Lamellar single crystals of both natural poly[(R)-β-hydroxybutyrate], PHB, and synthetic poly[(R,S)-β-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.

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A preliminary crystal structure of syndiotactic poly(β-hydroxybutyrate), syn PHB, was derived by using X-ray fiber and powder diffraction data in conjunction with computational modeling. The X-ray fiber diagram of highly stretched 69% syn PHB gave 23 reflections and a fiber repeat of 7.69 Å. The diffraction trace of a powder syn PHB octamer provided well-resolved d spacing measurements from which an orthorhombic unit cell of dimensions a = 7.84 Å, b = 14.90 Å, c = 7.69 Å is proposed. Conformational analysis with a syndiotactic diad repeat unit gave six low-energy chains with a fiber repeat of 7.69 Å. The most favorable chain conformation had dihedral angles similar to isotactic PHB but with a more extended integral helix. There is no energy preference for a right-handed or left-handed helix. The P212121 space group was selected based on observed systematic absences, the tentative orthorhombic unit cell, and favorable packing energies. The 21 symmetry in the chain direction is between neighboring sets of anticline packed helices of syn PHB. The structural model of syn PHB put forth is based on favorable packing energies and a reasonable fit of simulated X-ray diffraction data with the experimental results, which were further evaluated by a Rietveld analysis.

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