Phase transitions melting

When a substance undergoes a melting phase transition, the high degree of molecular arrangement existing in the solid becomes replaced by the disordered... 

F. Three-stage (partial melting, recrystallization, and total melting) phase transition theory... 

The chemical compositions and isomeric structures of the fatty acid chains of phospholipids is well known to have large effects on the physical properties of lipid bilayers, such as the temperatures of endothermic chain melting phase transitions. Lipid vesicles sensitized with lipid haptens can be agglutinated with specific antibodies directed against the haptens (see Fig. 1). 

The sharp change in the binding energy of DMPC molecule in the foam bilayer is obviously due to the occurrence of a chain-melting phase transition in the bilayer. The value of Q = 1.93-10 19 J relates to the gel state and the value of Q = 8.03-10 20 J concerns the liquid-crystalline state of the foam bilayer. The higher value of Q for the gel state is natural, as it refers to a state of higher degree of order. 

Another result shown in Table 3.15 is the slight shift of the main phase transition towards lower temperatures. Similar results have been found for water ethanol dispersions of DPPC [445,453,454]. The strong influence of ethanol on the enthalpy of the main phase transition of DMPC water-ethanol dispersions shown in Table 3.15 is similar to the substantial increase in this enthalpy in the case of water-ethanol dispersions of DPPC [445,453]. Thus, a correspondence is found for the temperature of the chain-melting phase transitions in the cases of foam bilayers and the fully hydrated water-ethanol dispersions of DMPC. 

Hence, the interaction between lipid molecules is very similar in these foam bilayers and it can be supposed that the AF foam bilayers are in the liquid crystalline state within the temperature range studied. This assumption is in agreement with the fact that amniotic fluid contains substantial amount of unsaturated phospholipids, which as known [45], lower considerably the temperature of the chain-melting phase transition. Bearing in mind the similarity of the phase behaviour of a phosphatidylcholine aqueous dispersion and foam bilayers [38-40], it can be supposed that at the temperatures which are important for in vivo systems, the foam bilayers are in the liquid crystalline state. This assumption allows to determine the critical concentration of phosphatidylcholines in amniotic fluid, necessary for formation of a foam bilayer by extrapolation of the Arrhenius dependence of C, for AF foam bilayers to 37°C. Thus, at 37°C C, = 19.9 jxg cm 3 and d, = 1.47. This value of C, at 37°C corresponds to the lower limit (found by other methods [46,47]) of phosphatidylcholine concentration which permits to classify as mature a sample of amniotic fluid. The above value... 

Membrane lipids are invariably polymorphic that is, they can exist in a variety of kinds of organized structures, especially when hydrated. The particular polymorphic form that predominates depends not only on the stmcture of the lipid molecule itself and on its degree of hydration, but also on such variables as temperature, pressure, ionic strength and pH (see References 11 and 12 and article Lipids, Phase Transitions of). However, under physiologically relevant conditions, most (but not all) membrane lipids exist in the lamellar or bilayer phase, usually in the lamellar liquid-crystalline phase but sometimes in the lamellar gel phase. It is not surprising, therefore, that the lamellar gel-to-liquid-crystalline or chain-melting phase transition has been the most intensively studied lipid phase transition... 

In the absence of the crystal field the quantum melting phase transition is second order for planar rotors, first order for linear rotors. When a crystal field is turned on the phase transition is absent for planar rotors, whereas a more... 

Figure 16.1 Experimental phase diagram of the iPP/EPDM blend as obtained by light scattering and DSC, showing the intersection of LCST and the crystal-melt phase transition. (From Reference (5) with permission from Elsevier.)...

Cevec, G. 1991. How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation an effective chain length model. Biochemistry, 30, 7186. 

R. E. Jacobs, B. Hudson, and H. C. Andersen, A Theory of the Chain Melting Phase Transition of Aqueous Phospholipid Dispersions, Proc. Natl. Acad. Sci. USA 11, 3993-3997 (1975). 

For the description of the chain-melting phase transition of pure lipid bilayer membranes the microscopic model of Pink and collaborators has been adopted. This model takes into account the acyl-chain conformational statistics and the van der Waals interaction between various conformers in a detailed way, while the excluded volume effect is accounted for by assigning each lipid chain to a site in a triangular lattice. The acyl chain conformations are represented by ten single chain states a , each described by a cross-sectional area A , an internal energy Ea and an internal degeneracy Da- The second membrane component is assumed to be a stiff, hydrophobically smooth molecule with no internal degrees of freedom and a cross-sectional area Ac- The model parameters will be chosen so that the mixture display the properties of the DPPC-cholesterol bilayer system for small concentrations of cholesterol [3]. The impurity will hereafter be called cholesterol . 

Impurity-modulated interface formation in lipid bilayer membranes near the chain-melting phase transition J. Hiort Ipsen and L. Cruzeiro-Hansson... 

The TG-DTA or TG-DSC combination allows obtaining the simultaneous TGA and DTA or DSC measurements on the same sample (Fig. 2.34). The heat effects are correlated with the mass variations. The DTA or DSC signal gives information on heat effects not correlated with mass variation (melting, phase transitions,...)... 

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