Enzymatic Degradability of Poly(β-Hydroxybutyrate) as a Function of Tacticity

The enzymatic degradation of films of synthetic poly[(R,S)-β-hydroxybutyrate], PHB, of various tacticities was investigated by weight loss measurement. Extracellular PHB depolymerases of the bacterium Pseudomonas lemoignei and the fungus Aspergillus fumigatus M2A were used. In both enzyme systems the nearly atactic samples showed the greatest weight loss; this maximum occurred at a slightly lower percent isotacticity in the P. lemoignei system than in the A. fumigatus. The maximum degradation rate for the P. lemoignei system was double that of the A. fumigatus; natural PHB degraded an order of magnitude more rapidly in both enzyme systems. As isotacticity increased, synthetic PHB showed decreased degradation while syndiotactic samples did not appreciably degrade. Results are interpreted in terms of both crystallinity and stereochemistry; the A. fumigatus system is more affected than the P. lemoignei by the presence of the S repeat units. Total weight loss data suggest that the enzymes are capable of endo cleavage.