Poly(Beta-Hydroxybutyrate) Stereoisomers: A Model Study of the Effects of Stereochemical and Morphological Variables on Polymer Biological Degradability

Abstract

IVOM`3 N'o 0704-0188 OCT 211993crystalline bacterial 100% IR)-PHB. However, this initial observed rate was followed by an abrupt decrease '4%21 993!') from substrata/zoenzyme incubations up to 21 days further confirmed that the 50% IRI-atactic-PHB sample r, was a poor substrate for the enztyme aftar the rapid initial degradation of (Ri-HR-rich polymer chain segments V.at the film surface. In contrast, a significant portion of the polymenc chains for both the predominantly syrdiotactic 50% Ifi)-PHJ3 and the 77% (Ri-PHlE ware degraded by the P. funiculown sifterseo to products .2 containing on average 3 * I HB repeat units. The relative degradability of these PHlE sterecioutmere has interesting implications on the acceptabdity of specific stereochermical sequences in the degradation of the PHBE by P. funscuilasum and other PHB dopolymerasse. ABSTRACT: In this study, we have prepared poly(fl-hydroxybutyrate), PHB, random stereocopolymers from 0-butyrolactone, BL, using a diethylzinc/water (1.0/0.6) catalyst system. (R)-and (S)-BL were synthesized in high enantiomeric purity (>98% ee (enantiomeric excess)), and approximately 5% racemization occurred at the methine stereocenters upon polymerization. The PHB stereoisomers produced had R repeat unit compositions of 95, 90,85,81,77,67, and 50 %. In addition, a 50% (R)-PHB stereoisomer with a predominantly syndiotactic repeat unit placement was prepared in our laboratory. The relative degradability of these PHB , stereoisomers was studied with a PHB depolymerase enzyme isolated from Penicillium funiculosum. This enzyme has been shown to catalyze the hydrolysis of (R)-PHB, but does not show activity for the enantiomeric substrate (S)-PHB. The P. funicolusum depolymerase/PHB stereocopolymer system, therefore, allowed the study of two opposing effects on the degradation rate: the increase due to the disruption of the crystalline phase, and the decrease due to a stereochemical enzyme impediment, as the ( crystalline bacterial 100% (R)-PHB. However, this initial observed rate was followed by an abrupt decrease I in the rate which was probably due to depletion of (R)-HB-rich segments on the polymer surface. Results. from substrate/exoenzyme incubations up to 21 days further confirmed that the 50% (R)-atactic-PHB sample 1"was a poor substrate for the enzyme after the rapid initial degradation of (R)-HB-rich polymer chain segments . at the film surface. In contrast, a significant portion of the polymeric chains for both the predominantly ii a1 syndiotactic 50 % (R)-PHB and the 77 % (R)-PHB were degraded by the P. funiculosum esterase to products S• e -; containing on average 3 :L 1 HB repeat units. The relative degradability of these PHB stereoisomers has interesting implications on the acceptability of specific stereochemical sequences in the degradation of the PHB by P. funiculosum and other PHB depolymerases.