Bilayer structure of glycolipid crystals. Thermal stability of the crystal and state of the alkyl chain

Abstract

The role of alkyl chains in the stabilization of a bilayer structure of glycolipids in the crystalline state has been studied for a series of methyl 6-O-acyl-β-D-glucopyranosides and methyl 6-O-acyl-α-D-galactopyranosides. Thermal properties and crystal structures of these glycolipids with an odd number of alkyl carbon atoms in the acyl moiety have been investigated by thermal analysis and X-ray methods. The melting temperatures of methyl 6-O-acyl-β-D-glucopyranosides, which are lower than those of the corresponding methyl 6-O-acyl-α-D-galactopyranosides, decrease with increasing number of alkyl carbon atoms from one to seven, but increase for longer alkyl chains. The α-galactosides show only a monotonic decrease in melting temperature with increasing alkyl chain length. In the crystals, the molecules form a bilayer structure with interdigitated alkyl chains. The arrangement of sugar moieties in the crystals differs between the β-glucosides and the α-galactosides, but they are almost the same among sugar derivatives with various alkyl chain lengths. The packing of the acetyl moiety in a crystal of methyl 6-O-α-D-galactoside is different from that of the other α-galactoside derivatives with longer alkyl chains. The carbonyl oxygen atom causes steric hindrance with adjacent alkyl chains, and such unfavorable intermolecular contact explains the relatively low thermal stability of the crystal. On the other hand, the contribution of the β-glucoside to thermal stability is ascribed to the well-ordered packing of the long alkyl chain moiety.