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

The quantum yield for light-induced H₂ generation was measured for a previously optimized bio-hybrid cytochrome c ₆-crosslinked PSI_C13G-1,8-octanedithiol-[FeFe]-H₂ase_C97G (PSI-H₂ase) nanoconstruct. The theoretical quantum yield for the PSI-H₂ase nanoconstruct is 0.50 molecules of H₂ per photon absorbed, which equates to a requirement of two photons per H₂ generated. Illumination of the PSI-H₂ase nanoconstruct with visible light between 400 and 700 nm resulted in an average quantum yield of 0.10-0.15 molecules of H₂ per photon absorbed, which equates to a requirement of 6.7-10 photons per H₂ generated. A possible reason for the difference between the theoretical and experimental quantum yield is the occurrence of non-productive PSI_C13G-1,8-octanedithiol-PSI_C13G (PSI-PSI) conjugates, which would absorb light without generating H₂. Assuming the thiol-Fe coupling is equally efficient at producing PSI-PSI conjugates as well as in producing PSI-H₂ase nanoconstructs, the theoretical quantum yield would decrease to 0.167 molecules of H₂ per photon absorbed, which equates to 6 photons per H₂ generated. This value is close to the range of measured values in the current study. A strategy that purifies the PSI-H₂ase nanoconstructs from the unproductive PSI-PSI conjugates or that incorporates different chemistries on the PSI and [FeFe]-H₂ase enzyme sites could potentially allow the PSI-H₂ase nanoconstruct to approach the expected theoretical quantum yield for light-induced H₂ generation.