H NMR Imaging Study of Enzymatic Degradation in Poly(3-hydroxybutyrate) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Evidence for Preferential Degradation of the Amorphous Phase by PHB Depolymerase B from Pseudomonas lemoignei

1 H NMR imaging has been used to monitor the enzymatic degradation of poly(3-hydroxybutyrate), PHB, and poly(3-hydroxybutyrate- co -23%-3-hydroxyvalerate), PHB/V, by PHB depolymerase B from Pseudomonas lemoignei . The samples examined were thin films of approximately 200 μm thickness produced by melt casting. Since only the amorphous regions of the semicrystalline films contribute to the 1 H NMR image intensity, this technique provides unique information regarding the degradation process in the amorphous regions of the films when combined with total weight loss measurements. It was found that although the total weight loss rate of both the PHB and PHB/V films was constant, as previously reported, the initial amorphous material consumption rate was exponential. During the initial stages of the degradation process, up to 40−60 h, preferential consumption of amorphous material by depolymerase B was found to take place. At later stages the preference for amorphous material diminished, and both crystalline and amorphous phases were degraded indiscriminately. This initial consumption of amorphous material supports evidence that this stage is necessary to provide access to lamellar crystalline regions. The initial amorphous polymer consumption was verified by optical microscopy of the PHB film surface, which revealed the well-known circular erosion pattern associated with this type of enzymatic activity. Values of 0.020 and 0.049 h -1 for the rate constant of amorphous PHB and PHB/V consumption by depolymerase B were calculated from the 1 H imaging data during the early stages of degradation. The factors responsible for the observed behavior of the depolymerase B enzyme and the implications for the mechanism of enzymatic degradation in PHAs are discussed.