(5) determining the physical and chemical rules that underlie the biological mechanisms for
repair and photoprotection.
The resolution of structural dynamics across the entire time scale of solar energy conversion is
central to the discovery of fundamental design principles for solar energy conversion. Just as
ground-state optical absorption spectra cannot reveal the complexity of excited-state reaction
dynamics for complex molecular ensembles, static molecular structures cannot reveal the
fundamental mechanisms underlying solar energy conversion. Efficient solar energy conversion
requires the discovery of mechanisms that control both the ground- and excited-state structural
landscapes of complex molecular assemblies — ranging from the attosecond electron dynamics
associated with nascent photon absorption and charge separation, to the minutes-and-longer
control of atomic motions during the catalytic production of solar fuels. Making these
discoveries will require new characterization tools (ultrafast optical, electron paramagnetic
resonance, advanced x-ray, neutron scattering, and imaging) to determine structure/function
relationships in photosynthetic proteins. Integration of experimental measurements of structural
and electronic dynamics with multi-scale theoretical approaches is essential for (1) achieving
fundamental breakthroughs in system design paradigms for solar energy capture and conversion
by supramolecular structures, and (2) mapping out and predicting optimized ground- and
excited-state structural and energy landscapes for efficient solar energy conversion.
Catalytic power and specificity, which are key attributes of enzyme-mediated catalysis, have
their origins in the active environment provided by the protein. The same is true in the primary
solar energy conversion reactions of photosynthesis. The proteins involved in the light-
harvesting complexes and the RCs are not just inert scaffolds. They provide much more than just
a means of optimally positioning the chromophores and the electron transfer cofactors. The
medium provided by the protein actively promotes, enhances, and indeed controls the light-
harvesting and electron-transfer reactions both within the protein and across interfacial protein
boundaries. This is a key feature of the natural system that allows it to operate so efficiently.
Furthermore, proteins can be readily engineered, via genetic modification, to explore a diversity
of structure/function scenarios. Because of major limitations imposed by covalent synthesis of
large assemblies, construction of the next generation of bio-inspired solar-energy conversion
devices will require placing the chromophores into a “smart matrix” to control their key
electronic properties by using weak interactions and self-assembly. Achieving efficient
integrated solar energy conversion systems using smart matrices will require the following:
(1) learning how natural protein matrices control and optimize energy and charge transport both
within a single protein and between proteins; (2) engineering proteins, polymers, membranes,
gels, and other ordered molecules to provide tailored active environments (i.e. smart matrices);
(3) incorporating bio-inspired cofactors within the designed matrix; (4) integrating multiple
cofactor-matrix assemblies to perform the overall function; (5) characterizing the coupling
between the cofactors and the matrix in natural and bio-inspired systems by using advanced
techniques; and (6) developing smart matrices that compartmentalize incompatible products
(e.g., O 2 and H 2 ).