Single-walled carbon nanotubes (SWNT) are promising candidates for use in energy conversion devices as an active
photo-collecting elements, for dissociation of bound excitons and charge-transfer from photo-excited chromophores, or
as molecular wires to transport charge. Hydrogenases are enzymes that efficiently catalyze the reduction of protons from
a variety of electron donors to produce molecular hydrogen. Hydrogenases together with SWNT suggest a novel biohybrid
material for direct conversion of sunlight into H<sub>2</sub>. Here, we report changes in SWNT optical properties upon
addition of recombinant [FeFe] hydrogenases from <i>Clostridium acetobutylicum</i> and <i>Chlamydomonas reinhardtii</i>. We
find evidence that novel and stable charge-transfer complexes are formed under conditions of the hydrogenase catalytic
turnover, providing spectroscopic handles for further study and application of this hybrid system.
The catalysts commonly used for the H<sub>2</sub> producing reaction in artificial solar systems are typically platinum or
particulate platinum composites. Biological catalysts, the hydrogenases, exist in a wide-variety of microbes and are
biosynthesized from abundant, non-precious metals. By virtue of a unique catalytic metallo-cluster that is composed of
iron and sulfur, [FeFe]-hydrogenases are capable of catalyzing H<sub>2</sub> production at turnover rates of millimoles-per-second.
In addition, these biological catalysts possess some of the characteristics that are desired for cost-effective solar H<sub>2</sub>
production systems, high solubilities in aqueous solutions and low activation energies, but are sensitive to CO and O<sub>2</sub>.
We are investigating ways to merge [FeFe]-hydrogenases with a variety of organic materials and nanomaterials for the
fabrication of electrodes and biohybrids as catalysts for use in artificial solar H<sub>2</sub> production systems. These efforts
include designs that allow for the integration of [FeFe]-hydrogenase in dye-solar cells as models to measure solar
conversion and H<sub>2</sub> production efficiencies. In support of a more fundamental understanding of [FeFe]-hydrogenase for
these and other applications the role of protein structure in catalysis is being investigated. Currently there is little known
about the mechanism of how these and other enzymes couple multi-electron transfer to proton reduction. To further the
mechanistic understanding of [FeFe]-hydrogenases, structural models for substrate transfer are being used to create
enzyme variants for biochemical analysis. Here results are presented on investigations of proton-transfer pathways in
[FeFe]-hydrogenase and their interaction with single-walled carbon nanotubes.
Research efforts to develop efficient systems for H<sub>2</sub> production encompass a variety of biological and chemical
approaches. For solar-driven H<sub>2</sub> production we are investigating an approach that integrates biological catalysts, the
[FeFe] hydrogenases, with a photoelectrochemical cell as a novel bio-hybrid system. Structurally the [FeFe]
hydrogenases consist of an iron-sulfur catalytic site that in some instances is electronically wired to accessory iron-sulfur
clusters proposed to function in electron transfer. The inherent structural complexity of most examples of these enzymes
is compensated by characteristics desired for bio-hybrid systems (i.e., low activation energy, high catalytic activity and
solubility) with the benefit of utilizing abundant, less costly non-precious metals. Redesign and modification of [FeFe]
hydrogenases is being undertaken to reduce complexity and to optimize structural properties for various integration
strategies. The least complex examples of [FeFe] hydrogenase are found in the species of photosynthetic green algae and
are being studied as design models for investigating the effects of structural minimization on substrate transfer, catalytic
activity and oxygen sensitivity. Redesigning hydrogenases for effective use in bio-hybrid systems requires a detailed
understanding of the relationship between structure and catalysis. To achieve better mechanistic understanding of [FeFe]
hydrogenases both structural and dynamic models are being used to identify potential substrate transfer mechanisms
which are tested in an experimental system. Here we report on recent progress of our investigations in the areas of
[FeFe] hydrogenase overexpression, minimization and biochemical characterization.