| Résumé | The themotropic and barotripic gel-phase polymophism of a homologous series of saturated, straight-chain β-D-glucosyldiacylglycerols was studied by Fourier transform infrared spectroscopy. Three spectroscopically distinct lamellar gel phases were detected thermotropically. Upon cooling to temperatures below the gel/liquid-crystalline phase transition temperature, all of these lipids form a metastable Lβ gel phase characterized by orientationally disordered all-trans acyl chains. The transformation of the metastable Lβ phase to a stable crystalline (Lc₂) phase first incolves the formation of an intermediate which itself is an ordered crystal-like (Lc₁) phase. In the intermediate Lc₁ phase, the zigzag planes of the polymethylene chains are nearly perpendicular to one another, and one of the ester carbonyl oxygens is engaged in a strong hydrogen bond, probably to the 2-hydroxyl of the sugar headgroup. The transformation of the Lc₁ phase to the Lc₂ phase involves a reorientation of the all-trans hydrocarbon chains and is probably driven by the strengthening of the hydrogen bond between the carbonyl ester oxygen and its proton donors. Since a "solid-state" reorganization of the acyl chains is an integral part of that process, it tends to become more sluggish as the chain length increases and is not observed with the longer chain homologues (N>16). The spectroscopic characteristics of the most stable gel phases of the odd- and even-numbered members of this homologous series of compounds exhibit only minor differences, indicating that the structures of these phases are generally similar. The barotropic phase behaviour of the shorter and longer chain β-D-glucosyldiacyl-glycerols is also different. Compression of the Lβ phase of the shorter chain compounds results in immediate conversion to their stable Lc phases, whereas compression of the Lβ phase of the longer chains does not. Furthermore, compression of the longer chain compounds may result in the formation of chain-interdigitated bilayers, whereas this is not the case for the shorter chain homologues. We suggest that the gel phase formed by any given homologue at a given temperature or pressure is that which maximizes the sometimes competing requirements for the optimal packing of the sugar headgroups and the hydrocarbon chains. |
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