Electron Transfer Kinetics in CdS Nanorod–[FeFe]-Hydrogenase Complexes and Implications for Photochemical H₂ Generation

Abstract

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H2 photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H2 generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H2 production. We found that the ET rate constant (k(ET)) and the electron relaxation rate constant in CdS NRs (k(CdS)) were comparable, with values of 10(7) s(-1), resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H2 production could be achieved by increasing k(ET) and/or decreasing k(CdS) through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS-CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron-sulfur cluster and is followed by transport through a series of accessory iron-sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H2 generation in CdS-CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H2 production efficiency.

Hydrogen production at high Faradaic efficiency by a bio-electrode based on TiO2 adsorption of a new [FeFe]-hydrogenase from Clostridium perfringens

“…Other very interesting semiconductors, which similarly have been coupled to hydrogenases for electro-and photo-production of hydrogen [8,10], although their higher 25 turnover frequencies, suffer from some drawbacks given by high toxicity. This has to be taken into account when comparing to TiO2 based systems: they might counterbalance a lower turnover number with a higher long-term sustainability.…”

“…Blue line represents the TiO2/CpHydA bioelectrode; grey line represents a control of the bare TiO2 electrode. 10 The current traces showed a current drop that is typical of immobilized redox enzymes and that has been attributed to both inactivation and desorption [9,19,35,36] Nevertheless, in this case the current drop after 30 minutes is much smaller when compared to another [FeFe]-hydrogenase adsorbed on TiO2 [9]; moreover, when the current drop is compared on the time-scale of hours, it is similar to that observed with [FeFe]-hydrogenases covalently immobilized on carbon electrodes [19]. 15 Gas chromatographic analysis of the gas phase showed the presence of H2 only when CpHydA was immobilized.…”

“…The turnover number of 4 s -1 was calculated on a total protein loading of 2 g (20 l of a 0.1 mg/ml protein solution loaded 10 onto the electrode, which means 30 pmol), i.e. a total biocatalyst amount overestimated by two order of magnitude.…”

“…5 To this end both [NiFe]-and [FeFe]-hydrogenases (H2ases) have been used on a wide variety of solid electrodes, such as pyrolytic graphite edge (PGE), glassy carbon and carbon felt electrodes, gold electrodes, TiO2 electrodes but also carbon nanotubes and nanowires, CdTe nanocrystals and CdS nanorods. Electrochemical, photoluminescence and Raman spectroscopy studies demonstrate that the hydrogenases remain catalytically active and therefore can be exploited as efficient catalysts [5][6][7][8][9][10]. Several electrochemical setups have been proposed exploiting the catalytic properties of both 10 [NiFe]-and [FeFe]-hydrogenases from various bacterial and some algal species [6,[11][12].…”

“…The Faradaic efficiency was calculated as the ratio between the moles of H2 produced by the electrode and the equivalent 10 of electrons passed through the electrode. The equivalents of electrons were measured by the mathematical integration of the current traces obtained during chronoamperometry experiments.…”