Thermodynamic Properties and Swelling Behavior of Glycolipid Monolayers at Interfaces

Abstract

Synthetic glycolipids with lactose headgroups (N = 1, 2, and 3) were synthesized, and their thermodynamic properties were systematically studied by Langmuir isotherms using a film balance. The molar transition entropy and the molar latent heat were calculated by applying the Clausius−Clapeyron equation. It has been demonstrated that the phase behavior of the glycolipid monolayers is comparable to that of ordinary phospholipids, despite the lower degree of cooperativity between the larger headgroups. The glycolipid monolayer was transferred onto a solid surface by Langmuir−Blodgett deposition, and the swelling behavior was investigated by ellipsometry. The surface grafting density was precisely controlled, and the water disjoining pressure inside the lactose layer was quantitatively measured. The measured swelling curves were analyzed in terms of the theoretical descriptions for the grafted polymer “brushes”. For the lipids with lactose units of N = 2 and 3, the disjoining pressure−thickness relation could fit very well to these theoretical approaches, even though the statistical limit N ≫ 1 is hardly fulfilled. The results suggest entropic effects of the headgroups on the interaction between the neighboring molecules. On the other hand, the theoretical description of the swelling behavior of the lipids with one lactose unit failed due to the “rodlike” structure of lactose. The unique properties of these glycolipids at interfaces, such as (i) the phase behavior comparable to that of ordinary phospholipids and (ii) the “polymer-like” swelling behavior, play very important roles in biological systems. Mimicking the complex interactions between oligosaccharide headgroups in the plasma membranes, the synthetic glycolipids designed in this study are quite realistic models for the glycocalix.