Differential Scanning Densitometry Research Articles

Cubic phases in phosphatidylethanolamine dispersions: Formation, stability and phase transitions

The non-lamellar phases formed by membrane lipids in diluted aqueous dispersions are mainly represented by the inverted hexagonal phase, HII, and phases of cubic symmetry, among them the bicontinuous cubic phases Pn3m (Q224), Im3m (Q229) and Ia3d (Q230). Here we report X-ray diffraction data on phosphatidylethanolamine (PE) dispersions forming highly stable Im3m and Pn3m cubic phases at ambient temperature as a result of a temperature cycling through the Lα − HII transition and complement the structural characterization of the PE phase transitions with thermodynamic data obtained by differential scanning calorimetry and differential scanning densitometry. All studied PEs displayed irreversible two-state Im3m → Pn3m phase transitions in the range ∼75–85°C with enthalpy of ∼100cal/mol. By contrast with the Lα − HII transition, the Im3m → Pn3m transition was not accompanied by a stepwise change of the specific volume. The cubic phases induced in dipalmitoleoyl PE dispersions are of particular interest because of their facile formation, especially in the presence of small amounts of charged lipid admixtures, and long-term stability at physiologically relevant conditions in a broad temperature range around room temperature.

Properties of mixed monolayers and bilayers of long-chain 5- n-alkylresorcinols and dipalmitoylphosphatidylcholine

5- n-Alkylresorcinols with different chain length are known to occur in different plants and procaryonts, for example in the cysts of the Azotobacter genus. To study the monolayer and bilayer behaviour of these amphiphilic compounds, we synthesized three analogues with different chain length, namely 5- n-octadecylresorcinol (18AR), 5- n-nonadecylresorcinol (19AR) and 5- n-heneicosylresorcinol (21AR). The pure alkylresorcinols form stable monolayers, 18AR and 19AR, show a phase transition from a liquid-condensed to a liquid-expanded state at temperatures between 20 and 40°C and at lateral pressures below 15 dyn/cm. In mixtures with dipalmitoylphosphatidylcholine (DPPC), monolayer isobars at higher lateral pressures (20–30 dyn/cm) show an increase in the transition temperature to values greater than 50°C for the liquid-condensed to liquid-expanded state of DPPC when the alkylresorcinol content is increased. Particularly in the liquid-expanded state, alkylresorcinols show a strong condensing effect on DPPC monolayers. 18AR/DPPC monolayers at a pressure of 5 dyn/cm show indications of an azeotropic behaviour. A similar increase in transition temperature is observed by differential scanning calorimetry (DSC) and differential scanning densitometry (DSD) in DPPC/alkylresorcinol bilayers. The transition enthalpy decreases with increasing alkylresorcinol content. All alkylresorcinols seem to be completely miscible with DPPC up to at least 50 mol%. The DSD data indicate a slight condensing effect of the alkylresorcinols on the liquid-crystalline phase of DPPC.

Polymerizable phospholipids with lipoic acid as head group: synthesis and phase properties

Lipoic acid-substituted phospholipids were synthesized from phosphatidyl cholines by exchanging the choline residue with a glycol group using phospholipase D, forming phosphatidylglycols, and coupling of lipoic acid (1,2-dithiolane-3-pentanoic acid) to the residual glycol hydroxyl group by esterification. The thermotropic properties of aqueous dispersions of the phosphatidylglycols and the phosphatidylglycolipoates with different chain lengths were studied by differential scanning calorimetry (DSC) and differential scanning densitometry (DSD). The exchange of choline with glycol increases the phase transition temperature T m for the gel to liquid-crystalline phase transition by ∼ 5°, independent of the chain length. Consecutive esterification of the glycol residue with lipoic acid causes a decrease of the transition temperature T m by ∼ 22° compared with the phosphatidylglycols. Polymerization of vesicles prepared from phosphatidylglycolipoates was induced by the addition of dithiothreitol and followed by DSC and DSD. Polymerization has only slight influence on the transition behavior, indicating that the polymer chain in the head group is effectively decoupled from the hydrocarbon region. Polymerized vesicles show increased stability towards aggregation and fusion.

