Research progress of additives in photobiological hydrogen production system to enhance biohydrogen

Photosynthetic biohydrogen has the advantages of extensive raw materials, clean and renewable, etc. But, its low substrate utilization rate limit its commercial application. It is reported that the use of additives in the process of biohydrogen by photofermentation is beneficial to increase biohydrogen. However, in practical application, the mechanism of additives in hydrogen production is not understood. This paper, the promotion effect of some additives on biohydrogen by photofermentation was reviewed. Whatever, the existing problems and development trends of various additives are also discussed. It is necessary to select appropriate additives according to the hydrogen-producing characteristics. The use of composite additives may further enhance biohydrogen, but the specific situation needs further exploration. The research results of this paper can help readers to further understand the role of additives in the crouse of photofermentative biohydrogen, provide reference for the research of photofermentative biohydrogen.

Fast Proton Transport in FeFe Hydrogenase via a Flexible Channel and a Proton Hole Mechanism

Di-iron hydrogenases are a class of enzymes that are capable of reducing protons to form molecular hydrogen with high efficiency. In addition to the catalytic site, these enzymes have evolved dedicated pathways to transport protons and electrons to the reaction center. Here, we present a detailed study of the most likely proton transfer pathway in such an enzyme using QM/MM molecular dynamics simulations. The protons are transported through a channel lined out from the protein exterior to the di-iron active site, by a series of hydrogen-bonded, weakly acidic or basic, amino acids and two incorporated water molecules. The channel shows remarkable flexibility, which is an essential feature to quickly reset the hydrogen-bond direction in the channel after each proton passing. Proton transport takes place via a “hole” mechanism, rather than an excess proton mechanism, the free energy landscape of which is remarkably flat, with a highest transition state barrier of only 5 kcal/mol. These results confirm our previous assumptions that proton transport is not rate limiting in the H₂ formation activity and that cysteine C299 may be considered protonated at physiological pH conditions. Detailed understanding of this proton transport may aid in the ongoing attempts to design artificial biomimetic hydrogenases for hydrogen fuel production.

Linear Scaling Relationships to Predict pKa's and Reduction Potentials for Bioinspired Hydrogenase Catalysis

Biomimetic catalysts inspired by the active site of the [FeFe] hydrogenase enzyme can convert protons into molecular hydrogen. Minimizing the overpotential of the electrocatalytic process remains a major challenge for practical application of the catalyst. The catalytic cycle of the hydrogen production follows an ECEC mechanism (E represents an electron transfer step, and C refers to a chemical step), in which the electron and proton transfer steps can be either sequential or coupled (PCET). In this study, we have calculated the pKa's and the reduction potentials for a series of commonly used ligands (80 different complexes) using density functional theory. We establish that the required acid strength for protonation at the Fe-Fe site correlates with the standard reduction potential of the di-iron complexes with a linear energy relationship. These linear relationships allow for fast screening of ligands and tuning of the properties of the catalyst. Our study also suggests that bridgehead ligand properties, such as bulkiness and aromaticity, can be exploited to alter or even break the linear scaling relationships.

Unraveling the mechanism of biomimetic hydrogen fuel production – a first principles molecular dynamics study

The Fe₂(bdt)(CO)₆ [bdt = benzenedithiolato] complex, a synthetic mimic of the [FeFe] hydrogenase enzyme can electrochemically convert protons into molecular hydrogen. The free energy landscape reveals a different mechanism for the biomimetic cycle.