In this study, 2-hydroxy-4-methylthiobutyrate (2H4MTB)-containing polyhydroxyalkanoate (PHA) copolymers were biosynthesized using methionine as the 2H4MTB precursor. 2H4MTB is a novel monomer unit that contains a sulfur atom in its side chain. The 2H4MTB-containing PHA was biosynthesized by functionalizing the leucine degradation and PHA synthesis pathways in recombinant Escherichia coli. The 2H4MTB fraction in the PHA copolymer was increased up to 30.6 mol% by increasing the methionine concentration in the medium. Using purified polymers containing 10.9 mol% 2H4MTB unit, the sulfide group of the 2H4MTB side chain was oxidized with hydrogen peroxide or peracetic acid, which resulted in the conversion of sulfide to sulfoxide and sulfone groups. The oxidized polymer was relatively hydrophilic, as revealed by water contact angle measurements, and swelled slightly when soaked in water. These results suggest that the 2H4MTB unit can be used as an oxidation site to impart hydrophilicity to the PHA copolymers.
Keywords: 2-hydroxyalkanoate; 3-hydroxybutyrate; Methionine; Polyhydroxyalkanoate; Sulfide; Sulfone; Sulfoxide.
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