Topotactic crystallization of isolated poly(β-hydroxybutyrate) granules from Alcaligenes eutrophus

Abstract

SUMMARYTransmission electron microscopy (TEM), differential scanning calorimetry (DSC), wide-angle X-ray powder diffractometry (WAXD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the crystallization behavior of PHB granules from Alcaligenes eutrophus isolated by enzymatic purification. TEM examination of freeze dried granules after mortar and pestle grinding at liquid nitrogen temperature revealed that the dry granules have a non-crystalline core/crystalline shell morphology. TEM micrographs of sections of PHB granules showed that upon annealing, the non-crystalline molecules in the core transform into stacks of lamellar crystals with a thickness of ~ 100 ,~. The FTIR results revealed the presence of bound water in a sample of freeze dried granules and WAXD of the same sample showed an increase in crystallinity after removal of this water by vacuum drying. The WAXD diffractograms showed an Correspondence to: R.H. increase in crystallinity of PHB granules when going from the in vivo to the dry state. In spite of the possibility of deforming them at very low temperatures (liquid nitrogen temperature) the glass transition temperature (Tg) of nascent PHB granules, as revealed by the DSC thermograms, was in the range -0.5-4°C. These results suggest that water is responsible for keeping the core of nascent PHB granules in a non-crystalline state. A model for biosynthesis where emerging PHB chains in an extended conformation are simultaneously hydrogen bonded to water molecules is proposed.

The chemomechanical properties of microbial polyhydroxyalkanoates

“…The proposal is that this hydrogen bonding prevents strong dipole-dipole interactions from occurring between the C O and CH 3 groups in P(3HB) [19,158,159]. Once this P(3HB)-bound water is removed, in situ crystallization can then occur due to increased hydrogen bonding, in a similar manner to the crystallization process on cooling for pure P(3HB) [19,158].…”

PHB and Other Polhydroxyalkanoic Acids

“…This is due to the formation of much larger PHA particles or to exogenous substances which get in excess to the PHA core of the granules to act as heterogeneous nucleation sites for crystallization (DE KONING and MAXWELL, 1993). Granules that never had been dried and that were obtained by the surfactant-hypochlorite treatment (RAMSAY et al, 1990), showed a crystalline shell at .the surface and an amorphous core (LAUZIER et al, 1992).…”

Candida tropicalis BPU1 produces polyhydroxybutyrate on raw starchy substrates

“…The spherulites—semi‐crystalline regions associated with crystallization of polymers from the melt —were clearly observed during crystallization, which grew radially with stacking. It was reported that physical treatments such as centrifugation may readily coalesce the PHB granules into larger masses which can lead to the apparent acceleration of its crystallization . The presence of PHB in the crystals was confirmed by TLC, UV‐Vis, FTIR and NMR.…”