Biosynthesis and property evaluation of poly(3-hydroxybutyrate-co-2-hydroxyalkanoate) containing 2-hydroxy-3-(4-hydroxyphenyl)propionate unit

Polyhydroxyalkanoate (PHA) biosynthesis has been performed using hydrophobic amino acids such as leucine, phenylalanine, and methionine as 2-hydroxyalkanoate (2HA) precursors, utilizing 2HA monomer-supplying enzymes (LdhA and HadA) derived from Clostridioides difficile. In this study, l-tyrosine was used as a 2HA precursor in the biosynthesis of poly(3-hydroxybutyrate-co-2HA) [P(3HB-co-2HA)] containing 2‑hydroxy-3-(4-hydroxyphenyl)propionate (2H3PhOHP), which shares the same carbon backbone as tyrosine and has hydroxy group in the side chain. The recombinant Escherichia coli DH5α, having the PHA biosynthesis pathway, was cultured by feeding glucose and l-tyrosine. The biosynthesized PHA was characterized by ¹H nuclear magnetic resonance (NMR), which revealed incorporation of the 2H3PhOHP unit into PHA up to 4.1 mol%. Modifying the phenol group of the 2H3PhOHP unit was performed using 2‑chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, and phosphitylation was confirmed by ³¹P NMR analysis. Furthermore, the polymers' thermal and mechanical properties and wettability were investigated. Introducing 2H3PhOHP units into PHA led to a decrease in crystallinity and an increase in glass transition temperature and surface wettability. Thus, the 2H3PhOHP unit is a new type of building block for PHA that changes material properties and provides a hydroxyl group in the side chain as a chemical modification site to further improve the material properties of PHA.