Recently, the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences (hereinafter referred to as "Dalian Institute of Chemical Physics"), led by Academician Li Can and Researcher Ding Chunmei's team, has achieved the efficient regeneration of bio-inspired NAD(P)H coenzyme through solar photocatalytic catalysis.

Among them, the selectivity of 1,4-NAD(P)H reached 99%, and the conversion rate was close to 100%, which can lay a certain foundation for constructing an efficient artificial photosynthesis system.

The relevant paper was published in JACS with the title "A Coupled System of Ni3S2 and Rh Complex with Biomimetic Function for Electrocatalytic 1,4-NAD(P)H Regeneration."

Advertisement

Ph.D. student Tian Shujie from the Dalian Institute of Chemical Physics is the first author, and Academician Li Can and Researcher Ding Chunmei from the Dalian Institute of Chemical Physics serve as co-corresponding authors.

It is reported that the reduced coenzyme I or II (referred to as NAD(P)H) is an important charge transfer medium and energy carrier, which is very expensive.After releasing electrons and protons, the NAD(P)H coenzyme turns into its oxidized form, making it difficult to use it in a stoichiometric manner. Therefore, when using the NAD(P)H coenzyme, a recycling regeneration method must be adopted.

In this study, the regenerated NAD(P)H can be used for hydrogenation reduction reactions such as enzyme-catalyzed glutamate synthesis. Previously, this research group also found that the electrocatalytically regenerated reduced coenzyme I can be directly used for imine hydrogenation reactions catalyzed by artificial catalysts [1].

Therefore, with the further development of the photoelectrocatalysis field, this achievement is expected to be used in synthetic biology enzyme-catalyzed reactions and the construction of solar artificial photosynthesis systems, thereby enabling the synthesis of high-value compounds such as amino acids and sugars through solar energy.

Developing efficient electrocatalytic NAD(P)H regeneration catalysts

It is understood that solar artificial photosynthesis is an important path to achieve the "dual carbon" goals and realize the transformation of new energy. The international community also attaches great importance to this, and countries and regions such as the United States, Japan, and the European Union have successively launched a series of major strategic plans.So-called artificial photosynthesis refers to the process of utilizing solar energy to convert water and carbon dioxide into hydrogen, methanol, and other high-value products. These high-value products are collectively referred to as "solar fuels."

Under the aforementioned background, Li Can's team began researching artificial photosynthesis powered by solar energy quite some time ago.

Starting in 2001, the research group successively laid out academic studies in the field of photocatalytic and photoelectrocatalytic artificial photosynthesis, and developed related advanced spectroscopic characterization techniques.

On this basis, a thousand-ton demonstration of liquid solar fuel was established, starting from water and carbon dioxide, achieving the large-scale artificial photosynthesis preparation of green hydrogen and methanol.

Despite significant progress in the field of artificial photosynthesis, people can still only obtain small molecule fuels, and it is difficult to generate more complex high-value compounds.Natural photosynthesis is capable of starting with carbon dioxide and water to synthesize carbohydrates such as glucose. For natural photosynthesis, it consists of two major parts: the light reactions and the dark reactions.

In the light reactions, after oxidizing water, electrons and protons are released and stored in the reducing power of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP).

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an important charge transfer medium and energy carrier, which can be used for enzyme-catalyzed Calvin cycle dark reactions.

Adhering to the principle of following nature, Li Can's team coupled artificial light reactions with bionic dark reactions to synthesize complex high-value products.

For artificial light reactions, they can capture solar energy and store it in high-energy reducing power, and then use synthetic biology enzymes to catalyze dark reactions to synthesize complex high-value products.In the process, the regeneration of NAD(P)H coenzyme is the most crucial link, serving as the bridge that connects the light reactions and the dark reactions.

In nature, 90% of redox enzymes, as well as about 400 types of dehydrogenase catalyzed reactions, are indispensable with the participation of NAD(P)H/NAD(P)+ coenzymes.

How to achieve an efficient NAD(P)H regeneration cycle through artificial catalysis has always been one of the challenges in the field.

