CRYSTAL-STRUCTURE OF SYNDIOTACTIC POLY(BETA-HYDROXYBUTYRATE) FROM X-RAY FIBER AND POWDER DIFFRACTION ANALYSES AND MOLECULAR MODELING

A preliminary crystal structure of syndiotactic poly(beta-hydroxybutyr ate), syn PHB, was derived by using X-ray fiber and powder diffraction data in conjunction with computational modeling. The X-ray fiber diag ram of highly stretched 69% syn PHB gave 23 reflections and a fiber re peat of 7.69 ii. The diffraction trace of a powder syn PHB octamer pro vided well-resolved d spacing measurements from which an orthorhombic unit cell of dimensions a = 7.84 Angstrom, b = 14.90 Angstrom, c = 7.6 9 Angstrom is proposed. Conformational analysis with a syndiotactic di ad repeat unit gave six low-energy chains with a fiber repeat of 7.69 Angstrom. The most favorable chain conformation had dihedral angles si milar to isotactic PHB but with a more extended integral helix. There is no energy preference for a right-handed or left-handed helix. The P 2(1)2(1)2(1) space group was selected based on observed systematic abs ences, the tentative orthorhombic unit cell, and favorable packing ene rgies. The 2(1) symmetry in the chain direction is between neighboring sets of anticline packed helices of syn PHB. The structural model of syn PHB put forth is based on favorable packing energies and a reasona ble fit of simulated X-ray diffraction data with the experimental resu lts, which were further evaluated by a Rietveld analysis.