Bolaamphiphilic Phosphocholines:  Structure and Phase Behavior in Aqueous Media

The phase behavior of 1,1‘-di- O -eicosamethylene-2,2‘-di- O -decylbis(glycero-3-phosphocholine) ( 1 ) and 1,1‘-di- O -hexadecamethylene-2,2‘-di- O -octylbis(glycero-3-phosphocholine) ( 2 ) was studied at the air−water interface and in aqueous dispersions using monolayer film balance, small-angle X-ray scattering, Raman spectroscopy, and cryo-TEM techniques. The limiting molecular areas (20 °C, pH 6.8) were observed to be 172 Å 2 for 2 and 105 Å 2 for 1, indicating a U-shaped configuration of the long polymethylene chains at the air−water interface. The larger molecular area of 2 relative to 1 at the air−water interface suggests that the alkyl chains of 2 are less ordered than those of 1 at similar pressures. The extrapolated d spacings for 1 from SAXS experiments were 32 Å in the liquid crystalline lamellar phase and 40 Å in the gel phase where the hydrocarbon chains are in their all trans configuration. The extrapolated d spacings for 2 were 27 and 33 Å in the liquid crystalline and gel phases, respectively. DSC analysis revealed phase transition temperatures of 1 and 2 in excess water at 15.3 and −23.5 °C, respectively, with observed transition enthalpies of 9.1 and 2.9 kJ/mol, respectively. These values are in good agreement with Raman spectroscopy experiments that detect alkyl melting transitions near these temperatures. Addition of 30 mol % cholesterol to both bolaamphiphile samples lowered the observed transition temperatures ( 1, 9.5 °C; 2, −25.1 °C) and enthalpies ( 1, 5.7 kJ/mol; 2, 2.5 kJ/mol), suggesting that cholesterol reduces the transition cooperativity. Extruded samples of pure 1 in buffer (30 mg/mL in 20 mM Tris, pH 8) produced vesicle structures that were variable in size (0.15−1.5 μm), with multilamellar and nonspherical morphologies apparent. Incorporation of cholesterol in the extruded samples (7:3 1 /Chol), however, gave stable spherical unilamellar vesicles with a mean diameter of 857 ± 237 Å. Similar results were observed for dispersions of 2 and 7:3 2 /Chol. These results suggest that stable, unilamellar monolayer membrane vesicles can be formed from bolaamphiphiles with relatively short (C 16 and C 20 ) membrane-spanning alkyl chains.