Splintering of poly(3-hydroxybutyrate) single crystals by PHB-depolymerase a from Pseudomonas lemoignei

With the aim of improved understanding of the mechanism of depolymerase action, folded chain lamellar single crystals were partially degraded with PWB-depolymerase A from Pseudomonas lemoignei. Enzymatically degraded single crystals of bacterial poly(3-hydroxybutyrate), PHB, were observed by transmission electron microscopy and were found to be splintered parallel to their long axes. Prior to degradation, the crystals were lamellar with macroscopic dimensions of approximately 2.5 mu m by 20 mu m and the classical baseplane single crystal diffraction pattern corresponding to bacterial PHB was recorded. When observed by TEM, the partially degraded crystals had been splintered longitudinally, to a needlelike morphology. The needlelike fragments of PHB still yielded the same crystalline baseplane diffraction pattern. These results support an ''edge attack'' model for the degradation of PHB single crystals and explain, at a molecular level. the lack of decrease in molecular weight during the degradation since the direction of chain folding is parallel to the long axis of the crystals. The proposed mechanism explains the conversion of PHB spherulite lamellae into a needlelike morphology and suggests that PHB-depolymerase A has both endo and exo activity.