Thermodynamic properties and swelling behavior of glycolipid monolayers atinterfaces

Synthetic glycolipids with lactose headgroups (N = 1, 2, and 3) were synthe sized, and their thermodynamic properties were systematically studied by La ngmuir isotherms using a film balance. The molar transition entropy and the molar latent heat were calculated by applying the Clausius-Clapeyron equat ion. It has been demonstrated that the phase behavior of the glycolipid mon olayers is comparable to that of ordinary phospholipids, despite the lower degree of cooperativity between the larger headgroups. The glycolipid monol ayer was transferred onto a solid surface by Langmuir-Blodgett deposition, and the swelling behavior was investigated by ellipsometry. The surface gra fting density was precisely controlled, and the water disjoining pressure i nside the lactose layer was quantitatively measured. The measured swelling curves were analyzed in terms of the theoretical descriptions for the graft ed polymer "brushes". For the lipids with lactose units of N = 2 and 3, the disjoining pressure-thickness relation could fit very well to these theore tical approaches, even though the statistical limit N much greater than 1 i s hardly fulfilled. The results suggest entropic effects of the headgroups on the interaction between the neighboring molecules. On the other hand, th e theoretical description of the swelling behavior of the lipids with one l actose unit failed due to the "rodlike" structure of lactose. The unique pr operties of these glycolipids at interfaces, such as (i) the phase behavior comparable to that of ordinary phospholipids and (ii) the "polymer-like" s welling behavior, play very important roles in biological systems. Mimickin g the complex interactions between oligosaccharide headgroups in the plasma membranes, the synthetic glycolipids designed in this study are quite real istic models for the glycocalix.