Molecular modelling of helical and extended-chain polyhydroxybutyrates and polytetramethylene succinate

Molecular modelling (MM) has been employed to investigate the crystalline chain conformations of a series of related aliphatic biodegradable polyesters. The bacterial (isotactic) poly (3-hydroxybutyrate) helical conformation of 2.98 Angstrom advance/repeat unit can be cold-drawn to a nearly extended chain conformation possessing an advance of 4.6 Angstrom/repeat unit versus 4.8 Angstrom for complete extension. Syndiotactic PHB has an extended helical chain conformation of 3.85 Angstrom per repeat on account of steric interference between substituents of contiguous R and S units. Molecular modelling techniques have confirmed this result and suggest that up to 25% of S configured units spread randomly or in a stereoblock fashion are isomorphous with the R repeats of isotactic helix segments. From X-ray fibre diffraction data, a low energy crystalline chain conformation for elastomeric poly(4-hydroxybutyrate), P4HB, with a pitch of 11.9 Angstrom per 2 repeat units has been proposed. A synthetic biodegradable polyester, poly(tetramethylene succinate) or Bionolle(R), undergoes a reversible strain-induced crystal modification in which the fibre identity period increases from 10.91 Angstrom to 11.97 Angstrom. Plausible conformers are proposed respecting these observed pitches. Modelled P4HB and Bionolle(R) chains in the 'all trans' planar zigzag form correspond to a pitch of 12.4 Angstrom, which overestimates the observed fibre repeat values. Analysis of the crystalline structure of synthetic and bacterial polyesters gives potential insight into their biodegradability, which is controlled by specific enzyme binding characteristics. (C) 1998 Elsevier Science Ltd. All rights reserved.