Structural effects on enzymatic degradabilities for poly[(R)-3-hydroxybutyric acid] and its copolymers

Poly[(R)-3-hydroxybutyric acid] and its copolymers were prepared by biosynthetic and chemosynthetic methods. The films of polyesters were prepared by both the solution-cast and melt-crystallized techniques. The enzymatic degradation of polyester films was carried out at 37 degrees C in an aqueous solution (pH 7.4) of PHB depolymerase from Alcaligenes faecalis. The rate of enzymatic erosion on the solution-cast films increased markedly with an increase in the fraction of second monomer units up to 10-20 mol% to reach a maximum value followed by a decrease in the erosion rate. Analysis of the water-soluble products liberated during the enzymatic degradation of polyester films showed the formation of a mixture of monomers and oligomers of (R)-3HB and hydroxyalkanoic acids units, suggesting that the active site of PHB depolymerase recognizes at least three monomeric units as substrate for the hydrolysis of ester bonds in a polymer chain. The rate of enzymatic erosion of melt-crystallized polyester films decreased with an increase in crystallinity. PHB depolymerase predominantly hydrolyzed the polymer chains in the amorphous phase and subsequently eroded crystalline phase. In addition, the enzymatic degradation of crystalline phase by PHB depolymerase progressed from the edges of crystalline lamellar stacks. The enzymatic erosion rate of crystalline phase in polyester films decreased with an increase in the lamellar thickness.