| Abstract |
The spontaneous colloidal nanostructures formed in water by the zwitterionic phospholipid DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) with the cationic detergent DTAC (n-dodecyltrimethylammonium chloride) were investigated at a fixed DMPC concentration and variable detergent:lipid total molar ratios (D:L). Apparent (neutral-sphere-equivalent) hydrodynamic diameters (Phi(e)) of liposomes and micelles were obtained by dynamic light scattering (DLS). Fluorescence lifetime imaging microscopy (FLIM), using chlorophyll-a as a probe, showed the morphology of giant vesicles and threadlike micelles. Micro-differential scanning calorimetry (micro-DSC) detected the presence of bilayers, in vesicles and discoidal micelles (disks). Pure DMPC liposomes are multilamellar and polydisperse (Phi(e) approximate to 100-10,000 nm). As D:L increased, smaller vesicles were found, due to the bigger spontaneous curvature of the bilayer: at D:L = 1, ULVs (unilamellar vesicles; (Phi(e) approximate to 100 nm) appeared and, at D:L = 2-10, ULVs coexisted with disks (Phi(e) approximate to 30 nm). Bilayers totally disappeared at D:L >= 15, giving rise to spheroidal (Phi(e) approximate to 2-16 nm) and threadlike (Phi(e) approximate to 100-10,000 nm) micelles. A quasi-equilibrium structural diagram for the DMPC-DTAC-water system shows equivalent diameters of the scattering nanoparticles as a function of D:L The results obtained herein for the system DMPC-DTAC show the role of electrostatic interactions in the formation of the mixed structures. (C) 2012 Elsevier Inc. All rights reserved. |
