Phase transition in lipids

A volume change is usually associated with phase transitions in lipid bilayers. 

Reiss-Husson, F., and Luzzati, V. Phase transitions in lipids in relation to the structure of membranes. Adv. in BioL Medical Physis//, 87-105 (1967). 

In addition, the reversibility of phase transition in lipid-water systems has been studied [30]. It was observed that the relaxation times in the transition region and the lifetimes of the metastable phases are similar, and sometimes significantly longer than the times characteristic of the biomembrane processes. The question arises as to the physiological significance of the equilibration that occurs a long time after lipid phase transition. 

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). 

Jain MK, Wu NW. Effect of small molecules on the dipalmitoyl 46. lecithin liposomal bilayer. III. Phase transitions in lipid bilayers. 

Phase transitions in lipid mixtures phase diagrams... 

Quinn PJ. Measurement of kinetics and mechanisms of phase transitions in lipid-water systems. J. Appl. Crystallogr. 1997 30 733-738. 

It is now well established that proteins can induce phase transitions in lipid membranes, resulting in new structures not found in pure lipid-water systems (c/. section 5.1). However, this property is not peculiar to proteins the same effect can be induced by virtually any amphiphilic molecule. Depending on the structure and nature of proteins, their interactions with lipid bilayers can be manifested in very different ways. We may further assume that the role of proteins in the biogenesis of cubic membranes is analogous to that in condensed systems, and lipids are necessary for the formation of a cubic membrane. This assumption is supported by studies of membrane oxidation, which induce a structure-less proteinaceous mass [113]. However, the existence of a lipid bilayer by itself does not guarantee the formation of a cubic membrane, as proteins may also play an essential role in setting the membrane curvature. In this context, note that the presence of chiral components e.g. proteins) may induce saddle-shaped structures characteristic of cubic membranes. (This feature of chiral packings has been discussed briefly in section 4.14)... 

In recent years interest in these materials has grown mainly for physical reasons. The layer perovskites are looked at as model compounds for the study of magnetic properties in two-dimensional systems (J2) and as models for the study of structural phase transitions in lipid bilayer-type arrays ( ). The use of layer perovskites as a matrix for organic solid state reactions represents a fairly new research topic. First experiments were carried out studying the photolysis of butadiyne (diacetylene) derivatives (li-ZSl) For a corresponding study of the butadiene derivatives the compounds listed in Table I were synthesized. 

An optical method for monitoring the concentration of general anaesthetics and other small organic molecules in biologically interesting systems has been based on phase sensing of a fluorescent hydrophobic probe . Phase transitions in lipid membranes are detected by this method. The technique shows promise. 

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]. 

Phase Transitions in Lipid Assemblies. The rich polymorphism of amphiphilic systems, of which the multilamellar and the Hn phases are only two structures, was made evident from the seminal work of Luzzati and co-workers. Since that early work, an immense variety of water-induced phase transitions have been observed and rationalized in terms of an apparently systematic connection between water content and polar group molecular area. Therefore, the recent observation of a double transition—Hn to lamellar back to Hn—from continual hydration of dioleoylphosphatidyl-ethanolamine (40) was a surprise. Furthermore, an estimate of the cost of uncurling the monolayer in the formation of bilayers based on the previously described bending modulus far exceeds the osmotic work that actually produced the transition. Although this transition sequence can successfully be accounted for by simple thermodynamical principles, it, in fact, contains many geometry-dependent free energy contributions that we simply do not yet understand (41). 

Trauble,H.,"Phase Transitions in Lipids" in "Biomembranes", Vol.3,p.l97,(F.Kreuzer. O.F.G.Slegers, ed.) Plenum Publ.Corp..New York... 

Two types of polymorphism occur in lipids. The forms are enantiotropic. When each form is thermodynamically stable in a definite range of temperature and pressure, the forms are enantiotropic. Thus one enantiotropic form changes to another at a certain transition temperature. If, on the other hand, one of two forms is thermodynamically unstable, the two forms are said to be monotropic. The phase transitions in lipids are usually of the monotropic type, for example in glycerides. 

Dilatometric methods are frequently used in studies of phase transitions in lipids as the dilatometric curves are very informative in that respect. Such a dilatometric curve is obtained by plotting specific volume versus temperature. It has been found that the ratio between specific heat and the coefficient of expansion is nearly constant at different temperatures. This can be a useful relationship. If, for example, complete data on specific heats are available a single determination of the coefficient of expansion can be used for estimation of the specific volume over a wide temperature range. 

In this chapter, studies involving surfactants are somewhat arbitrarily separated from those that concern lipids. Similarities or differences of behavior are pointed out whenever necessary. The chapter is organized as follows. Section II reviews the dynamics of the surfactant Lg phase and of transitions between various lyotropic phases. Section III deals with the dynamics of phase transitions in lipid/water mixtures. Section IV reviews the dynamics of phase transitions induced by shear. Section V concludes this chapter. 

III. DYNAMICS OF PHASE TRANSITIONS IN LIPID LYOTROPIC PHASES... 

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