Gel-liquid crystal transition

The temperature of the hydration step should be above the gel-liquid crystal transition temperature of the lipid to allow the lipid to hydrate in its fluid phase with adequate agitation. Hydration time may differ slightly among lipid species and structures. We believe that good hydration prior to sonication makes the sizing process easier and improves the homogeneity of the preparation. 

We note here that systematic studies of the melting transition of dry or nearly dry phospholipids bilayers (e.g., vesicles) have been scarce. While there is an abundant experimental and theoretical literature concerning the structure and properties of bilayers in water, less is known about their behavior when water is removed. We have therefore initiated a systematic experimental study of the gel-liquid crystal transition of pure DPPC and DPPC-cholesterol vesicles freeze-dried with and without disaccharides and oxyanion-disaccharide complexes. Some of our results to date are shown in Figure 9.3. 

In multicomponent systems difficulties arise from the overlapping of C-H features due to chemically different lipids, or in hpid-protein arrays, from protein C-H stretching bands. In these systems it becomes impossible to monitor a single lipid component. Mendelsohn et al. [64] showed how this problem csai be overcome by the use of deuterated components. They inserted a completely deuterated fatty acid into a model membrane system and followed the C-D stretching vibrations in the spectral window 2000-2220 cm which is uncluttered by modes from other components. As the membrane passed through a gel-liquid crystal transition the line-width of the C-D stretching vibrations of the bound fatty add was found to be a sensitive probe of membrane polymethylene chain order. 

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. 

The gel-liquid crystal transition in surfactant-based systems is the result of the cooperative melting of the hydrocarbon chains [58]. The melted state in a surfactant-based system is less disordered than that of liquid hydrocarbons due to the anchoring of one end of the molecule to the microstructure surface via its polar headgroup. This transition can be sharp in pure synthetic phospholipids and surfactants, but it is broad and ill-defined in natural phospholipids and surfactants, which are usually mixtures with a variety of hydrocarbon chain lengths. The transition temperature, T, depends on the nature of the polar headgroup and the... 

The hydrocarbon chain melting in surfactant-based systems may give rise to the formation of liquid crystals or molecular or micellar solutions, depending on concentration. The addition of water to surfactant decreases and the enthalpy associated with the transition. This is probably due to a reduction in the cohesion of the polar headgroup network. The effect is especially strong in transitions from solid to solid plus liquid crystals. In pure dioctadecyldimethylammonium bromide (DODAB), the polar layer melts at 86.5°C with a A// of 130 J/g. The transition temperature decreases as water content increases and disappears at about 75% DODAB. At this point, the gel-liquid crystal transition happens at a temperature that is independent of water content [46]. 

The inclusion of cholesterol disturbs the crystalline structure of the gel phase, and the phospholipid chains are more mobile than in its absence. This prevents the crystallization of the hydrocarbon chains into the rigid crystalline gel phase. In the more fluid liquid crystalline phase, the rigid cholesterol molecules restrict the movement of the hydrocarbon chains. In consequence, the addition of cholesterol to lipid bilayers or lamellar mesophases gradually diminishes the gel-liquid crystal transition temperature and the enthalpy and broadens the DSC transition peak [72,73]. No transition can be detected by DSC at 50% cholesterol [73,74] (curve/of Fig. 7), which is the maximum concentration of cholesterol that can be incorporated before phase separation. However, laser Raman spectroscopic studies show that a noncooperative transition occurs over a very wide temperature range [75]. 

In some cases, water transitions may be misinterpreted as other types of transitions. This has been the case for the transition at 0°C in the system Aerosol OT (AOT)-water, which was interpreted as the gel-liquid crystal transition. Using D2O instead of water shifted the transitions to 4°C, which demonstrated that these peaks are due to water melting [62,181]. AOT does not form a gel phase up to - 150°C. Another way to identify the melting transition of water is to plot the enthalpy per gram of sample versus composition, giving a value of 319.6 1/ g at 100% water, which corresponds to the melting enthalpy of pure water. 

The results of differential scanning calorimetry(DSC) indicate the change in aggregation state. The trans micelle showed a main endothermic peak at 14 2°C(A H =1.0 kcal/mol), corresponding to a gel-liquid crystal phase transition, whereas the transition temperature for the cis micelle appeared at 11.9°C( AH = 0.8 kcal/mol). This is unequivocal evidence that the trans-cis photoisomerization is a sufficient perturbation to alter the state of molecular aggregation. 

