration in Photosystem II are investigated. In an effort to identify and dissect the processes by which photosynthetic organisms cope with excessive light, focus is on nonphotochemical quenching which protects the organism from photooxidative damage. The genetic basis for nonphotochemical quenching is being explored and the relationship established between the quenching activity and the amount of protein encoded by a key regulatory gene. Femtosecond spectroscopy is used to probe the energy transfer in mutated photosynthetic organisms which show enhanced or diminished nonphotochemical quenching.

Understanding Photosystem II

Mechanistic studies on the light-induced water oxidation to oxygen (the key catalytic function of Photosystem II of green plants and cyanobacteria) focus on the elucidation of the intermediate states of the manganese complex as it advances through the four-electron oxidation cycle. Synchrotron-based EXAFS spectroscopy and newly developed X-ray methods, electron paramagnetic resonance, and FT-IR spectroscopy are employed to determine the oxidation states and characterize the structural changes of the oxo-bridged Mn4CaClx cluster during water oxidation. The main goals are to find out how water is incorporated into the catalytic site, and when and how the OO bond is formed. The wealth of structures of Mn clusters with oxygen bridges revealed by layered Mn oxide minerals, combined with constraints from EXAFS measurements are opening up a range of possibilities for the structure of the Photosystem II water oxidation complex. Exploration of solar light-driven synthesis of fuels in engineered systems focuses on photochemistry in nanoporous solids featuring bimetallic chromophore and redox sites. Binuclear units with visible light-absorbing charge-transfer states covalently anchored on the pore surface are probed for photoreactivity toward CO2 and H2O, with the gas-pore interface playing a critical role in the separation of primary redox products. We employ time-resolved FT-IR spectroscopy for the detection of transient intermediates, and elucidate the detailed structure of the bimetallic sites by optical, FT-IR, FT-Raman and XANES spectroscopy. The mechanistic understanding gained from the time-resolved studies assists in the design of bimetallic centers for CO2 reduction under visible light.