Gel-to-liquid-crystalline phase transition

FIGURE 9.12 All illustration of the gel-to-liquid crystalline phase transition, which occurs when a membrane is warmed through the transition temperature, T. Notice that the surface area must increase and the thickness must decrease as the membrane goes through a phase transition. The mobility of the lipid chains increases dramatically. 

The fluorescence lifetime is sensitive to the environment of the fluorophore, and in membranes this usually means the surrounding fatty acyl chains or the membrane protein interfacial region (see summary in Table 5.3). Generally, the lifetime of membrane-bound fluorophores is rather less sensitive to the types of subtle alterations which are encountered in membranes as compared to the fluorescence anisotropy parameters. The gel-to-liquid crystalline phase transition is a notable exception where most fluorophores show an alteration in lifetime properties. Although, again, the anisotropy (see below) is the most sensitive parameter in this regard, the fluorescence lifetime has been used with considerable success in the study of phase transitions and lateral phase separations. Fluorophores used to yield information on the... 

A representative uptake experiment is shown in Figure 5, where the effect of temperature is apparent (80). For both vinorelbine and vincristine, little uptake is observed at 25°C, far below the gel-to-liquid crystalline phase transition temperature of egg SPM (about 44°C), whereas 80% uptake is observed at 60°C where the membrane is fluid. Interestingly, the uptake of vinblastine at 25°C yields an end point nearly identical to that obtained... 

Lipid phase transitions are often slow on the timescale accessible to simulations, such as the gel to liquid-crystalline phase transition. Using the MARTINI model, liquid-crystalline to gel phase transitions and domain formation have been... 

Fig. 5.18 Effect of increasing molecular ratios of chlorpromazine and imipramine on the enthalpy, AH, of the main gel to liquid crystalline phase transition of dipalmitoylglycerophos-phoserine (DPPS) and dipalmitoylglycerophos-...

D. Gel-to-liquid-crystalline phase transition of liposomes by dynamic light scattering and anisotropy measurements... 

D. Gel-to-Liquid-Crystalline Phase Transition of Liposomes by Dynamic Light Scattering and Anisotropy Measurements... 

Figure 11.5 Schematic illustration of gel-to-liquid crystalline phase transition and DSC thermograms (a) 2Ci2N dispersed in 1, (b) 2Ci4ht dispersed in 1, (c) 2Ci dispersed in 3. Similar endothermic peaks were also observed for 2Cji (n= 12, 14)in2. [2C N ]= lOmM.

The C=0 stretching band in the IR absorption spectrum is equally useful. Phase transition shifts this band from 1738 cm towards lower wavenumbers it is found around 1733 cm as phospholipid vesicles undergo a gel to liquid crystalline phase transition Tm. Mantsch interpreted the shift towards lower wavenumbers of the overall band contour peak as resulting from the intensity increase of the component at 1727 cm upon a change in hydration (by hydrogen bonding) during the transition (Mantsch and Me Elhaney, 1991). 

Live and deproteinated plasma membranes of Acholeplasma laUawii were investigated by FTIR (Casal et al., 1980 Cameron et al., 1985). The temperature profiles of the gel to liquid crystalline phase transition of intact and deproteinated membranes, monitored by z/as(CD2), differ considerably. In intact membranes, the transition is broad and at temperatures within the range of the phase transition the live mycoplasma is able to keep the fluidity of its plasma membranes at a much higher level than that of the isolated plasma membrane. Native and reconstituted sarcoplasmic recticulum were investigated by Mendelsohn et al. (Mendelsohn et al., 1984). It appears that the protein Ca -ATPase interacts preferentially with the DOPC component of the membrane. A survey of these studies is available (Mantsch and McElhaney, 1991). 

The variation of excess specific heat (dE/dt) with temperature for a simple two-state, first-order endothermic process, such as the gel-to-liquid-crystalline phase transition of a single, highly pure phosphatidylcholine (PC), is illustrated schematically in Fig. 1. From such a DSC trace, several important parameters can be determined directly. The phase transition temperature, usually denoted Tm, is that temperature at which the excess specific heat reaches a maximum. For a symmetrical curve, Tm represents the temperature at which the transition from the... 

The sharpness or cooperativity of the gel-to-liquid-crystalline phase transition can also be evaluated from the DSC trace. The sharpness of the phase transition is often expressed as the temperature width at half-height, AT1/2, or as the temperature difference between the onset or lower boundary of the phase transition, Ts, and the completion or upper boundary, Ti, or AT = Ti — Tg. The AT1/2 values may range from <0.1°C for very pure synthetic phospholipids to as much as 10-15°C for biological membranes. From the Tm and AT1/2 values determined for a particular phase transition, the van t Hoff enthalpy, AHvh, can be approximately determined from the relationship ... 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

Although DSC and other physical techniques have made considerable contributions to the elucidation of the nature of lipid-protein interactions, several outstanding questions remain. For example, it remains to be dehnitively determined whether some integral, transmembrane proteins completely abolish the cooperative gel-to-liquid-crystalline phase transition of lipids with which they are in direct contact or whether only a partial abolition of this transition occurs, as is suggested by the studies of the interactions of the model transmembrane peptides with phospholipids bilayers (see above). The mechanism by which some integral, transmembrane proteins perturb the phase behavior of very large numbers of phospholipids also remains to be determined. Finally, the molecular basis of the complex and unusual behavior of proteins such as the concanavalin A receptor and the Acholeplasma laidlawii B ATPase is still obscure (see Reference 17). 

Incubation occurs by preference above the gel-to-liquid crystalline phase transition temperature which, for instance, equals... 

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