Carbohydrate liquid crystals

Abstract

There has been a marked increase during the past 5 years in the number of mesogenic compounds synthesized from the common, naturally occurring carbohydrates, particularly from glucose and glucitol. The structure of the mesophases depends on the shape of the molecules. Cyclic and acyclic carbohydrates with single n-alkyl or acyl chains of more than six carbon atoms are calamitic amphiphiles which form thermotropic smectic A_d phases. The more soluble of these mesogens are used as non-ionic surfactants for solubilizing and crystallizing membrane proteins. They form gels and lyotropic phases with water. The lamellar phase is most common, but some carbohydrate amphiphiles exhibit all of the classical lyotropic phases associated with the ionic surfactants, i.e. lamellar, cubic and hexagonal. Both the smectic A_d and the lamellar L_α phases are believed to be bilayers with interdigitized alkyl chains in the interior of the molecular clusters. The crystal structures of the alkylated cyclic sugars hitherto determined have head-to-head bilayers with interdigitizing alkyl chains. With the acyclic sugars, head-to-tail monolayer crystal structures are also observed. It is believed that these convert to bilayer structures prior to forming the bilayer smectic phases. Carbohydrates substituted with more than one hydrocarbon chain form hexagonal mesophases. Both cyclic and acyclic sugars, when monosubstituted with double alkyl or acyl chains, are wedge-shaped molecules which aggregate into discs that stack in hexagonally arranged columns. When full substituted, the flatter carbohydrate molecules, such as scyllo-inositol, form a large class of discotic mesophases. Those that are only partially substituted, leaving two hydroxyl groups free, are reported to form multilayer discotic mesophases which combine stacking forces with hydrogen bonding. The amphiphilic carbohydrates that form hexagonal phases thermotropically may form hexagonal, cubic or lamellar phases lyotropically.