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 tact icities were degraded using enzymes isolated from the fungus Aspergill us 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 t o degrade completely; no decrease in molecular weight was observed in the partly degraded polymer. By contrast, the single crystals of synth etic PHB showed only partial degradation, with some decrease in molecu lar weight; faster and more substantial degradation was observed for t he most isotactic material, whereas single crystals from a nearly atac tic sample were essentially inert. These results differ from those fou nd for PHB films, implying that tacticity response is linear when crys tallinity effects have been normalized. The observed results are consi stent 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 .