Reduced coenzyme I and reduced coenzyme II are both very complex molecules, and the regeneration of NAD(P)H requires the transfer of 2 electrons and 1 proton, which is prone to generating multiple by-products.

For the 1,4-reduced coenzyme II in biological systems, it is catalyzed by the flavin adenine dinucleotide molecule in ferredoxin-NADP reductase (referred to as FNR enzyme).Ferredoxin transfers electrons to flavin adenine dinucleotide, reducing it, and then 100% spatial selectivity can be achieved through hydrogen negative transfer.

Previously, the biggest challenge in artificial catalytic regeneration of reduced coenzymes was that the selectivity and activity of bioactive 1,4-NAD(P)H products were relatively low.

For the direct electrocatalytic reduction of NAD(P)+ to 1,4-NAD(P)H developed by the academic community before, its selectivity was usually below 70%, making it difficult for NAD(P)H to achieve both high activity and high selectivity in regeneration.

Therefore, the research group will develop efficient electrocatalytic NAD(P)H regeneration catalysts as the main goal of a series of recent studies.Selective yield reaches 99.1%, and the conversion rate is close to 100%.

Previously, in similar experiments, rhodium molecules were generally used as mediators. However, this team hopes to avoid the use of rhodium molecules and directly carry out photoelectrocatalytic or electrocatalytic regeneration of NAD(P)H coenzymes to prevent the impact of rhodium molecules on subsequent reactions.

In the study, they conducted research on the electrocatalytic regeneration of reduced coenzyme I coenzymes by using various metal catalysts and carbon materials.

However, they found that the products produced were very complex, including 1,4-reduced coenzyme I, 1,6-reduced coenzyme I, adenosine diphosphate-ribose, and other products.

The selectivity of the biologically active 1,4-reduced coenzyme I was only about 60%, and the research group did not observe the NAD2 by-products reported in previous literature.After further research, they found that for the reduction of NAD+ on the copper surface, it tends to adsorb a hydrogen-coupled electron transfer mechanism [2]. Through this, they discovered the importance of hydrogen adsorption on the electrode surface for the regeneration reaction of reduced coenzyme I.

Furthermore, to address the selectivity issue of the NAD(P)H regeneration reaction, the team conducted further research. The results showed that in biological systems, many hydrogenation and dehydrogenation enzymes have catalytic centers containing transition metal sulfide centers.

Inspired by this, they developed a series of metal sulfide catalysts, including one named CoMo2.75Sx.

Under conditions without the need for additional rhodium molecules as mediators, the selectivity of the direct electrocatalytic regeneration of 1,4-NAD(P)H by the CoMo2.75Sx catalyst can reach 91% [3].

The research group also found that the above process has a similar hydrogen transfer process to the biological system, which indicates that it is in line with the mechanism of enzyme-catalyzed regeneration of reduced coenzyme II in nature.However, the activity of the aforementioned process still needs to be improved. It remains a challenge to achieve the artificial catalysis of NAD(P)+ reduction to produce 1,4-NAD(P)H with both high activity and selectivity.

Previously, it has been reported in the literature that the precious metal rhodium complex [Cp*Rh(bpy)(H2O)]Cl2 can regenerate the artificial catalytic 1,4-reduction type coenzyme I with high regional selectivity. In this process, the rhodium-hydrogen is an active intermediate, and the formation of the rhodium-hydrogen species has a very slow kinetics, which leads to low activity.

At this time, the team noticed that metal sulfides with appropriate water/proton reduction performance and hydrogen adsorption performance can generate active hydrogen through electrochemical pathways.

The research group speculated that these active hydrogens might promote the rhodium molecular hydrogenation reaction to produce rhodium-hydrogen intermediates and the reduction process of NAD+.

Based on this speculation, the team developed different nickel sulfide (Ni3S2 and NiS2) catalysts and used them for the electrocatalytic NAD+ reduction reaction.At the same time, they have also enhanced the activity and selectivity of NAD(P)H regeneration by coupling the advantages of metal sulfide and organometallic complex catalysts.

Additionally, the research group also found that for the standalone nickel sulfide catalyst, it is actually a new type of NAD(P)H regeneration catalyst.

