Calorimetric measurement of phospholipid interaction with methyl-beta-cyclodextrin.

Cyclodextrins are able to bind hydrophobic molecules in their interior cavity and as such have received a great deal of attention as carriers of cholesterol, lipophilic drugs, and other sparingly soluble compounds. Despite the importance of these biochemical applications, relatively little is known about the interactions of cyclodextrins with phospholipid membranes. Here we characterize the binding of randomly methylated beta-cyclodextrin (m beta CD) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using right-angle light scattering and isothermal titration calorimetry. Existing models of lipophile-membrane interactions are inadequate to describe the observed binding; we introduce a modified chemical reaction model in which the chemical activity of the phospholipid is independent of its concentration. We find that an average of four m beta CD molecules bind to each POPC molecule with an enthalpy of reaction of 46 kJ mol(-1) and an equilibrium constant of 90 M(-3). These results are consistent with earlier qualitative observations and suggest that disruption of phospholipid membranes may be minimized if the concentration of m beta CD is kept below about 15 mM.

A thermodynamic model for extended complexes of cholesterol and phospholipid.

Studies of monolayer mixtures of certain phospholipids with cholesterol by epifluorescence microscopy and measurement of cholesterol desorption show evidence for the formation of "condensed complexes." A thermodynamic model of these complexes has been developed and has been shown to be generally consistent with observed phase diagrams, cholesterol desorption rates, and electric field susceptibility. Previous work has shown that complexes comprising 10-50 molecules provide good agreement with experimental results. The present study examines the calculated properties of complexes containing very large numbers of molecules and extends the condensed complex model to incorporate the formation of complexes of variable size. Trends in equilibrium composition are similar to those calculated for small complexes. Thermal transitions are continuous, with a strong composition dependence of the breadth of the transition. The average number of molecules in a large complex shows a pronounced dependence on the composition of the reaction mixture. Large complexes have properties of a separate thermodynamic phase.