Poly(alanine-nylon-alanine) as a bioplastic: chemoenzymatic synthesis, thermal properties and biological degradation effects

Abstract

Poly(amino acids) such as polypeptides and proteins are attractive biomass-based polymers that potentially contribute to circular economy for plastic.

Papain-Catalyzed, Sequence-Dependent Polymerization Yields Polypeptides Containing Periodic Histidine Residues

“…Transamidation and hydrolysis were sometimes observed in the chemoenzymatic polymerization of the di-or tripeptides containing Gly or Ala, which have a high affinity to papain. 34,35 Rapid chain propagation and product precipitation were reported to be important in minimizing or avoiding the transamidation reaction in polymerization of AlaGly-OEt. 32 The optimization of polymerization conditions such as the reaction pH, initial concentration of the monomer, and reaction time might suppress the transamidation side reactions to some extent, although complete suppression would be inherently unattainable.…”

“…We also attempted the polymerization of His-containing tripeptides, namely, GlyHisGly derivatives with and without hydrophobic side chains on the imidazole ring. We have previously reported that sandwiching with Gly residues dramatically improved the polymerizability of nonproteinogenic amino acids by mitigating the low affinity for papain. , Similar to previous reports, the His-sandwiching tripeptide monomer GlyHisGly-OEt underwent polymerization to give a precipitate in a much higher yield at 1.0 M than dipeptide monomers (57%, run 7), whereas polymerization of GlyHisGly-OEt did not proceed at 0.10 M (run 8). A 10-fold concentration of papain at 0.10 M compared with that at 1.0 M might promote transamidation and/or hydrolysis of polymerized product in situ because poly­(GlyHisGly) would be easily subjected to transamidation reaction as well as hydrolysis (described in detail later).…”

“…On the other hand, there was no peak attributed to the His or Gly residue-defective polypeptides. Considering the equal composition suggested by 1 H NMR, the side reactions such as transamidation and hydrolysis of the peptide chain scarcely occurred in chemoenzymatic polymerization of HisGly-OEt, although the transamidation products were sometimes observed in chemoenzymatic polymerization of the di- or tripeptides containing Gly or Ala. , In addition, peaks attributed to the dehydration products of poly­(HisGly) were observed, probably due to the formation of diketopiperazine and/or oxazolone at the C-terminus via protonation of the amide group of the His residue by imidazole . The 1 H NMR and MALDI-TOF mass spectra of poly­[His­(Bu)­Gly] are shown in Figure S22.…”

“…Low-affinity natural amino acids for the enzyme, such as proline and valine, can be incorporated into the polypeptide sequence using a tripeptide ethyl ester in which the low-affinity residue is sandwiched between two alanine (Ala) or glycine (Gly) residues as monomers . This technique is also applicable for the incorporation of unnatural amino acids with no specificity for enzymes, affording polypeptides containing α-aminoisobutyric acid (Aib) units, nylon units, or aromatic units, which have a similar structure to engineering plastics. − …”

Synthetic Formatotrophs for One‐Carbon Biorefinery

“…In addition, L-alanine is utilized to synthesize various polymers, such as co-polyamides. [76,77] L-alanine can be produced from pyruvate through a single step reaction by alanine dehydrogenase (Ald; Figure 4). In order to efficiently produce L-alanine using the formatotrophic E. coli strain, the pta, ack, ldhA, and pfl genes need to be deleted to increase the metabolic flux toward L-alanine formation by preventing pyruvate from being converted to various byproducts.…”