On the supermolecular structure and metastability of glycerol monostearate-amylose complex

Calorimetry of glycerol monostearate-amylose complexes reveals the presence of at least two thermally distinct metastable forms, which thus implies a definite connection between thermal properties and the supermolecular organization of these materials. On the basis of X-ray-diffraction, calorimetric, and structural-analysis data, it is postulated that form I (Tm 99·4°C), obtained at low crystallization temperatures, has a uniformly distributed partial order where no separate crystallites can be identified. Such a structure would have an internal energy and entropy between those of an amorphous melt and a structure of discrete crystallites, such as form II (Tm 116·6°C). Conversion of form I to form II is readily carried out by isothermal annealing at temperatures above its melting point. The conformational responses of the complex (forms I and II) in various stabilizing (Na2SO4, sucrose, CsCl) and destabilizing (urea, guanidine hydrochloride) environments further suggest that interconversion between the various forms can be explained by considering that structural orders at two levels are affected: association-dissociation of aggregated helices and helix-coil transitions; at high concentrations, CsCl caused disruption of the crystallites without altering the conformation of individual helices. The transition enthalpy of the complex shows very little change with increases in the long-range order of the supermolecular structure (X-ray-diffraction data) and is interpreted to reflect mainly contributions from conformational disordering of helices.