Structural Studies of the Monolayers and Bilayers Formed by a Novel Cholesterol-Phospholipid Chimera

Langmuir isotherm, neutron reflectivity, and small angle neutron scattering studies have been conducted to characterize the monolayers and vesicular bilayers formed by a novel chimeric phospholipid, ChemPPC, that incorporates a cholesteryl moeity and a C-16 aliphatic chain, each covalently linked via a glycerol backbone to phosphatidylcholine. The structures of the ChemPPC monolayers and bilayers are compared against those formed from pure dipalmitoylphoshatidylcholine (DPPC) and those formed from a 60:40 mol % mixture of DPPC and cholesterol. In accord with previous findings showing that very similar macroscopic properties were exhibited by ChemPPC and 60:40 mol % DPPC/cholesterol vesicles, it is found here that the chimeric lipid and lipid/sterol mixture have very similar monolayer structures (each having a monolayer thickness of 26 Å), and they also form vesicles with similar lamellar structure, each having a bilayer thickness of 50 Å and exhibiting a repeat spacing of 65 Å. The interfacial area of ChemPPC, however, is around 10 Å2 greater than that of the combined DPPC/cholesterol unit in the mixed lipid monolayer (viz., 57 ± 1 vs 46 ± 1 Å2, at 35 mN·m–1), and this difference in area is attributed to the succinyl linkage which joins the ChemPPC steroid and glyceryl moieties. The larger area of the ChemPPC is reflected in a slightly thicker monolayer solvent distribution width (9.5 vs 9 Å for the DPPC/cholesterol system) and by a marginal increase in the level of lipid headgroup hydration (16 vs 13 H2O per lipid, at 35 mN·m–1).