Thermotropic phase properties and structure of 1,2-distearoylgalactosylglycerols in aqueous systems

Differential scanning calorimetry and X-ray diffraction measurements indicate that aqueous dispersions of the 1,2-distearoyl derivatives of monogalactosyl and digalactosyldiacylglycerol exhibit complex polymorphic behaviour involving the formation of stable and metastable structural forms. The relative proportions of these forms are dependent on the thermal history of the samples. Two distinct gel phases can be identified for the monogalactosyl lipid; a stable highly ordered form (MGDG I ) and a metastable form (MGDG II ). There is also some indication of a low temperature modification of the stable form (MGDG´ I ). Analogous stable and metastable forms of the digalactosyl lipid (DGDG I and DGDG II ) can also be identified. In both cases, the metastable form appears on initial crystallization from the liquid-crystalline state and the stable form appears only after prolonged equilibration at room tempera­ture or after annealing the samples at higher temperatures. In the case of the monogalactosyl lipid, thermal equilibration leads to a complete, or nearly complete, conversion to the stable form. The equilibrium state in the digalactosyl lipid, however, lies much closer to the metastable form. Calorimetric measurements performed on samples containing limited water and thermogravimetric measurements indicate that the difference between the metastable and stable forms lies in their extents of hydration, and that the conversion of the metastable to the stable form involves a dehydration of the lipid. Comparison with other lipids showing similar polymorphic behaviour suggests that this process may be a common feature of lipids that tend to bind little water and of lipid systems in which extensive divalent ion bridging between neighbouring lipid headgroups can occur.