Lipids, thermal phase transition

Small-angle X-ray diffraction was used to identify the time-averaged location of amiodarone in a synthetic lipid bilayer. The drug was located about 6 A from the center of the lipid bilayer (Figure 4.13) [125, 126]. A dielectric constant of k = 2, which is similar to that of the bilayer hydrocarbon region, was used to calculate the minimum energy conformation of amiodarone bound to the membrane. The studies were performed below the thermal phase transition and at relatively low hydration of lipid. The calculated conformation differed from that of the crystal structure of amiodarone. Even though the specific steric effects of the lipid acyl chains on the confor-... 

Theoretical Calculations.- A molecular interpretation of the chain-length dependent thermotropic behaviour of saturated symmetrical-chain phosphatidylcholine bilavers has been put forward. Thermodynamic parameters of the thermal phase-transition were found to be linearly related to a perturbation parameter and could be used to predict the minimum number of carbon atoms in the acvl chain needed for a bilayer phase-transition to occur. A model has been developed, consistent with NMR data, for hydrocarbon-chain dynamics in lipid bilayers. Involving concerted rotations around at least two C-C bonds at a... 

Raison, J.K. and Wright, L.C. (1983) Thermal phase transitions in the polar lipids of plant membranes. Their induction by disaturated phospholipids and their possible relation to chilling injury. Biochim. Biophys. Acta 731, 69-78... 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

This chapter focuses on some aspects of phase transition behavior and other material properties of starch, particularly as they pertain to the structural order and interactions of the starch polysaccharides with water, lipids and other solutes. Understanding the thermally induced structural transitions of starch is helpful in controlling its physical properties and processing behaviors (e.g. plasticization, viscosity), as well as in designing products with improved properties (e.g. texture, stability). 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

A DSC heating scan of a fully hydrated aqueous dispersion of dipalmitoylphosphatidylcholine (DPPC), which has been annealed at 0°C for 3.5 days, is displayed in Fig. 2. The sample exhibits three endothermic transitions, termed (in order of increasing temperature) the subtransition, pretransition, and main phase transition. The thermodynamic parameters associated with each of these lipid phase transitions are presented in Table 1. The presence of three discrete thermotropic phase transitions indicates that four different phases can exist in aimealed, fully hydrated bilayers of this phospholipid, depending on temperature and thermal history. All of these phases are lamellar or bilayer phases differing only in their degree of organization. 

The passive permeability of lipid membranes is another fluidity related parameter. In general, two mechanisms of membrane permeability can operate in the membrane (8). For many nonpolar molecules, the predominant permeation pathway is solubility-diffusion, which is a combination of partitioning and diffusion across the bilayer, both of which depend on lipid fluidity. In a few cases, such as permeation of positively charged ions through thin bilayers, an alternative pathway prevails (9, 10). It is permeation through transient pores produced in the bilayer by thermal fluctuations. This mechanism, in general, correlates with membrane fluidity. However, for model membranes undergoing the main phase transition, permeation caused by this mechanism exhibits a clear maximum near the phase transition point (11). 

Differential scanning calorimetry is a well known technique in the study of the thermal behavior of lipids and can be used to assess purity and stability of lipids, perturbation of aggregate structures, phase transition temperatures, lipid mixing behavior, and influence of other molecules and ions on structure. ... 

The phase transition of lipid bilayers which comprise phospholipid mixtures, or phospholipids with different lengths of acyl chains, are of great importance due to their similarities with biomembranes or with lipid drug carriers such as liposomes, it is important to study the phase transitions and to detennine the exact crystalline mesophases of the mixed system. Thermal analysis studies indicated that the mi.xed lipids provide higher melting temperatures compared to those of pure lipids. This phenomenon occurrs w hen the lengths of the acyl chains are substantially different. 

Hendrich et al. [155] investigated the mechanism of incorporation of phe-nothiazines in the membrane bilayer lipids. They studied the influence of a particular phenothiazine derivative, TFZ, on the thermal properties of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylethanolamine by microcalorimetry. The main phase transition of both lipids was affected by this drug, depending on its concentration. The results suggest that TFZ was probably incorporated into both dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylethanolamine bilayers. The phase separation was presumably induced by the different modes of the drug-bilayer interactions of protonated and unprotonated forms of TFZ. Only phosphatidylcholine, which possesses polar heads less densely packed in bilayers than phosphatidylethanolamine ones, was able to distinguish between the different protonated forms of TFZ. 

Temperature has a dramatic and highly non-trivial effect on SPLA2 activation in the region of the main phase transition of saturated phospholipid bilayers [17, 19] (Fig. 3.2). As noted above, this is caused by dramatic lateral structural changes in the lipid bilayer [28]. It is possible to take advantage of this physical effect as a thermally activated release trigger mechanism in the biophysical drug-delivery model system, as illustrated by the data displayed in Fig. 3.6. As the temperature approaches the main phase transition temperature at 41 C of the DPPC Upid bilayer, the rate of calcein release is dramatically enhanced as quantified by the time of 50% calcein release (insert in Fig. 3.6). 

This expression reduces to the classical Clausius-Clapeyron equation when the difference in compressibility, thermal expansion and heat capacity vanish as is observed for most phase transitions in lipids [80]. 

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