Enzymatic and environmental degradation of racemic poly(3-hydroxybutyric acid)s with different stereoregularities

Racemic poly(3-hydroxybutyric acidi (P(3HB)) was prepared by the ring-opening polymerization of racemic beta-butyrolactone (beta-BL) in the presence of methylaluminoxane and fractionated into five components with various isotactic diad fractions ([i]) varying from 0.88 to 0.54. The degree of X-ray crystallinity of isotactic P(3HB) films decreased from 33 to 8% as the [i] value was decreased from 0.88 to 0.54. A predominantly syndiotactic P(3HB) ([i] = 0.30) was prepared by the polymerization of racemic beta-BL in the presence of 1-ethoxy-3-chlorotetrabutyldistannoxane, and its crystallinity was 26%. Enzymatic degradation of P(3HB) films with different stereoregularities was carried out at 37 degrees C in 0.1 M potassium phosphate buffer (pH 7.4) in the presence of PHB depolymerase purified from Alcaligenes faecalis. The highest rate of enzymatic hydrolysis was observed at an [i] value of 0.74. Syndiotactic P(3HB) film was hardly hydrolyzed by the enzyme. The enzymatic degradation products of racemic P(3HB) samples ([i] = 0.88-0.54) were a mixture of monomer, dimer, trimer, and tetramer of 3-hydroxybutyric acid and showed negative values (-11 +/- 1 degrees) of specific optical rotation, which suggests that PHB depolymerase hydrolyzes selectively the ester bond connecting the methine carbon of the (R)-3HB unit. The amount of PHB depolymerase adsorbed on the surface of P(3HB) films was measured at 37 degrees C. PHB depolymerase adsorbed on the surface of predominantly isotactic P(3HB) films, while little adsorption of enzyme took place on the surface of syndiotactic P(3HB) film, which suggests that the binding domain of PHB depolymerase has an affinity toward the isotactic P(3HB) crystalline phase. In addition, biodegradation tests of P(3HB) films were carried out under aerobic conditions in river water. The racemic P(3HB) film with [i] = 0.63 was degraded completely within 28 days at 25 degrees C in the river water, and the decomposed products containing (R)- and (S)-3HB units were metabolized by microorganisms.