Stereochemistry and size of sugar head groups determine structure and phase behavior of glycolipid membranes: densitometric, calorimetric, and x-ray studies

The role carbohydrate moieties play in determining the structure and energetics of glycolipid model membranes has been investigated by small- and wide-angle X-ray scattering, differential scanning densitometry (DSD), and differential scanning microcalorimetry (DSC). The dependence of a variety of thermodynamic and structural parameters on the stereochemistry of the OH groups in the pyranose ring and on the size of the sugar head group has been studied by using an homologous series of synthetic stereochemically uniform glyceroglycolipids having glucose, galactose, mannose, maltose, or trimaltose head groups and saturated ether-linked alkyl chains with 10, 12, 14, 16, or 18 carbon atoms per chain. The combined structural and thermodynamic data indicate that stereochemical changes of a single OH group in the pyranose ring can cause dramatic alterations in the stability and in the nature of the phase transitions of the membranes. The second equally important determinant of lipid interactions in the membrane is the size of the head group. A comparison of lipids with glucose, maltose, or trimaltose head groups and identical hydrophobic moieties has shown that increasing the size of the neutral carbohydrate head group strongly favors the bilayer-forming tendency of the glycolipids. These experimental results provide a verification of the geometric model advanced by Israelachvili et al. (1980) [Israelachvili, J. N., Marcelja, S., & Horn, R. G. (1980) Q. Rev. Biophys. 13, 121-200] to explain the preferences lipids exhibit for certain structures. Generally galactose head groups confer highest stability on the multilamellar model membranes as judged on the basis of the chain-melting transition. This is an interesting aspect in view of the fact that galactose moieties are frequently observed in membranes of thermophilic organisms. Glucose head groups provide lower stability but increase the number of stable intermediate structures that the corresponding lipids can adopt. Galactolipids do not even assume a stable intermediate L alpha phase for lipids with short chain length but perform only Lc----HII transitions in the first heating. The C2 isomer, mannose, modifies the phase preference in such a manner that only L beta----HII changes can occur. Maltose and trimaltose head groups prevent the adoption of the HII phase and permit only L beta----L alpha phase changes. The DSD studies resulted in a quantitative estimate for the volume change associated with the L alpha----HII transition of 14-Glc. The value of delta v = 0.005 mL/g supports the view that the volume difference between L alpha and HII is minute.(ABSTRACT TRUNCATED AT 400 WORDS)

Metastable behaviour of saturated phosphatidylethanolamines: a densitometric study

The specific volumes of saturated phosphatidylethanolamines having 12–18 carbon atoms per chain and the volume changes resulting from their temperatures-induced phase transitions have been investigated using differential scanning densitometry. No dependence on acyl-chain length has been found for the molar volume changes associated with the L c → L α transition, while the L β → L α transition showed ΔV-changes that increase with the length of the acyl-chains. The independence of acyl-chain length of the molar volume changes concomittant with the L c → L α transition can be rationalized by the assumption that the volume changes result from the combined effects of hydration and acyl chain melting. An interesting corollary of the present studies is the disappearance of metastability at chain lengths of about 20–22 carbon atoms.

DSC-studies of phospholipids containing iso-branched and ω-cyclohexane fatty acids

Differential Scanning Calorimetry (DSC) and Differential Scanning Densitometry (DSD) was employed to study the phase behavior of phospholipids with iso-branched and ω-cyclohexyl fatty acids. Phosphatidylcholines (PC) and phosphatidylethanolamines (PE) with iso-branched fatty acids show complicated phase behavior with several gel-gel transitions with odd-even effects. These odd-even effects are even more pronounced for phosphatidylcholines with ω-cyclohexyl fatty acids (cyPCs). Odd-numbered cyPCs show a large main transition with strong hysteresis while the even-numbered cyPCs have several transitions but show no hysteresis. These differences in thermotropic behavior are also reflected in their molecular volumes. The odd-numbered cyPCs display much tighter packing in the gel phase than the even-numbered analogues. Binary mixtures of cyPCs with either straight chain PCs or iso-branched PCs show eutectic behavior. This is also observed when cyPCs of different chain length are mixed.