Quantum yield measurements of light-induced H2 generation in a photosystem I-[FeFe]-H2ase nanoconstruct

The quantum yield for light-induced H-2 generation was measured for a previously optimized bio-hybrid cytochrome c (6)-crosslinked PSIC13G-1,8-octanedithiol-[FeFe]-H(2)ase(C97G) (PSI-H(2)ase) nanoconstruct. The theoretical quantum yield for the PSI-H(2)ase nanoconstruct is 0.50 molecules of H-2 per photon absorbed, which equates to a requirement of two photons per H-2 generated. Illumination of the PSI-H(2)ase nanoconstruct with visible light between 400 and 700 nm resulted in an average quantum yield of 0.10-0.15 molecules of H-2 per photon absorbed, which equates to a requirement of 6.7-10 photons per H-2 generated. A possible reason for the difference between the theoretical and experimental quantum yield is the occurrence of non-productive PSIC13G-1,8-octanedithiol-PSIC13G (PSI-PSI) conjugates, which would absorb light without generating H-2. Assuming the thiol-Fe coupling is equally efficient at producing PSI-PSI conjugates as well as in producing PSI-H(2)ase nanoconstructs, the theoretical quantum yield would decrease to 0.167 molecules of H-2 per photon absorbed, which equates to 6 photons per H-2 generated. This value is close to the range of measured values in the current study. A strategy that purifies the PSI-H(2)ase nanoconstructs from the unproductive PSI-PSI conjugates or that incorporates different chemistries on the PSI and [FeFe]-H(2)ase enzyme sites could potentially allow the PSI-H(2)ase nanoconstruct to approach the expected theoretical quantum yield for light-induced H-2 generation.