(General Student Poster Award Winner, 3rd Place) Synthesis of Nylon Resin Precursor Materials from Biomass-Derived Pyruvate with Photo/Biocatalyst Using Visible Light As an Energy Source
Authors: Yamada Kyosuke , Amao Yutaka 
Published: December 2024

Plastics have become extremely indispensable in modern society due to their high durability and ease of processing. They are made from petroleum and emit carbon dioxide, a greenhouse gas, at the time of manufacture and disposal. Because of their high durability, they are difficult to decompose in the natural environment, and the influence of waste plastics on the environment has become a serious problem. Major example of such environmental issues is the soil and ocean pollution. This pollution caused by discarded plastics has increased the significance for biodegradable plastics, which decomposes into low-molecular compounds such as carbon dioxide and water through natural processes. Therefore, systems synthesizing precursor materials for biodegradable plastics from biomass-derived pyruvate with visible light contained in sunlight, a renewable energy source, have been constructed1. However, previous visible-light driven organic synthesis systems have focused on building C-C bonds and have not been able to synthesize nylon precursor materials requiring C-N bonds. Amino acids, organic nitrogen compounds, can be made into biodegradable nylon through polymerization or copolymerization, as typified by silk materials. Among them, L-alanine has a similar structure to L-lactic acid, which is a monomer of poly-L-lactic acid (PLA), a biodegradable plastic, so L-alanine has the potential to become a biodegradable plastic.

Based on these backgrounds, in this study, we have developed a visible-light driven L-alanine synthesis system from biomass-derived pyruvate (Figure 1) and attempted to improve the efficiency of the system. The system is composed of a visible-light driven NADH regeneration system consisting of an electron donor (TEOA: triethanolamine), a photosensitizer (water-soluble zinc porphyrin) and an electron mediator (Rh complex)2; and an enzyme, L-alanine dehydrogenase (AlaDH), that uses NADH as a coenzyme. Enzymes have the advantage of being environmentally friendly, as they have high reaction selectivity and the reaction proceeds under mild conditions such as room temperature, normal pressure, and near neutrality.

The sample solution (5.0 mL) contained 0.20 M TEOA, 10 μM ZnTPPS: zinc tetraphenylporphyrin tetrasulfonate, 10 μM [Cp*Rh(bpy)(H2O)]2+, 2.0 mM NAD+, 0.10 U AlaDH, 0.50 mM sodium pyruvate and 5.0 mM NH4HCO3 in 500 mM HEPES-NaOH (pH 8.0). The reaction mixture was deaerated by freeze-pump-thaw cycles repeated 6 times, and the gas phase was replaced with argon. Then, the solution was irradiated with 250 W halogen lamp at 30.5 oC. As a control, it was reacted under the dark. The produced L-alanine was qualitatively and quantitatively determined by high-performance liquid chromatograph system. To reduce the amount of expensive coenzyme NAD+ usage, we measured the L-alanine production rate and production concentration at low NAD+ concentrations and investigated the dependence of the reaction rate on pyruvate and ammonia concentrations under these conditions.

Figure 2 shows the time course of L-alanine and pyruvate concentrations. The L-alanine concentration increased linearly with the pyruvate concentration decreasing linearly. This means that pyruvate was converted to L-alanine by this system. The conversion yield for pyruvate to L-alanine was ca. 100 % after 24 hours irradiation. In addition, no increase in L-alanine concentration is observed under dark condition, which means that electrons excited by visible light reduced NAD+ to NADH via the electron mediator, and NADH functioned as a coenzyme for AlaDH. Figure 3 shows the L-alanine production concentration after 24 hours irradiation at each initial NAD+ concentration. Even for reducing the initial NAD+ concentration, the L-alanine production concentration remained unchanged, indicating that the regeneration efficiency of the expensive coenzyme NAD+ was improved.

We have succeeded in synthesizing L-alanine, a precursor material for biodegradable nylon, from biomass-derived pyruvate with visible-light irradiation using photo/biocatalyst and in improving the efficiency of using the expensive coenzyme NAD+. In the future, ammonia will be required to be derived from environmentally friendly precursor materials in contrast to pyruvate derived from biomass.

References

1. M. Takeuchi, Y. Amao, Sustainable Energy Fuels, 2023, 7, 355.

2. S. H. Lee, J. H. Kim, C. B. Park, Chem. Eur. J., 2013, 19, 4392.

Figure 1