Lipid melting

Lipid Melting Temperatures Membrane lipids in tissue samples obtained from different parts of the leg of a reindeer have different fatty acid compositions. Membrane lipids from tissue near the hooves contain a larger proportion of unsaturated fatty acids than those from tissue in the upper leg. What is the significance of this observation ... 

Gibbs, A. G. (2002). Lipid melting and cuticular permeability new insights into an old problem../. Insect Physiol., 48, 391 400. 

Figure 6.3 Effects of hydrocarbon chain modifications on melting points of similar-sized cuticular lipids. When lipids melt, the absorption frequency of C-H symmetric stretching vibrations increases from -2849 cm1 to -2854 cm4. From right to left, compounds are (chemical change relative to n-alkane, molecular mass in daltons) filled circles, n-dotnacontane (no change, 450) open circles, palmitic acid myristyl ester (wax ester, 452) filled triangles, 13-methylhentriacontane (methyl-branched alkane, 450) open triangles, (Z)-13-tritriacontene (double bond, 462) filled squares, 9,13-dimethylhentriacontane (2 methyl branches, 464) open squares, oleic acid oleyl ester (2 double bonds and an ester link, 532). Data from Gibbs and Pomonis (1995) and Patel el al. (2001).

Finally, even when HC composition and cuticular transpiration are correlated, causation cannot be assumed. For example, higher cuticular water-loss rates in the desert ant, Pogonomyrmex barbatus, are correlated with a decrease in abundance of an n-alkane and an increase in a methylalkane (Figure 6.2 Johnson and Gibbs, 2004). This is exactly what one would expect if lipid melting points affect cuticular permeability, but this increase is also accompanied by a change in mating status. Mated, de-alate queens that have founded... 

Permeability changes in full-thickness skin have been associated with temperature or solvent pretreatment. The molecular basis of these permeability changes has been attributed to lipid melting or protein conformational changes. The current studies have probed the role of lipid fluidity in the permeability of lipophilic solutes, and examined the effects of temperature on the physical nature of the stratum corneum by differential scanning calorimetry and thermal perturbation infrared spectroscopy. Combining molecular level studies that probe the physical nature of the stratum corneum with permeability results, a correlation between flux of lipophilic solutes and nature of the stratum corneum barrier emerges. 

In the solvent method the separation of the solubilised or dispersed material from the solvent phase can be explained by precipitation or phase change induced by solvent evaporation, addition of electrolyte, pH modification or heat treatment (Krochta and McHugh 1997). Such treatments can be adjusted to enhance film formation or specific properties. For composite emulsion-based films or coatings a lipid material and most likely a surfactant, is added to the solution, which is then heated above the lipid melting point and homogenised. The prepared solution is then applied on an appropriate support and the solvent evaporates. 

Additionally, we would expect a lipid melting transition to be accompanied by a change in the degree of molecular order and hence accompanied by a change in entropy. For a reversible two-state transition, T represents the temperature at which both phases are in equilibrium and hence the free energy change (AG) associated with the process is zero. The expression... 

Apart from different preparation procedures, factors determining the loading capacity of a drug in lipid carriers have been found to be the matrix composition and thus the solubility of drug in melted lipid, the miscibility of drug melt and lipid melt, the chemical and physical structure of the solid lipid matrix, and the polymorphic state of the lipid material [16],... 

C. butyricum appears to regulate the stability of the bilayer arrangement of membranes by altering the ratio of ether versus acyl ethanolamine phospholipids in response to changes in the degree of lipid unsaturation of the membranes. Experiments with bacteria indicate that substitution of plasmenylethanolamine for phosphatidylethanolamine in biomembranes would have only small effects on lipid melting transitions, whereas the tendency to form non-lamellar lipid structures would be significantly increased. 

In the microemulsion method, the lipid component (fatty acids and/or glycerides) is melted, a mixture of water, cosurfactant(s) and a surfactant is heated to the same temperature as the lipid and added under mild stirring to the lipid melt. Once a thermodynamically stable miaoemulsion is produced, it is dispersed in cold aqueous media under constant mechanical mixing to avoid any... 

In the last decade, accurate microcalorimetric and atomic force microscopy experiments have provided detailed information on the lipid melting processes [113-118]. Interestingly, a linear relationship between the isobaric heat capacity and the volume expansion with temperature has been observed for a variety of lipids [115-118] leading to the interpretation that proportional enthalpy and volume changes at the melting transition would be driven by intrinsic structural changes within the lipid molecules, whereas the changes of free volumes and intermolecular interactions could be considered as perturbations [116]. 

Ebel H, Grabitz P, Heimburg T (2001) Enthalpy and volume changes in lipid membranes. I. The proportionality of heat and volume changes in the lipid melting transition and its implication for the elastic constants. J Phys Chem B 105 7353... 

The homogenization process can be carried out at a temperature above the melting point of the lipid (melt homogenization) or at a temperature below the melting point of the lipid (cold homogenization). 

First, the drug is dispersed or dissolved in the lipid melt and the molten lipid is then dispersed in the hot aqueous phase by high-shear mixing (e.g. Ultra-Turrax vortexing) to obtain the crude pre-emulsion. Depending on their lipophilicities, the stabilizers can be dispersed or dissolved in the water phase or in the lipid melt. The quality of the pre-emulsion strongly influences the quality of the final dispersion whereas the size of the droplets in the pre-emulsion should preferably be in the lower m-range. 

Recently Al-Saiden et al [181] examined the thermal transitions in desiccated stratum comeum membranes of neonatal rats, rabbits and adult abdominal human skin. Four endothermic transitions were observed at 39-45°C, 55-58°C, 68-74°C and 77-86°C. The transitions at 39-45°, 68-74° and 77-86°C were attributed to phase charges in the intercellular lipid bilayers. A fifth transition at 48° was seen only in the neonatal rabbit stratum corneum and was attributed to lipid melting. 

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