Crystal structure of syndiotactic poly(β-hydroxybutyrate) from X-ray fiber and powder diffraction analyses and molecular modeling

A preliminary crystal structure of syndiotactic poly(beta-hydroxybutyrate), syn PHB, was derived by using X-ray fiber and powder diffraction data in conjunction with computational modeling. The X-ray fiber diagram of highly stretched 69% syn PHB gave 23 reflections and a fiber repeat of 7.69 ii. The diffraction trace of a powder syn PHB octamer provided well-resolved d spacing measurements from which an orthorhombic unit cell of dimensions a = 7.84 Angstrom, b = 14.90 Angstrom, c = 7.69 Angstrom is proposed. Conformational analysis with a syndiotactic diad repeat unit gave six low-energy chains with a fiber repeat of 7.69 Angstrom. The most favorable chain conformation had dihedral angles similar to isotactic PHB but with a more extended integral helix. There is no energy preference for a right-handed or left-handed helix. The P2(1)2(1)2(1) space group was selected based on observed systematic absences, the tentative orthorhombic unit cell, and favorable packing energies. 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 reasonable fit of simulated X-ray diffraction data with the experimental results, which were further evaluated by a Rietveld analysis.