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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - MatEnSAP (Semi-Artificial Photosynthesis with Wired Enzymes)

Teaser

Summary

The motivation of this research is to develop new toolsets for driving energy demanding (endergonic) reactions via a new interdisciplinary approach: semi-artificial photosynthesis. Focus would be placed firstly to study solar-to-fuel conversion reactions since they form the basis of renewable fuel generation, which is urgently needed to reduce greenhouse gas emissions and provide a practical route of storing solar energy.

Currently, solar fuel conversion is studied via â€˜artificial photosynthesisâ€™, which utilises synthetic, often biomimetic, components to convert and store solar energy, but is often constrained by inefficient catalysis as well as costly and toxic materials. Nature, on the other hand has already produced many bio-catalysts that can efficiently and selectively perform thermodynamically and kinetically demanding multi-electron transformations, which drive life-sustaining reactions such as photosynthesis. Semi-artificial photosynthesis bridges the rapidly developing fields of synthetic biology and artificial photosynthesis, and is an unexplored platform for understanding solar fuel generation.

The objectives of this research are to: 1) develop the toolbox (in the form of for example: electrodes, redox connections, and complementary photosensitisers) needed for bridging artificial and biological photosynthesis; 2) modify and develop techniques that can be used in a complementary manner to provide a holistic picture of the biotic-abiotic interface, and thus aid rational design in semi-artificial photosynthesis; and 3) produce novel and efficient proof-of-concept solar fuel generation pathways that cannot be accessed via artificial or natural photosynthesis alone.

Work performed

Aim 1 aims to establish the toolbox needed to interface the biotic with the abiotic components:

i) We expanded current state-of-the-art indium tin oxide electrode design to allow for the incorporation of larger/more complex biocatalysts (cyanobacteria) [Zhang et al. (2018), J. Am. Chem. Soc., 140, 6-9].

ii) We integrated a state-of-the-art photosensitiser, p-Si, with a titanium dioxide electrode scaffold that can host enzymes such as hydrogenase. We showed that the integrated photoelectrode can transfer the photoexcited electrons to the hydrogenase, resulting in sustained light-driven hydrogen production [Leung et al. (2017), Chem. Sci., 8, 5172-5180; Nam et al (2018), Angew. Chem. Int. Ed., 57, 10595-10599].

iii) We developed new inverse opal graphene-based electrodes and performed a full comparison study with the indium tin oxide analogous electrodes [Fang et al. (2019), Nano Lett., 19, 1844-1850]. We also explored porous titanium dioxide electrodes [Miller et al. (2019), Angew. Chem. Int. Ed., 58, 4601-1605].

iv) A review has been published [Kornienko et al. (2018), Nature Nanotechn., 13, 890-899].

Aim 2 develops new approaches to understand the biotic-abiotic interface:

i) A Mo-containing formate dehydrogenase, which can be used to selectively catalyse CO2 reduction to formate, was characterised using a combination of inhibition studies, electrochemical output, and modelling [Robinson et al. (2017), J. Am. Chem. Soc., 139, 29, 9927-9936]. This adds new mechanistic understanding of the CO2 reduction enzyme and a fruther in-depth mechanistic investigation is currently in progress.

ii) The discovery that a protein conduit, MtrC, is a high performance H2O2 reductant was made when developing Raman spectroscopy as a means to monitor protein electrochemistry in vitro [Reuillard et al. (2017), J. Am. Chem. Soc., 139, 9, 3324-3327]. This work established Raman spectroscopy as a powerful technique for studying protein films in situ, and will be employed to characterise subsequent systems.

iii) We employed the rotating ring disk electrode technique as a means to study photo-induced charge conversion events by protein-films and reported the characterisation of additional energy transfer pathways in photosystem II in the production of H2O2 [Kornienko et al. (2018), J. Am. Chem. Soc., 140, 17923-17931].

iv) Comprehensive work to systematically study how electrode morphology and surface chemistry influence photosystem II loading and photoactivity, employing a range of techniques including fluorescence microscopy, photoelectrochemistry and ATR-IR, in line with aim 2, has been reported [Fang et al. (2019), Nano Lett., 19, 1844-1850].

v) Reversible electrochemistry for a formate dehydrogenase on a metal oxide electrode has been demonstrated and the enzyme-electrode interface be characterised by QCM and ATR-IR spectroscopy [Miller et al. (2019), Angew. Chem. Int. Ed., 58, 4601-1605].

vi) We have contributed to the development of protein film electrochemical EPR spectroscopy [Abdiaziz et al. (2019), 55, 8840-8843]

vii) A review has been published [Kornienko et al. (2019), Acc. Chem. Res., 52, 1439-1448].

Aim 3 establishes novel proof-of-concept solar fuel generation pathways via semi-artificial photosynthesis:

i) We accomplished the wiring of enzymes for solar-driven overall water splitting, first by using a small bias [Dong et al. (2018), Angew. Chem. Int. Ed., 57, 10595-10599] and without external bias [Sokol et al. (2018), Nature Energy, 3, 944-951].

ii) Solar-driven CO2 reduction to water oxidation has been achieved by wiring Photosystem II to formate dehydrogenase [Sokol et al. (2018), J. Am. Chem. Soc., 140, 16418-16422].

iii) Reversible interconversion of formate into H2/CO2 has been accomplished with a pair of wired redox enzymes (Sokol et al (2019), J. Am. Chem. Soc., 141, 17498-17502]

Final results

Progress has been made with respect to all three goals. As for the combination of biological catalysts with synthetic materials (aim 1), we have already integrated isolated enzymes with semiconductors, conducting metal oxides, 3D electrode architectures, polymers and synthetic dyes as proposed. We are currently exploring the possibility to use advanced light absorbers such as perovskites and combine live (non-photosynthetic) bacteria with metal oxide electrodes for electrogenesis and organic synthesis. We believe that this to be important as it may increase the longevity and allow for complex organic synthesis in our solar fuels systems.

As for the development of techniques to investigate the enzyme-electrode interface (aim 2), we have already successfully demonstrated the usefulness of advanced electrochemistry (e.g., rotating ring disk electrochemistry; photoelectrochemistry), advanced vibrational spectroscopy (IR and Resonance Raman spectroscopy) and quartz crystal microbalance with dissipation. We have also explored AFM-SECM, but the use of this technique for porous electrodes is not possible and single-enzyme analysis is extremely challenging even using state of the art instrumentation. Nevertheless, we will further attempt to use AFM-SECM in collaboration. In addition, we have also explored other techniques that have not been listed in the proposal such as protein film electron paramagnetic resonance spectroscopy. Transient absorption spectroscopy is also being pursued as a new means to gain insights into the electron transfer dynamics for interfaced enzymes with semiconductors in collaboration.

Following the demonstration of bias-free solar H2 synthesis using semi-artificial photosynthesis (aim 3), the focus will be on the development of bias-free CO2 reduction to formate and more complex products using enzyme cascades or live cells.