SYNTHESIS OF LINEAR OLIGOMERS OF (R)-3-HYDROXYBUTYRATE AND SOLID-STATE STRUCTURAL INVESTIGATIONS BY ELECTRON-MICROSCOPY AND X-RAY-SCATTERING

Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 10( 5) Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at -70 degrees in THF leads to a mixture of oligomers with inc reasingly sharp distribution around a 15-, 30-, and 45mer. Discrete Fr agments are also isolated when P(3-HB) is heated under reflux (89 degr ees) in neat Et(3)N. Linear oligo(S-HB) derivatives (3-7) containing u p to 96 3-HB units are synthesized using an exponential segment-coupli ng strategy. These oligomers are used to calibrate size-exclusion chro matography columns for the analysis of oligo(3-HB) samples from the di fferent sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron micros copy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAX S). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Angstrom, respe ctively, indicating that the 32mer molecules are folded once, very tig htly, into a 'hair-pin'-type conformation. High-molecular-weight P(3-H B), which was crystallized in a similar way, also has a lamellar cryst allite thickness of ca. 50-65 Angstrom. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions , an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Angstrom packing within the tight folds o f P(3-HB) is discussed in view of its possible function in ion transpo rt through cell membranes.