Main phase transition

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

Figure 4 Spectrum of 6-d DPPC Belov (34°C) and Above (49°C) the Main Phase Transition.

Figure 5 Gel Phase CD- Rocking Mode Spectrum of 4-d, DPPE. The molecule has lts main phase transition at 6A°C. The residual gel phase disorder is substantial.

Fig. 3.12 Tracing of DSC thermograms of DPPS alone (control) and in the presence of MDR modifiers at a lipid-drug molar ratio of 1 0.05 (0.05 corresponds to the concentration of the drug in the ionized form). On the thermograms the small peaks on the left of the main phase transition peaks represent the...

Fig. 3.46 Partition coefficients oflOpM tamoxifen (O) and 4-hydroxytamoxifen ( ) in DM PC bilayers as a function of temperature. Maximal partitioning is observed at main phase transition temperatures. (Reprinted from Fig. 3 of ref. 147] with permission from Academic Press.)...

Tab. 3.15 The effects of calcium channel-blocking drugs on the main phase transition ofdimyristoylphosphatidylcholine (DMPC) adapted from ref. 155.

Phase transition is an important property of membranes. Below the phase transition temperature, lipids are tilted and highly ordered. They are in their solid or "gel" state. Increasing the temperature leads to a pre-transition, characterized by periodic undulations and straightening of the hydrocarbon chain. Further increase of the temperature causes the main phase transition. Above the main phase transition temperature, lipids are fluid or "liquid crystalline." Figure 3 shows the phase diagram for the interaction of water with a lipid as well as its inferred arrangements in a model membrane (5). Phase transitions in membranes and membrane models have been extensively studied by spectroscopic techniques and by differential scanning calorimetry. 

Figure 3. Schematic representation of a phospholipid-water phase diagram. The temperature scale is arbitrary and varies from lipid to lipid. For the sake of clarity phase separations and other complexities in the 20-99% water region are not indicated. Structures proposed for the phospholipid bilayers at different temperatures are shown on the right-hand side. At low temperature, the lipids are arranged in tilted one-dimensional lattices. At the pre-transition temperature, two-dimensional arrangements are formed with periodic undulations. Above the main phase, transitions lipids revert to one-dimensional lattice arrangements, separated somewhat from each other, and assume mobile liquid-like conformations.

An important result is the coincidence of the temperature of the main phase transition determined for the water-ethanol dispersion by DSC (see below) with the temperature of the steep change in the foam bilayer thickness (23°C). Within the range from 22 to 23°C the foam bilayer thickness variation is similar to that of the interlamellar distance in water dispersions of DMPC [443]. These facts show that both in the bulk phase and in the foam bilayer a chainmelting phase transition occurs which is characterised by a sharp shift in the number of gauche conformations of carbon-carbon bonds [430,444]. 

It is well known that the main phase transition is due to the melting hydrocarbon tails of amphiphile molecules [430,436], The average number of gauche conformations for each DMPC molecule is about 7 in the liquid-crystalline state [444], In view of this the thickness of the hydrocarbon layers of a foam bilayer can be estimated to be h = 1.13 nm. Then for the liquid-crystalline state of the foam bilayer, the thickness of the polar inner layers is (i2 = 3.5 nm and the total thickness of the foam bilayer is h = 5.7 nm. These values are relevant to the liquid-crystalline state of the foam bilayers (24-30°C) and are shown in Table 3.14. The value obtained for the thickness of the DMPC foam bilayer in the liquid-crystalline state seems reasonable when compared with the value of the interlamellar distance in the liquid crystalline DPPC-water-alcohol dispersions [445], extrapolated to high ethanol concentrations. 

Thermodynamic characteristics of the pretransition and of the main phase transition in the system DMPC/H20/NaCI/C2H50H (2 mg dm 3 DMPC and 7 I0 2 mol dm 3 NaCI)... 

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. 

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. 

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