Mixed Fluorocarbon/Hydrocarbon Surfactant Vesicles as Carriers of Metalloproteins: Scattering and Magnetic Resonance Experiments

Comparison of vesicle formation in zwitanionic and catanionic mixtures of hydrocarbon and fluorocarbon surfactants: phase behavior and structural progression

The phase behavior of zwitanionic and catanionic mixtures of perfluoro and hydrocarbon surfactants has been studied for the case of lithium perfluorooctanoate (LiPFO) as anionic surfactant and tetradecyldimethylamine oxide (TDMAO) as zwitterionic or tetradecyltrimethyl ammonium bromide (TTAB) as cationic surfactant. Samples as a function of the mixing ratio were characterized by means of electric conductivity, light scattering, cryo-TEM, and rheology. Despite the equal chain length of the zwitterionic and cationic surfactant we observe largely different phase behavior in these systems with the formation of precipitates around equimolar mixing for the catanionic system, while no such precipitation is observed for the zwitanionic system. This can be rationalized in terms of the much lower interaction parameter beta (from cmc measurements) for the case of the zwitanionic system. Accordingly, in the zwitanionic mixture a larger range of stable unilamellar vesicles is observed, while for the catanionic system larger vesicles are present, with a much stronger tendency for formation of multilamellar vesicles. Another interesting effect is that only for the case of the zwitanionic mixture a substantial increase of viscosity is observed prior to the formation of vesicles, which means that here the transition from spherical micelles to vesicles takes place via strongly anisometric micellar aggregates, which are not observed for the case of the catanionic mixtures. From these results it can be concluded that the structure and stability of vesicles formed in these mixtures are largely controlled by the extent of the electrostatic interactions which allows modification accordingly.

Curvature elasticity of mixed amphiphilic bilayers

Elastic bending constants of mixed amphiphilic bilayers are calculated using a molecular approach. The free energy is expanded up to quadratic order in curvatures and compositions, choosing a flat symmetrical bilayer as the reference state. Bending constants are then calculated from the derivatives of the free energy evaluated at this reference state. Two-component bilayers are considered. As a novelty, the local compositions are allowed to fully relax upon bending so that the 2 monolayers are at chemical equilibrium with each other at every curvature. The compositional degree of freedom is shown to affect the bending constant k, but not the saddle-splay constant k. The influence on the membrane elastic properties of various chain structural features, such as length, volume, and stiffness, is investigated. This may prove useful to model mixed bilayers composed of hydrocarbon/hydrocarbon and hydrocarbon/fluorocarbon chains.

Fluorescence and surface-enhanced Raman study of 9-aminoacridine in relation to its aggregation and excimer emission in aqueous solution and on silver surface

Fluorescence spectroscopy and surface-enhanced Raman spectroscopy (SERS) have been applied to study the aggregation and excimer emission of 9-aminoacridine (9AA) and 9-aminoacridine hydrochloride (9AA-HCl) in aqueous solution and on silver colloids. The effect of the drug concentration, pH, and chloride concentration on these processes has been investigated. The excimer emission of 9AA is connected to the dimerization of this drug in solution: the formation of 9AA dimers is greatly favored when the drug is under the amino form at neutral and acidic pH, while at alkaline pH the imino 9AA form tends to form large-size aggregates which cannot be excited to render excimer emission. 9AA is adsorbed on the silver surface under two different forms: strongly and weakly attached 9AA, each one corresponding to the different drug tautomers: imino and amino. The interaction of 9AA with silver induces a charge transfer from the adsorbate to the metal leading to a remarkable fluorescence quenching, a basicity decrease of the adsorbed drug and a considerable weakening of the dimer-excimer emission. Furthermore, an attribution of the main Raman features appearing in the SERS spectra has been proposed, providing marker bands for the imino and amino 9AA tautomers, and a mechanism for the molecular dimerization is also suggested.