Phase transition in glycolipid monolayers induced by attractions between oligosaccharide head groups

The surface property of two kinds of synthetic glyceroglycolipids was investigated to confirm the stereoeffect of sugar residues on the phase behavior in monolayers: one is the maltooligosaccharide-containing lipids [Mal(N)(C-12)(2)] and the other is cellooligosaccharide-containing lipids [Cel(N)(C-12)(2)]. The two kinds of glycolipids exhibit the opposite dependence of the surface pressure-area (II-A) isotherm on the number of glucose residues (N), in that Mal(N)(C-12)(2) tends to be more expanded as N increases, while Cel(N)(C-12)(2) tends to be more condensed. The liquid-expanded/liquid-condensed phase transition in Cel(N)(C-12)(2) was evaluated by the Brewster angle microscopy and the latent heat analysis with the Clausius-Clapeyron equation, as a model of film condensation induced by the attraction between oligosaccharide head groups. The film morphology of Cel(4)(C-12)(2) was unique in that it exhibits the optically isotropic and flexible fluid type of the LC phase, while Glc(C-16)(2) forms a two-dimensional tilted crystalline phase in the same way as fatty acids and phospholipids. The heat of transition (Delta H) of Cel(4)(C-12)(2) does not converge toward zero even at high temperature. In other words, the main transition of Cel(4)(C-12)(2) does not become second order in contrast to the behavior usually encountered in monolayers. The refractive index of the oligosaccharide head groups in water as well as the alkyl chains in air must be properly taken into account to interpret the reflectivity.