X-ray grazing incidence diffraction and Langmuir monolayer studies of the interaction of β-cyclodextrin with model lipid membranes

Abstract

The interactions of β-CD with one component monolayers of cholesterol (chol), 1-stearoyl-sn-glycero-3-phosphocholine (lyso-PC), 1,2-dipalmitpyl-sn-phosphocholine (DPPC), sphingomyelin (SM) and the SM/chol and DPPC/chol mixtures have been investigated by the Langmuir monolayer technique and the synchrotron grazing incidence X-ray diffraction (GIXD). The investigated lipid monolayers have been studied with and without the 10 −3 M solution of β-CD in the aqueous subphase. The surface pressure–area ( π– A) isotherms and the relaxation of the monolayers (surface pressure–time curves) were monitored. Our experiments reveal that there is not impact of β-CD on the packing properties of the DPPC monolayers, while the presence of β-CD in subphase changes the in-plane organization of SM molecules. Monolayers composed of pure chol molecules have been rapidly affected by the presence of the β-CD in the subphase. Our data show that β-CD can complex and desorb one-chain phospholipid (lyso-PC) but this process is relatively slow and, as indicated by the GIXD data, β-CD molecules are present at the air/water interface. Subtraction of cholesterol by the β-CD from mixed binary systems containing SM/chol (70/30, 50/50 and 30/70 mol ratio) and DPPC/chol (70/30 and 50/50 mol ratio) has also been investigated. Our experiments proved that cholesterol can be removed from the mixed monolayers only when it is unbound. The β-CD was not capable to distract the monolayers of the SM/chol, forming a stable complex of the 2:1 stoichiometry (as observed in the model lipid raft). Interestingly, at the surface pressure of 30 mN/m also at the molar proportion of 50/50 no cholesterol removal was observed. This was interpreted by relatively strong SM/chol interactions and the tight packing of the mixed monolayer. For model membranes, in which cholesterol was in large excess (SM/chol, 30/70) the β-CD extraction of cholesterol was observed, and the membrane composition evolves towards the lipid proportion corresponding to the stable complex stoichiometry (SM/chol 2:1).