Quantitative analysis of the packing of alkyl glycosides: a comparison of linear and branched alkyl chains

Abstract

In an attempt to relate the geometry of glycolipid assemblies with molecular packing constraints, the surface areas per molecule for straight and branched-chain alkyl glycosides with varying chain length are calculated. Effects of temperature, water content, sugar size and paraffin chain length are analysed based on closest packing assumption. The results show a continuous increase of the interface between the hydrophilic and the hydrophobic domain per molecule with growing dominance in bulkiness of either domain, until it reaches a maximum in hexagonal phases. The surface area per molecule, on the other hand, exhibits a sudden jump upon the phase transition from a lamellar to a hexagonal phase, reflecting different values of the packing parameter in both assemblies. This increase is primarily based on the assembly, rather than on molecule-based domain sizes. Therefore, estimations of molecular region sizes can serve only to determine the principal ability of compounds to form certain phases, but not predict the actual phase exhibited under given conditions. Within straight-chain glycosides the surface area per molecule is practically constant, whereas it increases with growing chain length for branched-chain analogues. This can be explained with differences in the volume–length ratio of the hydrocarbon domain.

Keywords:

• packing
• alkyl glycoside
• branched chain
• geometric analysis
• SAXS

Acknowledgements

This work was supported by the Malaysian Ministry of Science, Technology and Innovation under the HCD-EXPT program and e-Science grant 03-01-03SF0083/13-02-03-3021, as well as by the University of Malaya under research grant FS349/2008A.