Enzymatic degradation of single crystals of bacterial and synthetic poly(beta-hydroxybutyrate)

Lamellar single crystals of both natural poly[(R)-beta-hydroxybutyrate], PHB, and synthetic poly[(R,S)-beta-hydroxybutyrate] of various tacticities were degraded using enzymes isolated from the fungus Aspergillus fumigatus and the bacterium Pseudomonas lemoignei. Degradation was monitored by both turbidimetric and titrimetric assays. Despite their highly ordered state, single crystals of bacterial PHB were observed to degrade completely; no decrease in molecular weight was observed in the partly degraded polymer. By contrast, the single crystals of synthetic PHB showed only partial degradation, with some decrease in molecular weight; faster and more substantial degradation was observed for the most isotactic material, whereas single crystals from a nearly atactic sample were essentially inert. These results differ from those found for PHB films, implying that tacticity response is linear when crystallinity effects have been normalized. The observed results are consistent with preferential degradation from the crystal edges rather than the chain folds of the lamellar surface and support the hypothesis of a combined endo-exo degradation mechanism for these two depolymerases.