Construction of higher-ordered monolayer membranes derived from archaeal membrane lipid-inspired cyclic lipids with longer alkyl chains

A series of artificial cyclic lipids that mimic archaeal membrane ones has been synthesized. The structural features of these molecules include a longer cyclic framework, in which the alkyl chain length ranges from 24 to 32 in carbon number, which is longer than our first analogous molecule with 20-carbon long alkyl chains [K. Miyawaki, T. Takagi, M. Shibakami, Synlett 8 (2002) 1326]. Microscopic observation reveals that these molecules have a self-assembling ability: hydration of the lipids yields multilamellar vesicles in aqueous solution and monolayer sheets on solid supports. High-sensitivity differential scanning calorimetry (24- and 28-carbon alkyl chain lipids) indicates that (i) the alkyl chain length affects their phase behavior and (ii) the enthalpies of endothermic peaks accompanied by phase transition were considerably lower than those of their monomeric phospholipid analogs. Fluorescence polarization measurements suggest that the membranes made from the 24-carbon alkyl chain lipid have a higher polarization factor than membranes composed of DMPC and DMPC plus cholesterol. These findings imply that the cyclic lipids containing 24- and 28-carbon alkyl chain construct well-organized monolayer membranes and, in particular, that the molecular order of the 24-carbon alkyl chain lipid is higher than that of bilayer membranes in the liquid-ordered phase.