A Hybrid Biological-Organic Photochemical Half-Cell for Generating Dihydrogen
- Pennsylvania State Univ., University Park, PA (United States)
In this final report we describe our work on the fabrication of a hybrid biological/organic photochemical half-cell that couples Photosystem I, which produces electrons at a highly reducing redox potential, with a hydrogenase enzyme, which catalyzes H2 evolution at high rates when provided with a source of electrons. We considered it far too ambitious at the present stage of knowledge to attempt to design a system that carries out the reaction 2H2O + 8hv → 2H2 + O2 in a single photochemical step, hence, we focused our work on optimizing the half-cell reaction 2H+ + 2e- + 2hv → H2. The challenge has been to deliver low-potential electrons from PS I to H2ase rapidly, at high quantum yield, and with high thermodynamic efficiency. To accomplish this, we engineered a molecular wire that connects the [4Fe-4S] clusters of PS I with those of the hydrogenase enzyme. In doing so, the highly reducing electrons generated by PS I are transferred by quantum mechanical tunneling to the enzyme. The core idea was to connect the terminal Fe/S cluster of Photosystem I to the distal Fe/S cluster of H2ase through a covalently bonded “molecular wire” consisting of an 8-carbon hydrocarbon wire terminated on both ends with a sulfhydryl group. By providing a direct electron transfer pathway through signa bonds, diffusion chemistry is avoided, and, because the tether length and redox potentials were properly chosen, a rate of electron transfer between the two enzymes was achieved that twice exceeded the rate of electron transfer in natural photosynthesis. We also devised a way to utilize the FA and FB clusters of Photosystem I to serve as a reservoir so that electrons could be extracted from the quinone binding sites and passed through a similar molecular wire to the bound hydrogenase. The additional induction of the IsiA antenna pigments by iron starvation led to an optical cross section twice that of the naturally-occurring Photosystem I, thus further raising the rates of electron transfer to the enzyme. We were likewise able to re-engineer a naturally-occurring dicluster ferredoxin to replace the hydrocarbon molecular wire as the tether between Photosystem I and the hydrogenase enzyme. Finally, we developed techniques to place Photosystem I on electrodes and in block copolymer interfaces and we were able to measure photocurrents approaching 35.0 ± 3.5 mA cm-2 upon illumination of these assembled devices, which are among the highest reported so far for such systems on a per protein basis.
- Research Organization:
- Pennsylvania State Univ., University Park, PA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- FG02-05ER46222
- OSTI ID:
- 1483277
- Report Number(s):
- DOE-PSU-46222-F
- Country of Publication:
- United States
- Language:
- English
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