Crystal structures of glycolipids with odd and even numbered alkyl chains

The effect of the alkyl chain on the thermal stability of crystals for a series of glycolipids, methyl 6-O-alkanoylglycopyranosides, was investigated using thermal analysis, X-ray structures and molecular mechanics. Glycolipids with two types of sugar moieties, alpha-galactopyranoside and beta-glucopyranoside carrying an alkyl chain with various numbers of carbon atoms, 1-11, and 1-13, respectively, were used for the analyses. All the crystals have bilayer structures with interdigitated alkyl chains. The crystal packing is significantly affected by the sugar moieties but the same for individual sugar derivatives for a variety of alkyl chain lengths. Alkyl chains with an even number of carbon atoms in the alpha-galactoside derivatives have temperature factors (thermal parameters) which are higher than those for alkyl chains with an odd number of carbon atoms. Alkyl chains with an even number of carbon atoms in the beta-glucosides have a disordered structure with two alternate conformations while the remaining derivatives have well ordered alkyl chains. The mean lengths of the hydrogen bonds in the beta-glucosides with an even number of carbon atoms in the alkyl chain are longer than those with an odd number of carbon atoms, but a slight difference only is observed in the alpha-galactosides. The thermal stability of the alpha-galactosides with an odd number of carbon atoms in the alkyl chain is higher than that of alpha-galactosides with an even number of carbon atoms. On the other hand, the opposite behavior is observed for the beta-glucosides. The difference in the melting points between beta-glucosides with an odd and even number of carbon atoms in the alkyl chain is greater than the difference for the corresponding alpha-galactosides. The difference in the thermal stability is suggested to be related to the packing of the sugar moieties in the crystal based on molecular mechanics calculations.