Bolaamphiphilic Phosphocholines: Structure and Phase Behavior in Aqueous Media†
journal contributionposted on 11.12.1999, 00:00 by Ciro Di Meglio, Shankar B. Rananavare, Sönke Svenson, David H. Thompson
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 Å2 for 2 and 105 Å2 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 (C16 and C20) membrane-spanning alkyl chains.
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33 Åtrans configurationgel phaseditransition enthalpiesphase behaviorRaman spectroscopy experimentscholesterolÅ.phase transition temperaturesbolaamphiphile samplesgel phasesvesicle structuresRaman spectroscopyspacinginterfacemonolayer film balancelamellar phaseSAXS experimentsC 20CholPhase BehaviorpHtransition cooperativityDSC analysisC 16dispersiontransition temperaturesnonspherical morphologiesunilamellar monolayer membrane vesiclesunilamellar vesicles20 mM Trishydrocarbon chainsalkyl chains40 Å105 Å 2kJ172 Å 232 Åpolymethylene chains