Lipid transitions

Comparson of the transitions observed by differential scanning calorimetry in membranes of M. laidlawii and in water dispersions of the lipids from the membranes support the concept that most of the lipids exist as a smectic mesophase in the membranes. The evidence for a bilayer structure is straightforward in this case. Lipid transition temperatures are a function of fatty acid composition and correlate well with biological properties. The calorimeter possesses advantages over high resolution NMR for M. laidlawii, and perhaps in many other systems, because the data can be interpreted less ambiguously. In M. laidlawii membranes the bilayer appears to be compatible with the same physical properties observed in other membranes—a red-shifted ORD, lack of ft structure in the infrared, reversible dissociation by detergents, and poorly... 

Release of liposome-encapsulated CF from HA/PLL films has been observed at temperatures above the lipid transition temperature (Fig. 4f). Below this temperature, the vesicles were stable at least for a few hours. The polyelectrolyte network destabilizes the embedded vesicles, which show higher lipidic bilayer permeability upon heating than do vesicles in solution [84], No change in film properties upon heating has been reported as proof of the polyelectrolyte destabilization effect. 

Thermally induced permeability enhancement of the more lipophilic solutes (butanol, octanol and hydrocortisone) through hairless mouse stratum corneum occurred in the temperature range also associated with lipid transitions in the calorimetry studies. Therefore, it seems likely enhanced permeabilities and lipid mobility within the stratum corneum are correlated. However, these macroscopic studies are unable to provide more specific information concerning the molecular origins of the thermal transitions. The studies provide even less information concerning possible irreversible alterations of the keratinized protein components of the stratum corneum. 

While a temperature-dependent IR spectrum allows one to examine specific elements of a transition, a DSC thermogram enables the visualization of transitions in their entirety and the calculation of associated thermodynamic parameters. The IR and DSC thermal profiles for identically treated samples of hydrated porcine SC are shown in Fig. 3. The results of a series of thermograms for intact, delipidized, fractionated, and reheated SC as well as extracted lipids suggest that these three major transitions near 60,70, and 95°C in intact SC are due to intercellular lipid, a lipid-protein complex associated with the comeocyte membrane, and intracellular keratin, respectively. Evidence supporting these deductions is elegantly presented by Golden et al. [33]. More recently, the presence of a subzero lipid transition at -9°C has also been reported [34]. 

Thermal lipid transitions in the physiologic temperature range have also been reported for porcine [32] and neonatal murine [43] SC. Based on arguments similar to those presented above, the transition near 20°C in porcine SC detected by IR has been attributed, not to sebaceous lipids, but to a solid-to-fluid phase change for a small fraction of SC lipids. 

For one-step probe sonication (SUV formation) the lipid or lipids in solid form are placed in an appropriate test tube (with dimension that ensure proper placement on the probe) together with the hydration solution. The mixture is heated above the lipid transition temperature and subsequently probe sonicated. This method may not be applicable in some cases of lipids and when high lipid concentrations are used (>20 mg/mL). 

Remove ethanol by rotary evaporation, under a nitrogen stream, at a suitable temperature above the lipid transition temperature. Room temperature is suitable for this formulation. This will leave a film of lipids deposited on the wall of the flask. 

After removing the residual chloroform, dry lipid film is hydrated carefully with B-octylglucoside (50 mg.mL ) in HBS (pH 7.4) by gently swirling around the wall of the vessel. This must be carried out above the lipid transition temperature. 

DSC thermograms of embryos, defatted embryos and extracted lipids from embryos. Scanned from -125°C to 100°C at 10°C/min. Peaks in the range -90°C to 10°C correspond to lipid transitions. These peaks were independent of relative humidity (RH). This pattern was observed in the three analysed genotypes. 

Although embryos and seeds are complex systems, the thermal transitions of their main components can be analyzed by DSC. The peaks generated by the melting of freezable water (shown in Figure 43.1) interfered with the observation of lipid transitions. Therefore, the extraction of lipidic components resulted from an interesting approach to analyze these complex systems, as it allows identification of the transitions of the different components occurring at similar temperatures. 

DSC has been used to investigate thermal transitions in the SC, including thermotropic lipid transitions in the SC [2]. 

