MOLECULAR FORCES BETWEEN MEMBRANES DISPLAYING NEUTRAL GLYCOSPHINGOLIPIDS - EVIDENCE FOR CARBOHYDRATE ATTRACTION

The surface force apparatus was used to determine the fundamental forc es governing the adhesion between mixed bilayer membranes comprising l actosyl ceramide (LacCer) and di-tridecanoyl-phosphatidyl choline. For ces between membranes were quantified as a function of the glycolipid surface densities, which ranged from 0 to 30 mol %. Control measuremen ts of the forces between pure phosphatidylcholine membranes and mixed bilayers of lactosyl ceramide with phosphocholine showed that the ster ic thickness of the carbohydrate headgroups increased from 19 to 25 An gstrom when the glycolipid density increased from 10 to 20 mol %. The layer compressibility also decreased with increasing carbohydrate cove rage, but the corresponding adhesion between lactosyl ceramide-contain ing membranes increased with increasing amounts of glycosphingolipid i n them. The nonspecific van der Waals forces accounted for the attract ion measured in the control experiments and that between identical 10 mol % LacCer bilayers. However, the increase in the adhesion with incr easing glycolipid density was 2-4 times greater than predicted by Lifs chitz theory. Additionally, the forces measured during separation of m embranes containing 20 and 30 mol % glycosphingolipid indicated that t he headgroups bind and rearrange during bilayer detachment. The intera ctions between the carbohydrates are weak and apparently dynamic, and they generate an additional density-dependent intermembrane attraction that is on the order of the van der Waals force.