Thermotropic lipid transitions

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

We stress here that although DSC is in principle a relatively straightforward physical technique, its theoretical thermodynamical and kinetic basis is not trivial but should be well understood as it applies to equilibrium and nonequlibrium thermotropic lipid phase transitions of various types and to either heat conduction or power compensation instruments. Moreover, some care must be taken in sample preparation, selection of sample size, and sample equilibration before data acquisition in the choice of suitable scan rates, starting temperatures, and ending temperatures during data acquisition and in the analysis and interpretation of the DSC thermograms obtained. An adequate treatment of these issues is not possible in this brief... 

Mention should be made here of the recently developed technique of pressure perturbation calorimetry (PPC), which measures the temperature-dependent volume change of a solute or colloidal particle in aqueous solution. PPC can also be used to detect thermotropic phase transitions in lipid model membranes and to characterize the accompanying volume changes and the kinetics of the phase transition. PPC essentially measures the heat change that results from small pressure changes at a constant temperature in a high-sensitivity isothermal calorimeter. For an excellent recent review on PPC as applied to lipid systems, the reader is referred to Heerklotz (19). 

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. 

Prenner EJ, Lewis RNAH, McElhaney RN. The interaction of the antimicrobial peptide gramicidin S with lipid bilayer model and biological membranes. Biochim. Biophys. Acta 1999 1462 201-221. Papahadjonponlos D, Moscarello M, Eylar EH, Isaac T. Effects of proteins on thermotropic phase transitions of phospholipid membranes. Biochim. Biophys. Acta 1975 401 317-335. 

An illustration of a lipid-water phase diagram, in which the transitions are driven by water content, is shown in Fig. 2 (43). A similar phase sequence can be produced by changes in temperature as well, and phospholipid phase diagrams generally exhibit pronounced temperature dependence. A generalized phase sequence of thermotropic phase transitions for the typical membrane lipids can be defined (44) ... 

Membrane fluidity is determined by following anisotropic rotation of fluorescent or spin probes. Liquid-crystalline (or fluid) to gel thermotropic phase transition of lipids (Figure 1) (cf. Section 3.1.1 l)in liposomes or intact biomembranes can be followed by Fourier transform infrared (FTIR) spectroscopy or differential scanning calorimetry (DSC). 

The thermotropic phase transition temperature of a vesicle composed of a mixture of dipalmitoyl and dimyristoyl phosphatidylcholine (DPPC and DMPC, respectively) is intermediate between the phase transition temperatures of the single lipid vesicles and reflects the relative concentrations of the two lipids in the vesicle. This can be used to determine the rate of exchange of phosphatidylcholine between two unilamellar vesicles of initially pure phospholipid. 

Polar carotenoids decrease cooperativity of the main thermotropic phase transition and the pretransition of synthetic phosphatidylcholines while incorporated to membranes at low concentrations. This effect is realized by increasing motional freedom of lipid molecules in the ordered phase and decreasing the rate of lipid motion in the fluid phase at temperatures above the phase transition. The decrease of the cooperativity of the phase transition is a concentration-dependent process and leads to a complete removal of the phase transition at concentrations as high as 10 mol% carotenoid with respect to lipid (Subczynski et al. 1992, 1993 Strzalka and Gruszecki, 1994). Eigure 6 presents the effect ofviolaxanthin on the phase transition of the series of PC membranes monitored by 5-SASL ESR technique. 

As mentioned above, thermotropic phase transitions may be brought about at constant pressure by adding or removing heat. (In biology, atmospheric pressure is considered constant for all practical purposes.) This is the case when ice is heated up to give liquid water, or when a lipid bilayer in the Lp phase is heated up to give an La phase. In thermodynamics, the amount of heat gained or lost by a system at constant pressure is called enthalpy (H). We can write ... 

As was mentioned in previous sections, mixing lipids, and particularly sterols, often has the effect of broadening the phase transitions. Intrinsic proteins have the same effect. Thus it is not surprising that cell membranes, containing highly complex mixtures of lipids and proteins, do not usually exhibit measurable thermotropic phase transitions. With a few exceptions, transitions in cell membranes can only be seen with high-sensitivity DSC instruments. 