Lipid transitions in the skin have been studied by Guia et al. [2] using DSC experiments on fractionated skin samples and solvent extracts. Three major transitions were observed in all samples at 65, 80 and 95 °C as shown in Figure 2, while a small peak (not observed in all samples) is seen at 35 °C. When these samples were cooled and reheated, the transitions at 35 and 65 °C remained unchanged and the peak at 95 °C disappeared, while the peak at 80 C was decreased in size and shifted to a lower temperature. To explain these observations, fractionated samples and samples extracted using solvents were studied. On extraction, all transitions below 90°C disappeared, while the transition at 95 C remained in the extracted sample. On concentration of the extracts, a peak remained at 65 C [2]. When homy cell membranes are prepared from the stratum comeum, the thermal profile shows 2 peaks at 65 and 75 °C (as shown in Figure 3), while a reheat of this sample shows a peak at 65 C and a minor peak at 70 C. 

DSC studies supported this result shov g lipid transitions in die imtreated rat SC at 41.9, 55.1, 70.2, and 77.5 °C, while SC pre-treated with disodium hydrogen phosphate and ibuprofen showed only the last lipid transition. This confirms that ibuprofen disrupts the SC and, through its ability to act as an anionic surfactant, gives rise to self permeation enhancement [12],... 

DSC studies undertaken to investigate the effect of propylene glycol (PG), glycerol and isopropyl myristate (IPM), and combinations thereof on the SC, showed a decrease in transition temperatures of the lipid fraction. The combination of PG/IPM, affected the SC microstructure, although the decreases in lipid transition temperatures were not as much as expected fi om the results for individual PG and IPM [18]. 

Phase coexistence - based in part on the phenomenon of nonideal miscibility, but also for other reasons, real phase boundaries in the phase diagram are often not lines, but strips or areas in which two or more different phases coexist. Lipid transitions are not sharp, but have instead a finite width, i.e. they extend over a certain temperature range. Specific examples will also be presented below. 

In the maps corresponding to the lipid (Figure 10.7) a clear difference can be seen between LDL and HDL. Whereas in LDL an autopeak is seen in the synchronous map at 1636 cm and a cross-peak 1736/1742 (-F), in HDL only noise is produced. As shown earlier, (Section 10.3), in LDL one peak is changing, but not only in intensity. Infrared studies of phospholipids have shown the presence of two bands corresponding to two different hydration states of the interface carbonyl. The asynchronous map would point to a change in the lipid core where different hydration states could be involved. In the case of HDL, no lipid transition was proposed, and the 2D-IR agrees with the absence of conformational changes in the lipid in the interval 20-40 °C. 

Fig. 3. Differential scanning calorimetry scans of biomembrane transitions, all obtained with 50% ethylene glycol/water as solvent. (A) A. laidlawii membranes from cells grown in tryptose medium at 37 C (B) lysodeikticus membranes from cells grown in brain heart infusion at 37 C (C) JE. coli K12W945 whole cells grown in minimal salts with glucose at 20 C (D) the same cells as in (C), but scanned after thermal protein (E) rat liver microsomes (F) rat liver In all cases a lower temperature reversible lipid transition is followed by a higher temperature irreversible protein peak. The protein denaturation peaks are featureless in (A), (E), and (F), but show fine structure in (B), (C), and (D). Unlike other organisms, coli after heating shows two lipid transitions and residual reversible protein denaturation, as seen in (D).

Gray GM, Yardley HJ (1975) Lipid compositions of cells isolated from pig, human, and rat epidermis. J Lipid Res 16 434-440 Guy CL, Guy RH, Golden GM, Mak VHW, Francoeur ML (1994) Characterisation of low-temperature (i.e. <65 °C) lipid transitions in human stratum corneum. J Invest Dermatol... 

In an elegant modeling study of lipid bilayers, Yagisawa et al. showed that oscillations can be induced by a transmembrane pH and salt gradient, with no electrical stimulation or pressure gradients [51]. Briefly, the pH difference leads to a transmembrane dipole and electrical stress on the nonpolar interior of the bilayer, triggering a gel/liquid crystal transition. Following this transition, permeability to salt increases and there is a relaxation of electrical stress, followed by reversal of the lipid transition, restoration of membrane potential, and reinitiation of the cycle. 

Figure 7 shows a series of typical DSC curves for samples of the DPPC-water mixture with increasing wato content expressed as [(g water)/(g lipid + g wato )] X 100 and designated as The ice-melting peaks are followed by two lipid transition peaks of the gel (Lp-)-to-gel (Fp ) and subsequent gel (Fp )-to-liquid crystal phase transitions, generally called the Tp and transitions, respectively. 

---