However, when it comes to the regeneration of NAD(P)H coenzyme through direct electrocatalysis, the selectivity of 1,4-NAD(P)H is only about 80%.

Although the use of rhodium molecular catalysis can indeed selectively produce 1,4-NAD(P)H, the activity that can be achieved is very low.

Interestingly, the coupling system of the Ni3S2 catalyst and rhodium molecules can simultaneously achieve the reduction of NAD(P)+ to 1,4-NAD(P)H with high efficiency and selectivity.The experimental data show that the selectivity of the above process reaches 99.1%, and the conversion rate is close to 100%.

After "normalization" treatment, the NAD+ reduction activity of the Ni3S2-rhodium system is 5.8 times and 13.2 times that of the Ni3S2 system and the rhodium molecular system, respectively, mainly due to the synergistic effect between Ni3S2 and rhodium molecules.

Moreover, other various transition metal sulfides can also produce a synergistic effect with rhodium molecules.

The team stated: In the Ni3S2-rhodium system, as a medium for synergistic electron-proton transfer, Ni3S2 can promote the formation of rhodium-hydrogen active species, which can then promote the occurrence of NAD(P)+ reduction reactions.

The rhodium-hydrogen species can interact with the nicotinamide of NAD(P)+, which can transfer hydrogen negatively in a directional manner, thereby selectively generating 1,4-NAD(P)H with high selectivity.In general, the coupling system of Ni3S2 and rhodium molecules can simulate the function of biological redox enzymes, providing new ideas for designing efficient and highly selective artificial systems for 1,4-NAD(P)H regeneration.

Additionally, the research group found that other various metal and rhodium molecular catalysts can also exhibit similar synergistic effects.

Therefore, they coupled proton reduction electrocatalysts with highly selective molecular catalysts to create a universal strategy capable of achieving efficient NAD(P)H regeneration [4].

There is hope to construct an efficient artificial photosynthesis system to produce more high-value artificial photosynthesis products.

In summary, the team has completed relevant research on the synergistic effects of sulfide electrocatalysts and rhodium molecular catalysts on electrocatalytic NAD(P)+ reduction reactions.Through this, they revealed a biomimetic mechanism for the formation of rhodium-hydrogen active species promoted by a concerted electron-proton transfer process, achieving the regeneration of 1,4-NAD(P)H with high activity and high selectivity.

However, the level of electrocatalytic NAD(P)H regeneration activity still needs to be improved. Currently, the current density of product formation is about 1 mA/cm², and the concentration of the produced NAD(P)H is approximately 1 mM.

The NAD(P)H regeneration reaction is a reduction hydrogenation process, which is accompanied by a strong hydrogen production competitive reaction.

In the aforementioned studies, when electrocatalytically reducing NAD(P)+ with Ni3S2 alone, the Faradaic efficiency of the NAD(P)H product is generally low, at around 10%, with serious hydrogen production side reactions occurring.

Within the Ni3S2-rhodium coupled system, the Faradaic efficiency of the NAD(P)H product can reach 56%, indicating that the hydrogen production competitive reaction has been significantly suppressed.Even so, the Faraday efficiency still needs to be further improved. To achieve this goal, the top priority is to design electrode materials with appropriate hydrogen adsorption properties and to effectively use active hydrogen for NAD(P)+ reduction reactions.

Therefore, the team will continue to adhere to the principle of following the natural way, carry out bionic design from multiple aspects such as structure and function, and strive to enhance the regeneration performance of NAD(P)H.

In addition, in the aforementioned research, they used the enzyme-catalyzed glutamate synthesis reaction as a probe reaction to detect the selectivity of 1,4-NAD(P)H.

At the same time, the research group has demonstrated that the electrocatalytically regenerated reduced coenzyme I can be effectively used for the imine hydrogenation reduction reaction catalyzed by artificial catalysts[5], laying the foundation for the future transformation of carbon dioxide and synthesis of amino acids through the coupling of enzyme-catalyzed dark reactions.

If the results of this study are used for the photoelectrocatalytic regeneration of NAD(P)H and coupled with various downstream enzyme-catalyzed reactions, it is expected to build an efficient artificial photosynthesis system to produce more high-value artificial photosynthesis products.