Observations of thermotropic phase transitions even in simple lipid mixtures are complicated by factors such as nonideal miscibility, coexistence of different phases under certain conditions, and specific interactions between lipids, leading to complex formation. A well-studied, yet incompletely understood, binary lipid mixture that exemplifies these difficulties is the one formed by phosphatidylcholine and cholesterol. 

John R. Silvius, Thermotropic Phase Transitions of Pure Lipids in Model Membranes and Their Modifications by Membrane Proteins , in Lipid-Protein Interactions, John Wiley Sons, Inc. New York, 1982. 

Silvius, J.R. Thermotropic phase transitions of pure lipids in model membranes and their modification by membrane proteins, in P.C. lost and O.H. Griffith (eds.), Lipid-Protein Interactions. Vol. 2, Wiley, New York, (1982), pp. 239-281. Somerville, C. Direct tests of the role of membrane lipid composition in low-temperature-induced photoinhibition and chilling sensitivity in plants and cyanobacteria. Proc. Natl. Acad. Sci. U.S.A. 92 (1995), 6215-6218. 

NIST Standard Reference Database 34, Lipid Thermotropic Phase Transition Database, National Institute of Standards and Technology, Gaithersburg, MD, 221/ A320. 

Our understanding of the thermotropic behavior of the lamallae lipids is better. Four transitions near 40 C, 70 C, 80 C, and 95 C have been detected by differential scanning calorimetry [52]. The two lowest transitions reflect lipid-water and lipid-lipid interactions because they are reversible and are missing in lipid-extracted stratum comeum. The transition at the highest temperature is irreversible and is believed to reflect protein denatur-ation. Finally, the transition at 80 C is attributed to lipid-protein interaction because it is missing from lipid-extracted stratum comeum. The relevance of these lipid transitions to the permeability properties of the skin will be discussed in Section V. 

Fully hydrated bilayers composed of a single phospholipid species undergo a well-defined thermotropic phase transition (see Fig. 2.1) in which the lipid chains change from an ordered, pseudocrystalline or gel state to a fluid or liquid crystalline state which is similar to that found in biological membranes. The lipid chain configuration in the high-temperature, fluid phase is that described in the final section of the previous chapter. In the low temperature, ordered phase, the chains are essentially in the parallel, all-trans state,and may be tilted relative to the bilayer normal, the state as in phosphatidylcholines, or untilted the Lq state as in phosphatidylethanolamines. Additionally, an inter-... 

Evidence presented throughout the course of this review has emphasized the interaction of proteins and lipids in carrying out membrane-associated function. Thus, it is no longer admissible to conceive of lipids as acting merely as a matrix in which proteins exist rather, there is evidence that proteins can immobilize the lipids which bind them and that lipids can influence the temperature dependence of a number of membrane-protein functions. This mutual interaction of lipids and proteins is often viewed in the context of solvent effects. To a significant degree, this point of view derives from studies based on thermotropic phase transitions occurring in biomembranes. This subject has recently been discussed by Melchior and Steim (1976), Papa-hadjopoulos et al. (1975), and Lee (1977). They cite abundant evidence... 

The existence of phospholipid bilayers in biological membranes has since been well established by numerous experimental data using newly improved methods. Melchior and Steim (1976) have observed thermotropic phase transitions in membrane lipids and have postulated that these transitions come from a melting of hydrocarbon chains associated with one another. While lipids might exist in one of several liquid-crystalline phases, the physical data indicate that a bilayer is the most probable configuration. Other physical data, obtained by differential thermal analysis, NMR spectroscopy. X-ray diffraction, and light microscopy, support the view that the reversible thermotropic-gel-liq-uid-crystal phase transition arises from the melting of the hydrocarbon interiors of lipid bilayers (Chapman, 1970 Oseroff et al., 1973). 

Thermotropic phase transitions of pure lipids in excess water... 

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