Incomplete lipid chain freezing of sonicated vesicular dispersions of double-tailed ionic surfactants

Structural insights on biologically relevant cationic membranes by ESR spectroscopy

Cationic bilayers have been used as models to study membrane fusion, templates for polymerization and deposition of materials, carriers of nucleic acids and hydrophobic drugs, microbicidal agents and vaccine adjuvants. The versatility of these membranes depends on their structure. Electron spin resonance (ESR) spectroscopy is a powerful technique that employs hydrophobic spin labels to probe membrane structure and packing. The focus of this review is the extensive structural characterization of cationic membranes prepared with dioctadecyldimethylammonium bromide or diC14-amidine to illustrate how ESR spectroscopy can provide important structural information on bilayer thermotropic behavior, gel and fluid phases, phase coexistence, presence of bilayer interdigitation, membrane fusion and interactions with other biologically relevant molecules.

The effect of an oligonucleotide on the structure of cationic DODAB vesicles

The effect of a small single-stranded oligonucleotide (ODN) on the structure of cationic DODAB vesicles was investigated by means of differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and electron spin resonance (ESR) spectroscopy. ODN adsorption induced coalescence of vesicles and formation of multilamellar structures with close contact between lamellae. It also increased the phase transition temperature by 10 °C but decreased transition cooperativity. The ODN rigidified and stabilized the gel phase. In the fluid phase, a simultaneous decrease of ordering close to the bilayer surface and increase in bilayer core rigidity was observed in the presence of the ODN. These effects may be due not only to electrostatic shielding of DODAB head groups but also to superficial dehydration of the bilayers. The data suggest that oligonucleotides may induce the formation of a multilamellar poorly hydrated coagel-like phase below phase transition. These effects should be taken into account when planning ODN delivery employing cationic bilayer carriers.

1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-rich domain formation in binary phospholipid vesicle membranes: two-dimensional nucleation and growth

Decades of study have probed phase transitions in model phospholipid bilayers and vesicles, especially in the context of the equilibrium phase diagram. Critical to the response of vesicles to environmental triggers, to the ultimate domain morphology, and to the approach to equilibrium (or not), we present here a study of domain formation in vesicles, focusing on a mechanism by which the cooling rate, tension, and composition affect the first appearance (nucleation) and subsequent growth of solid membrane domains. Employing a popular mixed membrane model based on DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively), we examined phase separation in giant two-component vesicles that were cooled from the one-phase fluid (Lα) region of the phase diagram into a region of fluid (Lα)-solid coexistence. At moderate and low membrane tensions, cooling produced solid DPPC-rich domains appearing as compact patches or irregular hexagons and likely with a Pβ' (ripple) arrangement. (The compact solid domains in this study differed distinctly from striped domains in vesicles of the same composition, in terms of molecular organization and conditions of first appearance during cooling.) The amounts of these solid domains were shown to adhere to the lever arm rule for a tie line on the phase diagram, with a solid composition near 95 mol % DPPC. The nucleation of the compact solid domains occurred in a short period, followed by rapid addition of ordered molecules to the nucleated domains, excluding tracer dye. The two-dimensional nucleation density of these compact solid domains (in the range of 10(-2)-10(-1) μm(-2)) was found to increase with the cooling rate (equivalent to the quench depth) with a greater than linear dependence. The 2-D nucleation density was also seen to decrease with membrane tension, presumably because membrane tension increases the line tension around a domain that opposes nucleation. A sigmoidal dependence of the nucleation density on the DPPC concentration was also found. With cooling rates in excess of ∼1 °C/min, solid domains persisted down to room temperature, likely passing from a preferred equilibrium to a local equilibrium with continued cooling. As a result of the persistence of the originally nucleated domains and the conservation of DPPC in the membrane, we observed an increasingly greater number of smaller domains with increased cooling rates. The domains in these vesicles were compact or hexagonal-shaped in contrast to flower-shaped dendritic domains in the same membrane system in a supported membrane configuration.

Spectroscopic investigation of fluorinated phenols as pH-sensitive probes in mixed liposomal systems

The pK_a values of three fluorinated phenols, 2,4,6-trifluorophenol (3FP), 2,3,5,6-tetrafluorophenol (4FP) and 2,3,4,5,6-pentafluorophenol (5FP) have been measured by using UV-vis and ¹⁹F-NMR spectroscopy at 25 °C in water and in the presence of pure POPC, pure DDAB and mixed POPC–DDAB liposomes.

Enclosed volume determination of concentrated dioctadecyldimethylammonium chloride (DODAC) vesicular dispersions by low-resolution proton NMR diffusometry and T2 relaxometry

The enclosed volume of concentrated dioctadecyldimethylammonium chloride (DODAC) dispersions has been determined by means of low-resolution NMR pfg-diffusometry and T(2) relaxometry. The pfg-NMR diffusometry method is based on the different diffusion behaviors of water in the external and internal phases and as such does not require the addition of a tracer. On the other hand, T(2) relaxometry is based on the different relaxation behaviors of water fractions upon addition of manganese chloride as external (paramagnetic) probe. It was noticed that reliable results are found only for temperatures below the phase transition temperature of DODAC, when the exchange between the two water compartments can be neglected. At 5 °C, these two independent methods resulted in similar enclosed volume values, meaning that the results are reliable and reflect the real enclosed volume. In addition, the T(2) relaxometry method has been proven to be useful in the investigation of the DODAC membrane permeability.

Influence of the vesicular bilayer structure on the sorption of ethylbenzyl alcohol

The influence of the physicochemical properties of the vesicular bilayer on the sorption of poorly water soluble compounds was investigated with pulsed field gradient 1H nuclear magnetic resonance (PFG-NMR) for the case of phosphatidylcholine and dioctadecyldimethylammonium bromide (DODAB), using 4-ethylbenzyl alcohol as a model compound. Hereby, the effect of bilayer thickness at a constant physicochemical state was studied using a range of phosphatidylcholines of varying chain lengths, whereas DODAB was preferred to check the influence of the bilayer physicochemical state since this cationic lipid is characterized by three different states within the studied temperature range. When the phospholipid alkyl chain length was changed, no differences were observed in the sorption which was linked to the surface-mediated sorption. On the other hand, when the chemical composition was preserved but the temperature and thus the physical state of the bilayer were changed, the sorption in dioctadecyldimethylammonium bromide (DODAB) vesicles changed dramatically. From those experiments, a strong relationship between the ordering of the surfactant molecules and the sorption can be assumed.