Synthesis of paramylon ester-graft-PLA copolymers and its two-step enzymatic degradation by proteinase K and β-1,3-glucanase

Abstract

Paramylon ester-graft-poly(lactic acid) (PLA) copolymers were synthesized by the ring opening polymerization of l-lactide with paramylon acetate (PaAc) samples having an acetate degree of substitution (DS) from 0.5 to 2.2 (PaAc-g-PLA) or paramylon propionate (PaPr) samples with a propionate DS of 1.6 or 2.0 (PaPr-g-PLA). These copolymers exhibited thermoplasticity and some formed self-supporting melt-pressed films. Analyses of annealed films using differential scanning calorimetry showed that their thermal properties varied with the PLA DS. The enzymatic degradation behaviors of the neat paramylon as well as the PaAc and PaPr were investigated by treating these materials with β-1,3-glucanase followed by spectrometric analysis of the glucose levels in the buffer solutions, prior to the investigation of the copolymers. The data showed that 9 wt% glucose was obtained from the neat paramylon after 7 d and that the PaAc and PaPr were also degraded by β-1,3-glucanase. The concentration of liberated glucose was also found to decrease as the DS increased. Two-step enzymatic degradation tests were subsequently carried out with PaAc-g-PLA and PaPr-g-PLA specimens. In the first step, proteinase K was applied to degrade the PLA side-chains and high performance liquid chromatography (HPLC) was used to assess the release of lactic acid. Both HPLC and nuclear magnetic resonance (NMR) analyses established that the PLA side-chains of both the PaAc-g-PLA and PaPr-g-PLA specimens were successfully degraded. In the second step, β-1,3-glucanase was employed to decompose the remaining paramylon ester backbones. Monitoring the release of glucose showed that these backbones were successfully degraded in trials using the PaAc0.5-g-PLA and PaAc1.0-g-PLAs after 7 d These results confirmed that both copolymers could be broken down via this two-step enzymatic treatment, and that degradation by β-1,3-glucanase was dependent on the DS of the paramylon ester backbone. The results of this work show that PaAc1.0-g-PLA is a plastic material capable of thermal processing and is also completely biodegradable.