Tacticity Characterization of Biosynthesized Polyhydroxyalkanoates Containing ( S )- and ( R )-3-Hydroxy-2-Methylpropionate Units.

Polyhydroxyalkanoates (PHAs), aliphatic polyesters synthesized by microorganisms, have gained considerable attention as biodegradable plastics. Recently, α-carbon-methylated PHAs have been shown to exhibit several interesting properties that differ from those of conventional PHAs, such as their crystallization behavior and material properties. This study investigated α-carbon methylated ( S )- and ( R )-3-hydroxy-2-methylpropionate (3H2MP) as new repeating units. 3H2MP units were homopolymerized or copolymerized with ( R )-3-hydroxybutyrate (3HB) by manipulating the culture conditions of recombinant Escherichia coli LSBJ. Consequently, PHAs with 3H2MP units ranging from 5 to 100 mol % were synthesized by external addition of ( R )- and ( S )-enantiomers or the racemic form of 3H2MPNa. The ( S )-3H2MP precursor supplemented into the culture medium was almost directly polymerized into PHA while maintaining its chirality. Therefore, a highly isotactic P(3H2MP) ( R : S = 1:99) was synthesized, which displayed a melting temperature of 114-119 °C and a relatively high enthalpy of fusion (68 J/g). In contrast, in cultures supplemented with ( R )-3H2MP, the precursor was racemized and polymerized into PHA, resulting in the synthesis of the amorphous polymer atactic P(3H2MP) ( R : S = 40:60). However, racemization was not observed at a low concentration of the ( R )-3H2MP precursor, thereby synthesizing P(3HB- co -8 mol % 3H2MP) with 100% ( R )-3H2MP units. The thermogravimetric analysis revealed that the thermal degradation temperatures at 5% weight loss of P(3H2MP)s occurred at approximately 313 °C, independent of tacticity, which is substantially higher than that of P(3HB) (257 °C). This study demonstrates a new concept for controlling the physical properties of biosynthesized PHA by manipulating the polymers' tacticity using 3H2MP units.