Phase transition, lipid bilayers

Nagle, J. F. (1980). Theory of the main lipid bilayer phase transition, Ann. Rev. Phys. Chem., 31, 157-195. 

Nagle IF. Theory of the main lipid bilayer phase-transition. Annu. 

Several independent studies indicate that a lipid bilayer phase transition takes place in conjunction with transmission of the action potential. Forty... 

The increase in sensitivity over standard DSC instruments and the ability to study reactions occurring in solution directly mean that HSDSC may be applied to the study of a range of systems not open to study by standard DSC. Typical examples include the denaturation of proteins, phase changes in lipid bilayers, phase transitions in dilute polymer solutions, and changes in structure of creams and emulsions. Although there are many systems that have been studied using HSDSC, the discussion that follows will concentrate on systems of biological or pharmaceutical relevance. 

R. G. Priest, Semiphenological Model for the Lipid Bilayer Phase Transition Finite Chains in Three Dimensions, Chem. Phys. 66, 722-725 (1977). 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

Phase transitions have been characterized in a number of different pure and mixed lipid systems. Table 9.1 shows a comparison of the transition temperatures observed for several different phosphatidylcholines with different fatty acyl chain compositions. General characteristics of bilayer phase transitions include the following ... 

Bilayer phase transitions are sensitive to the presence of solutes that interact with lipids, including multivalent cations, lipid-soluble agents, peptides, and proteins. 

Fig. 2 The phase behavior of mixed-chain phosphatidylethanolamine in excess water. At low temperature this lipid exists as crystalline bilayers (the gel or Lq state). With increasing temperature the bilayers undergo a reversible endothermic transition commonly called the bilayer phase transition. The resulting state (the liquid crystalline or L3 state) is composed of fluid bilayers. The lipid hydrocarbon chains are more disordered in fluid bilayers than in crystalline bilayers. This results in more cone-shaped molecules as illustrated in the top of the figure. With further increase in temperature the lipid molecules become still more cone-shaped and their preferred conformation is the inverted hexagonal En state.

J.F. Nagle, Theory of Lipid Monolayer and Bilayer Phase Transitions Effect of Headgroup Interactions, T. Membr. Biol. 27 233 (1976). 

Theoretical Calculations.- A molecular interpretation of the chain-length dependent thermotropic behaviour of saturated symmetrical-chain phosphatidylcholine bilavers has been put forward. Thermodynamic parameters of the thermal phase-transition were found to be linearly related to a perturbation parameter and could be used to predict the minimum number of carbon atoms in the acvl chain needed for a bilayer phase-transition to occur. A model has been developed, consistent with NMR data, for hydrocarbon-chain dynamics in lipid bilayers. Involving concerted rotations around at least two C-C bonds at a... 

Electroformed GUVs have proved to be a unique system for visualizing the effects of lipid membrane phase transitions on a dosed single bilayer. In addition, DMPC GUVs have been used in studies of interdigitated phase formation, induced by ethanol and temperature variations [22]. 

While the v-a plots for ionized monolayers often show no distinguishing features, it is entirely possible for such to be present and, in fact, for actual phase transitions to be observed. This was the case for films of poly(4-vinylpyri-dinium) bromide at the air-aqueous electrolyte interface [118]. In addition, electrostatic interactions play a large role in the stabilization of solid-supported lipid monolayers [119] as well as in the interactions between bilayers [120]. 

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

With certain lipid bilayers, a change of physical state referred to as a pretransition occurs 5° to 15°C below the phase transition itself. These pretransitions involve a tilting of the hydrocarbon chains. 

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

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

Membranes are composed of phospholipids and proteins. The fatty acid composition of the phospholipids in a membrane influences how it is affected by the cold. In general, as the temperature of a cell is lowered the lipids in the membrane bilayer undergo a phase transition from a liquid crystalline (fluid) state to a gel (more solid) state. The temperature at which this transition takes place is very narrow for phospholipids composed of a simple mixture of fatty acids, but is quite broad for the phospholipids in cellular membranes. It is usually implied from various methods... 

Liposomes can be prepared from pure lipids or mixtures of lipids. Cholesterol is known to serve as a "fluidity buffer" it enhances the fluidity of the gel state bilayer, while it decreases the fluidity of the fluid state bilayer. Increasing concentrations of cholesterol in bilayers cause a broadening and gradual disappearance of the phase transition (Demel and De Kruyff, 1976). 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

---