Melting of the hydrocarbon

As it is well known [36,37], the natural lipid/protein mixtures (such as amniotic fluid) can undergo different phase transitions due to variation in temperature or composition. Of special importance for the natural bilayer lipid membranes is the so-called main phase transition between the lipid crystalline and gel states at which a melting of the hydrocarbon tails of the lipid molecules occurs. For example, it has been demonstrated [36] that there exists an upper limit of the gel phase content in membranes above which the membrane morphology and permeability change dramatically thus making the execution of the physiological functions of the membrane impossible. 

Conclusion (b) was of particular importance since it had been predicted by some that horizontal stacking of the cores, and hence better 7t-7t overlap, in the mesophase should result in a substantial increase in the charge mobility. The negative effect actually observed, which is now generally accepted, can be attributed to the structural fluctuations within the core that result on melting of the hydrocarbon side chains. Important in the general acceptance of this effect was a comparative study of PR-TRMC results with DC time-of-flight results on... 

The gel-to-liquid-crystalline phase transition is accompanied by physical and struaural changes in the bilayer (Figure 3d,g). The surface area increases and the bilayer thickness decreases as a result of the melting of the hydrocarbon chains. The net result is a small increase in volume accompanying the... 

Brain phospholipid can exist in the presence of water in two different phases (15), the usual so-called lamellar phase and a hexagonal phase dependent upon temperature and concentration. The temperature at which a particular phase of this type can exist for a given phospholipid will also depend upon the transition temperature for melting of the hydrocarbon chains. 

Most of the reported structural transformations of PC lipid bilayers with respect to environmental conditions such as temperature, pressure, pH, etc., are associated with isomerizations of the constituent lipid molecules (mostly 14—24 carbon acyl chains). For instance, the transition from the ordered gel to fluid liquid crystalline phase, called the main phase transition, has been related to the melting of the hydrocarbon chains in the gel phase, phospholipids with all trans alkyl chains are present, whereas in the disordered liquid crystalline phase the most populated conformational states correspond to gauche forms in the alkyl chains. 

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... 

In monosodium n-decanephosphonate-water and disodium n-decanephospho-nate-water, the main transitions are the melting of the hydrocarbon network followed by the melting of the polar headgroups. The temperatures at which both transitions occur, as well as their enthalpies, increase as the water content in the system decreases [65]. 

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). 

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