Solutions at concentrations above the cmc may contain significant concentrations of both monomeric and micellar surfactant. Previous researchers have used FT-IR to investigate monomer-to-micelle transitions (27) and gel-to-liquid crystal transitions of lipid bilayers (28,29). These studies have demonstrated that, in the case of such two-state transitions, linear combinations of the infrared spectra of the initial and final states characterize the spectra of the intermediate states, where both forms coexist. However, changes in band frequency or width are not necessarily linear with the extent of the transition. Linear combinations of two highly overlapped Lorenztion bands can give rise to non-linear shifts in the band frequency and width (27-29). 

The rate and character of the molecular motions of both the molecules embedded in the lipid bilayer and lipid molecules themselves are strongly dependent on the temperature [19, 203], At a certain temperature tm, the gel-liquid crystal phase transition is known to occur for the membrane made of a synthetic lipid. For example, tm = 41.5 °C for the membranes from DPL. In the vesicles formed by a mixture of lipids, e.g. egg lecithin, the phase transition occurs smoothly rather than jumpwise and starts below 0 °C. Note that the permeability of lipid membranes increases notably upon transition from the liquid crystal state to the gel state [204]. 

The lipid multilayers with a homogeneous composition generally show a transition of gel-liquid crystal. When the temperature is raised to 42 °C, which is higher than the phase transition of 41.4 °C, the released amount of 5-FU increased, while the amount of drug delivered decreased at 37 °C, which is lower... 

Albon N, Sturtevant JM. Nature of Gel to Liquid-Crystal Transition of Synthetic Phosphatidylcholines. Proc. Natl. Acad. Sci. U. 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

The DSC thermogram of the liposome-embedded heme was measured to estimate the phase transition of the lipid bilayer of the liposome-embedded heme . The DMPC-liposome showed the endothermic peak at 24 °C, which was corresponding to the gel-liquid crystal phase transition temperature (T,) of the liposome . But for the liposome in which a simple heme such as I was embedded, the phase transition peak was broadened and shifted to lower temperature (22 °C). On the other hand, the peak was also observed at 24 °C for the DMPC liposome/lipid-heme. This suggests that the orientation of the phospholipid in the liposome is equivalent for the DMPC-liposome and the DMPC liposome/lipid-heme and that the compatibility of the lipid-heme with the phospholipid is large enough to form a stable liposome. 

M. J. Blandamer, B. Briggs, P. M. CuUis, B. J. Rawlings, J. B. F. N. Engberts, Vesicle-cholesterol interactions effects of added cholesterol on gel-to-liquid crystal transitions in a phosphohpid membrane and five dialkyl-based vesicles as monitored using DSC, Phys. Chem. Chem. Phys., 2003, 5, 5309-5312. 

Figure 7.8. The change of stmcture at the gel to liquid crystal transition.

K.-I., and Kambara, T. (1993) A model for self-sustained potential oscillation of lipid bilayer membranes induced by the gel-liquid crystal phase transitions. Biophys.]., 64, 1461-1475. 

Fujiwara et al. [147] used DSC to follow the interactions between multilamel-lar vesicles of dipalmitoylphosphatidylcholine (1.4 mM) and polyacrylic acid as a function of pH. The thermograms showed the gradual disappearance of a small pretransition peak at 35 C and of the main transition peak (sharp) at 41°C, with the simultaneous appearance of a broad peak at 44°C as the pH was lowered from 7.6 to 3.8. At the latter pH value, only the 44°C peak was seen (Fig. 16). The peak at 41°C corresponds to pure vesicles, whereas the one at 44°C is due to the vesicles interacting with polyacrylic acid. The sum of the enthalpies of these peaks remained constant. Hence, the gel to liquid crystal transition tempera-... 

Finally, we discuss the role of interlamellar water in lipid phase transitions. As shown in Fig. 36, the phase behavior of the lipid in the DMPE-water system is complex in the absence of freezable interlamellar water [21], Presumably, in a region of such low water content, the lipid bilayers exist as hydrated crystals containing only nonfreezable interlamellar water. However, with the appearance of freezable interlamellar water (curves d-m), the lipid phase transition comes to be characterized by a certain peak that is gradually shifted to lower temperatures with increasing water content and finally converges to a fixed temperature, generally ascribed to the gel-to-liquid crystal phase transition. Such phase behavior suggests that freezable interlamellar water is absolutely necessary for the formation of the gel phase of lipid-water systems. In this respect, another noticeable point is that the fixed peak of the gel-to-liquid crystal transition is obtained above a certain water content where a maximum uptake of the freezable interlamellar... 

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