Pretransition temperature

Between the pretransition temperature and Tm solid and liquid regions may coexist within a bilayer.101 The term lateral phase separation has been applied to this phenomenon.105 106 Since changes in the equilibrium between solid and liquid can be induced readily, e.g., by changes in the ionic environment surrounding the bilayer, lateral phase separation may be of significance in such phenomena as nerve conduction.107... 

EPR spectrum of tetramethylpiperidine-l-oxyl dissolved in an aqueous dispersion of phospholipids. (Top) above the main bilayer transition temperature Tt (center) between Tt and pretransition temperature (bottom) below pretransition temperature. From Shimshick and McConnell/... 

Much of the interpretation of the observed changes in EPR spectra of spin labels is empirical. For example, the spectra in the accompanying figure can be interpreted to indicate that the spin label dissolves in the lipid to a greater extent at higher temperatures. The ratio/(defined in the figure) is an empirical quantity whose change can be monitored as a function of temperature. Plots of/vs T have been used to identify transition and pretransition temperatures in bilayers.1... 

The effect of halo thane (CF3CHBrCl) on the lateral surface conductance and membrane hydration has been studied by Yoshida and coworkers [41]. Below the pretransition temperature, the activation energy of the ion movement (H30++0FT) was 18.1 kj/mol, which corresponds to that of the spin-lattice relaxation time of water (18kJ/mol) above pretransition the activation energy increased to 51.3 kj/mol. Halothane did not show any effect on the ion movement when the temperature was below the pretransition temperature. When the temperature exceeded the pretransition temperature, the authors observed at 0.35 mM halothane (equilibrium concen-... 

Fig. 3.96 also shows an increase in pretransition temperature with increase in ethanol concentration, leading to overlapping of the pretransition peak with that of the main phase... 

Additional increases in temperature result in a marked increase in the long-axis rotational rates of the hydrocarbon chains, and at the pretransition temperature, the Lp/ phase converts to the so-called ripple or Pp/ phase. In the Pp/ phase, the extended hydrocarbon chains seem to remain tilted with respect to the normal to the local bilayer plane but behave as if they... 

The rote of cholesterol in the fluidity of biological membranes is characterized as essential. Cholesterol is a silent molecule and in the case of lipid bilayers and liposomes it is included into bilayers to control the rate of the release of encapsulated molecules or to influence the stability of liposomes. The addition of cholesterol in lipid bilayers composed of DPPC, at concentrations more than 20%, results in the decrease of the Tin and elimination of the pretransition temperature. 

While the main transition occurs close to the expected temperature and is merely broadened, the pretransition temperature (35 °C) lies by about 2 °C higher than the value found at slow heating rates reflection of the kinetic hindrance... 

In the pretransition temperature region, where the nematic phase has a certain degree of short-range smectic A order, it is easy to detect both the motion of the liquid and the formation of the domain patterns [122, 123, 126]. The latter have marked differences from the domain patterns arising in the pure nematic phases. For instance, in the temperature region where the ratio of the electrical conductivity cr /crx becomes less than 1, the Kapustin-Williams domains are not observed with an initial planar orientation. Instead, different domains are formed at low frequencies (a < ujc) of the applied field. They are arranged parallel to the initial orientation of the director and have a well-defined field strength threshold, Uth oc d [123]. 

The Kerr effect in the pretransition temperature region has been investigated very actively [17, 34-39], Tsvetkov et al. [39] determined the values for B >0 (phenyl-benzoates) and S <0 (cyanostilbenes) close to Tn-i- These values exceed the absolute value of the Kerr constant for nitrobenzene by 200 and 6.5 times, respeetively. Such high values of B enable the control voltage of the Kerr cell to be reduced several fold and give a decrease of an order of magnitude or more in power consumption [40]. 

In the pretransition temperature region above the smectic A-nematic phase transition, where the nematic phase has a certain degree of the short-range smectic A order, it is easy to detect both the motion of a liquid and the formation of domain patterns. The latter show marked differences from the... 

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