Effect of tacticity on enzymatic degradability of poly(beta-hydroxybutyrate)

Films of synthetic poly(beta-hydroxybutyrate)s (PHB)s ranging from highly isotactic to moderately syndiotactic were cast from each of several polymer fractions isolated from ring-opening polymerizations of racemic beta-butyrolactone and were subjected to degradation by the extracellular PHB depolymerases of the bacterium Pseudomonas lemoignei and the fungus Aspergillus fumigatus M2A. Films of bacterially produced PHB were also degraded for comparison. Gravimetric determinations of enzymatic degradation were measured relative to control experiments in which no depolymerase was added. Each sample was exposed to the same concentration of enzyme, as determined by the activity of the enzyme solutions towards bacterial PHB powder, and the degradation behavior varied strongly with the tacticity of the samples. Weight loss was greatest for the bacterial polymer, with films being completely degraded by either enzyme in 48 h. The degradation of the synthetic samples by both enzymes followed similar trends, although the fungal depolymerase effected less rapid degradation for all synthetic samples than did the bacterial enzyme. Of the synthetic PHB samples, those of intermediate tacticity (55-60% isotactic diads) exhibited the greatest rate of weight loss, while those most syndiotactic in nature (34-45% isotactic) degraded the most slowly. The observed rates are interpreted in terms of isotactic contents, tacticity sequence distributions, and sample crystallinities.