Understanding Carbon-neutral Hydrogen Production by Nature’s Hydrogen Evolution Catalyst

Project Information

Nat-HEC

Grant agreement ID: 897555

DOI

10.3030/897555

Project closed

EC signature date 25 March 2020

Start date 1 April 2020

End date 31 March 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 191 852,16

EU contribution

€ 191 852,16

Coordinated by

UPPSALA UNIVERSITET
Sweden

Periodic Reporting for period 1 - Nat-HEC (Understanding Carbon-neutral Hydrogen Production by Nature’s Hydrogen Evolution Catalyst)

Reporting period: 2020-04-01 to 2022-03-31

Summary of the context and overall objectives of the project

Climate change is one of the biggest challenges faced by us as well as by future generations. Converting solar energy into chemical energy may be a promising perspective to reduce CO2 emissions. Here, energy is catalytically stored in chemical bonds. Following this principle, alternative fuels and chemical raw materials could replace fossil sources. However, catalysts composed of earth abundant elements that work at low over potential are necessary to accomplish this global challenge. Redox enzymes meet these requirements and catalyse in multi-electron reactions e.g. the fixation of nitrogen (N2, nitrogenase), carbon dioxide (CO- and formate-dehydrogenase), or the evolution of molecular hydrogen (H2, hydrogenases). Such enzymes serve as inspiring blueprints for the design of high-efficiency catalysts. The high energy density of H2 offers multiple possibilities for utilization as a carbon neutral fuel and chemical raw material. However, today about 95% of the industrially utilized H2 originates from steam reformation of fossil resources and is thus associated with significant CO2 release. This illustrates the need for alternative catalysts for hydrogen generation. Nature’s Hydrogen Evolution Catalyst, the redox enzyme [FeFe] hydrogenase, is found in both bacteria and algae. They catalyses the release of H2 with high rates, at neutral pH, and low overpotentials. [FeFe] hydrogenases inspired the design of numerous synthetic compounds mimicking its unique iron sulphur site, however never meeting the efficiency of the native system.

The overall aim of the project is to elucidate the reaction mechanism of Nature’s Hydrogen Evolution Catalyst (Nat-HEC), the redox enzyme [FeFe] hydrogenase, to inspire the design of synthetic catalysts for the generation of molecular hydrogen (H2).

Specific objectives are:
Objective 1: The action aims to elucidate the catalytic mechanism of [FeFe] hydrogenases by transient absorption spectroscopy.
Objective 2: In transient absorption spectroscopy experiments, ensemble samples have to be measured. Ideally, the reaction starts in all molecules of the ensemble at the same time from the same starting state. To approach this scenario, a homogenous starting state will be adjusted via a newly developed transmission cell.
Objective 3: The reduction effectivity of photoinduced electron transfer will be optimized to enhance the signal to noise ratio, e.g. chemically altering or co-immobilizing photosensitizers and enzyme.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

It is assumed that all [FeFe]-hydrogenases share a common reaction mechanism (catalytic cycle). To elucidate this common catalytic mechanism, we investigated [FeFe]-hydrogenases from two different groups, the group A enzyme CrHydA1 and group D enzyme TamHydS. While group A is heavily investigated, group D hydrogenases are barley characterised.
A major difference between group A and D represents the proton transfer mechanism crucial for H2 catalysis. While group A enzymes share a conserved proton transfer pathway, this pathway is absent in group D. We identified and characterised an alternative proton transfer pathway, its impact on the redox states adopted and on the oxygen tolerance of the enzyme. In amino acid variants of TamHydS we could confirm the reactivity of the active site in an apparent oxygen tolerant configuration by photoreduction. Oxygen tolerance represents the ultimate challenge in the application of [FeFe]-hydrogenases for sustainable catalysis applications.
The newly discovered proton transfer pathway in group D hydrogenase leads from the opposite direction to the active site, in fact the site where a µH would be located. Additionally, TamHydS has an unusual preference to accumulate the diiron site reduced state, the state µH was proposed for. However, spectroscopically we could neither confirm nor exclude the µH/tH geometry in TamHydS. In contrast, in group A hydrogenase (CrHydA1) we could show that a µH geometry can be excluded for the diiron site reduced state, an observation that was only made at non-ambient conditions (cryogenic temperatures) before. These findings strongly support the notion that the catalytic cycle is composed of conserved active site geometry states only.
Regarding the involvement of the tH geometry in catalysis we could show that the terminal hydride redox state (Hhyd) is not an artefact to non-native reductant omnipresent in samples of earlier studies, supporting its catalytic relevance.
We developed a whole cell spectroscopy approach where we were able to characterise an additionally protonated version of the hydride state inside E. coli cells that is proposed to be the last intermediate before H2 release.
Involved in a more applied H2 production approaches we investigated the photodamage on CrHydA1 in the framework of a semi-artificial photosynthesis concept that involved light harvesting via photo-polymers (as photosensitisers) in combination with [FeFe]-hydrogenase as bio-catalysts.
Moving even deeper in applied bio-catalysis we moved the photoreduction approach into living E. coli cells. These cells containing Eosin Y, sacrificial reductant and CrHydA1 were producing H2 upon illumination. We were able to analyse the redox state composition adopted by the [FeFe]-hydrogenase inside the living cells and draw conclusions on the catalytic mechanism in a more native environment.

Results were published open access and disseminated on international conferences and the European Researchers Night 2021.
The project addresses issues related to climate change and the environment, and thereby benefits for society as well as it contributes towards European policy objectives and strategies to be climate neutral in 2050. Potential users of the project are primarily found in the research field of synthetic catalyst design, whole cell catalysis and artificial catalysis.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The main technological achievement and innovation of the experimental WPs is the establishment of photoreduction as a tool for the investigation of redox enzymes as an alternative to spectro-electrochemical investigation and addition to the aerosol technique. The innovative approach allowed to investigate a new group of [FeFe]-hydrogenases and relate to a universal catalytic cycle of bio H2 production in nature, extending the state of the art in this field. Intermediates, possible intermediates and new intermediates of the catalytic cycle could be further characterised and contributed to a better understanding of the hydrogen chemistry in [FeFe]-hydrogenases. The results were of high scientific quality and were/are/will be published in top scientific journals. The project addresses issues related to climate change and the environment, and thereby benefits for society as well as it contributes towards European policy objectives and strategies to be climate neutral in 2050.

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