Thermotropic phase behavior

The influence of cholesterol concentration at various temperatures on the thermotropic phase behavior and organization of saturated PE bilayers has been studied by combining DSC, FT-IR, and 31P-NMR. It was found that incorporation of low levels of cholesterol into the bilayer caused a progressive reduction in the temperature, enthalpy, and overall cooperativity of the lipid hydrocarbon chain melting transition... 

Krill, S. L., Knutson, K. and Higuchi, W. I. The stratum comeum lipid thermotropic phase behavior. Biochimica et Biophysica Acta 7772(2) 281-286, 1992. Golden, G. M. et al. Lipid thermotropic transitions in human stratum comeum. Journal of Investigative Dermatology 86 255-259, 1986. 

The pattern of thermotropic phase behavior exhibited by an aqueous dispersion of any lipid molecular species will vary considerably, depending on the length and structure of the hydrocarbon chains, the structure and charge of the polar headgroup, the nature of the link (ester or ether) of the hydrocarbon chains to the glycerol or sphingosine backbone, and other chemical features of the lipid under study. Also, the degree of hydration and the pH and ionic composition of the aqueous phase can affect lipid thermotropic phase behavior profoundly. However, even a cursory discussion of this topic is beyond the scope of this article, and the reader is referred to recent reviews for more detailed information (16-18). 

Although studies of the thermotropic phase behavior of singlecomponent multilamellar phospholipid vesicles are necessary and valuable, these systems are not realistic models for biological membranes that normally contain at least several different types of phospholipids and a variety of fatty acyl chains. As a first step toward understanding the interactions of both the polar and apolar portions of different lipids in mixtures, DSC studies of various binary and ternary phospholipid systems have been carried out. Phase diagrams can be constructed by specifying the onset and completion temperatures for the phase transition of a series of mixtures and by an inspection of the shapes of the calorimetric traces. A comparison of the observed transition curves with the theoretical curves supports... 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

The effect of cholesterol on the thermotropic phase behavior of PC bilayer also varies significantly with the structure, particularly the degree of unsaturation, of the hydrocarbon chains, with more highly unsaturated PCs exhibiting a reduced miscibility with cholesterol and other sterols. Moreover, the structure of the lipid polar headgroup is also important in determining the effect of cholesterol on the host lipid, as is the structure of the sterol molecule itself. For more information on the application of DSC to the biologically important area of lipid-sterol interactions, the reader is referred to recent reviews (23-25). 

DSC has also been used to study the effects of a wide variety of antimicrobial peptides on the thermotropic phase behavior of different lipid bilayers. These studies again are highly biologically relevant because the primary mode of action of most antimicrobial peptides is the perturbation and permeabilization of the lipid bilayers of the target membrane, and these agents have considerable promise as antibiotics, especially to treat multiple dmg-resistant pathogenic bacteria. Again, the reader should consult recent reviews for more information on this topic (30, 31). 

Seguin C, Lewis RNAH, Mantsch HH, McElhaney RN. Calorimetric studies of the thermotropic phase behavior of cells, membranes... 

McMullen TPW, Lewis RNAH, McElhaney RN. Differential scan- 45. ning calorimetric study of the effect of cholesterol on the thermotropic phase behavior of a homologous series of linear saturated phosphatidylcholines. Biochemistry 1993 32 516-522. 

Lewis RNAH, Mak N, McElhaney RN. A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains. Biochemistry 1987 26 6118-6126. 

Zhang, Y.P., Lewis, R.N., and McElhaney, R.N. Calorimetric and spectroscopic studies of the thermotropic phase behavior of the M-saturated 1,2-diacylphosphatidyl-glycerols, Biophys.., 72, 779,1997. 

Huang C, Li S. Calorimetric and molecular mechanics studies of the thermotropic phase behavior of membrane phospholipids. Biochim Biophys Acta 1999 1422 273-307. 

The influence of the amount of cholesterol at various temperatures on the thermotropic phase behavior and organization of saturated phosphatidylethanolamine bilayers has been... 

Thermotropic Liquid Crystals. - Organic molecules, having aromatic rings or unsaturations thus producing elongated shapes, and also polymeric molecules often show thermotropic phase behavior. Mesomorphism comprises typically nematic, smectic A and B, and cholesteric thermotropic phases, but in several cases columnar, discotic and rod-disk self-assembly shapes of thermosensitive mesogens have been observed. 

Seddon et al. have studied the lyotropic (and thermotropic) phase behavior of n-oc-tyl-l-O-P-D-glucopyranoside and its thio derivative n-octyl-1 -5-P-o-glucopyrano-... 

He X, Zhang H, Yan D, Wang X. 2003. Synthesis of side chain liquid crystalline homopolymers and triblock copolymers with p methoxyazobenzene moieties and poly(ethylene glycol) as coil segments by atom transfer radical polymer zation and their thermotropic phase behavior. J Polym Sci Part A Polym Chem 41 2854 2864. 

Lewis R. N., R. N. McElhaney, P. E. Harper, D. C. Turner, and S. M. Gruner. 1994. Studies of the thermotropic phase behavior of phosphatidylcholines containing 2-alkyl substituted fatty acyl chains A new class of phosphatidylcholines forming inverted nonlamellar phases. Biophysical Journal 66(4) 1088-1103. 

Effect of Polymers on Thermotropic Phase Behavior. In order to determine if the polymers had any direct effect on the bilayers, the gel-to-liquid crystalline phase transition temperature was measured for the samples. The gel-to-liquid crystalline phase transition temperature has been found to be a very sensitive measure of bilayer packing and order. If the polymers were directly interacting with the bilayer, it should be observed in the transition temperature and enthalpy. 

Deperrdirrg on their stmcture, some PILs can be used as catiorric sttrfaclants in a solvent, can be ttsed as self-assembly media for amphiphiles (lyotropic phase behavior), or can tmdergo phase transitions with changes in temperature (thermotropic phase behavior). These three behaviors are described below. 

Thermotropic Liquid Crystals. The thermotropic phase behavior is usually observed in molecules possessing aromatic rings, conjugated... 

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