Lipid lateral segregation driven by diacyl cyclodextrin interactions at the membrane surface

Cyclodextrins are hydrophilic molecular cages with a hydrophobic interior allowing the inclusion of water-insoluble drugs. Amphiphilic cyclodextrins obtained by appending a hydrophobic anchor were designed to improve the cell targeting of the drug-containing cavities through their liposome transportation in the organism. After insertion in model membranes, they were found to induce a lateral phase separation into a pure lipid phase and a fluid cyclodextrin-rich phase (L-CD) with reduced acyl chain order parameters, as observed with a derivative containing a cholesterol anchor (M. Roux, R. Auzely-Velty, F. Djedaini-Pilard, and B. Perly. 2002. Biophysical Journal, 8:813-822). We present another class of amphiphilic cyclodextrins obtained by grafting aspartic acid esterified by two lauryl chains on the oligosaccharide core via a succinyl spacer. The obtained dilauryl-beta-cyclodextrin (beta DLC) was inserted in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) membranes and studied by deuterium NMR (H-2-NMR). A laterally segregated mixed phase was found to sequester three times more lipids than the cholesteryl derivative (similar to 4-5 lipids per monomer of beta DLC), and a quasipure LCD phase could be obtained with a 20% molar concentration of beta DLC. When cooled below the main fluid-to-gel transition of DMPC-d54 the beta DLC-rich phase stays fluid, coexisting with pure lipid in the gel state, and exhibits a sharp transition to a gel phase with frozen DMPC acyl chains at 12.5 degrees C. No lateral phase separation was observed with partially or fully methylated beta DLC, confirming that the stability of the segregated LCD phase was governed through hydrogen-bond-mediated intermolecular interactions between cyclodextrin headgroups at the membrane surface. As opposed to native beta DLC, the methylated derivatives were found to strongly increase the orientational order of DMPC acyl chains as the temperature reaches the membrane fluid-to-gel transition. The results are discussed in relation to the "anomalous swelling'' of saturated phosphatidylcholine multilamellar membranes known to occur in the vicinity of the main fluid-to-gel transition.