Transitions thermotropic

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

The major transition in membranes of log-phase cells is the broad gel to liquid-ciys-talline transition of the phospholipids that begins below 0 °C and ends at 20-24 °C. While observing the thermotropic transitions in bacterial membranes at stages of cell... 

Figure 9. A. Thermotropic fluorescence spectra of E. coli DH1 cells using the hydrophobic probe, N-phenyl-1 -naphthylamine (NPN). (a) Mid-log phase cells (b) stationary phase cells (c) cells made genetically transformable by the method of Hanahan.146 NPN was added to 4 mL of cell culture to a final concentration of 1 pM and the thermotropic fluorescence spectra were recorded.24 Measurements were made at increasing temperature (ca. 2 °C per min). Excitation 360 nm emission 410 nm. Measurements were made at increasing temperature (ca. 2 °C per min). B. Effects of physical treatments on the thermotropic transitions in genetically competent E. coli DH1. (a) Thermotropic transitions at descending temperature (b) cells pelleted at low speed and suspended in supernatant (c) as in b but suspended in equal volume of distilled water (d) as in (b) but suspended in 10 mM phosphate buffer, pH 7.4. Excitation 360 nm emission 410 nm. Fluorescent probe was NPN. Measurement (a) was made at decreasing temperature and (b), (c), (d) at increasing temperatures (ca. 2 °C per min).

Freeze-fracture electron microscopy studies of the membranes of E. coli and A. vinelandii by Reusch et al.24 provide evidence of structural changes that support the fluorescence data (Figure 10). Freeze-fracture micrographs of log-phase cells show a typical mosaic of particles and pits on both concave and convex surfaces of the plasma membranes. However, as complexed PHB was increasingly incorporated into the membranes, as determined by analysis of the purified membranes and evidenced by the intensity of the thermotropic transition at - 56 °C, the micrographs revealed the formation of small semi-regular plaques in the plasma membranes (arrows) that possess shallow particles. The plaques grew in size and frequency as the concentration of membrane PHB and intensity of the PHB/polyP transition increased. 

Figure 10. Representative freeze-fracture electron micrograph of competent E. coli DH1. The micrograph shows the typical appearance of small semi-regular plaques (arrows) in the plasma membranes of E. coli DH1 cells after treatment to make them genetically transformable by the method of Hanahan.146 These cells have sharp thermotropic transitions at -56 °C when examined as in Figure 9A.24...

Golden, G., Guzek, D., Harris, R., McKie, J. and Potts, R. Lipid thermotropic transitions in human stratum comeum. J. Invest. Derm. 86 255-259, 1986. Friberg, S., Osborne, D. and Tombridge, T. X-ray diffraction of human stratum comeum. J. Soc. Cosmet. Chem. 36 349-354, 1985. 

Melchior, D. L. and Steim, J. M. Thermotropic transitions in biomembranes. Annual Reviews of Biophysics and Bioengineering 5 205, 1976. 

Krill, S. L., Knutson, K. and Higuchi, W. I. The stratum comeum lipid thermotropic phase behavior. Biochimica et Biophysica Acta 7772(2) 281-286, 1992. Golden, G. M. et al. Lipid thermotropic transitions in human stratum comeum. Journal of Investigative Dermatology 86 255-259, 1986. 

Lin, S.-Y., Liang, R.-C. and Lin, T.-C. Lipid and protein thermotropic transition of porcine stratum comeum by microscopic calorimetry and infrared spectroscopy. Journal of the Chinese Chemical Society 47 425-429, 1994. 

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

Side-chain liquid-crystalline polymers with controlled molecular weights have been obtained by the polymerization of FM-25 with 1-22 (X = Br)/CuBr/ L-3 in the bulk at 100 °C, to examine the thermotropic transition as a function of the MWD.324 Second-order nonlinear optical materials with branched structure were prepared by the copper-catalyzed radical polymerization of FM-26 and FM-27 using hyperbranched poly[4-(chloromethyl)styrene] as a multifunctional initiator.325... 

Differential scanning calorimetry (DSC) is widely used as a means of detecting the temperatures of thermotropic transitions, but the technique cannot be used to identify the mesophase. X-ray diffraction measurements on ahgned samples obtained on cooling from the isotropic liquid phase under the influence of a strong magnetic field or by drawing a fiber from the mesophase are able to discriminate between different types of mesophases. 

Most glycerolipids and sphingolipids in aqueous dispersions form closed vesicles, limited by lipids in the lamellar (bilayer) disposition. Depending on the lipid structure, different thermotropic transitions may be observed, of which the following are the most common. 

Saturated glycero- and sphingophospholipids other than phosphatidylcholines and phos-phatidylglycerols, that cannot form tilted Lp- phases, form instead the gel Lp phase. In these cases, the gel-to-fluid Lp-to-La thermotropic transition (without a rippled intermediate) has very similar properties to that of the main transition in, e.g. DPPC (see Table 4.1). 

Figure 4.4. In this phase diagram we find the Lp-, Pp-, Lp and phases that have already been described, plus a few new ones, namely Iv, Lo and L p. W (denoted c in the figure) is the pure DPPC crystalline or subgel phase, that is only observed in annealed (i.e. kept at 4 °C for several days) aqueous multilamellar dispersions of DPPC. A thermotropic transition, the subtransition , converts the Lc- into the Lp- phase at ca. 18 °C. Lo and Lop are two different hquid ordered (Lo) regions, respectively called liquid-crystalUne-like liquid ordered (Loo) and gel-like liquid ordered (Lop). The latter two phases differ in the orientational order of the hydrocarbon chains, and in the relative position of the cholesterol molecule in the host PC bUayer.

The lamellar phase represents the structure of cell membrane lipids under steady-state conditions. However in certain circumstances, particularly in membrane fusion events (e.g. in egg fertilization, or cell infection by some viruses), membrane lipids abandon transiently the lamellar disposition, adopting nonlamellar architectures, of which the best known is the so-called inverted hexagonal , or Hn, phase. Nonlamellar structures are at the origin of the lipid stalk , a structural intermediate that connects two bUayers in the membrane fusion process. Only certain lipids, or lipid mixtures, can undergo the Lo(-Hii thermotropic transition, and the latter can be detected by DSC. Hu, like other nonlamellar phases, has received particular attention lately because of its possible implication in important phenomena such as cell membrane fusion, or protein insertion into membranes. High-sensitivity DSC instruments allow the detection of La-Hn transitions with phospholipid suspensions of concentration 5 him or even less. 

This implies a fast, and complete, reorientation of the side groups, as in the molten state. Nevertheless, local anisotropy of motion is retained up to the much higher isotropization temperature (Ti). Great heat absorption is involved in the Tc transition, while a minor DSC signal is detected at the melting. Such a dramatic transformation of a large mass of the solid at Tc temperature supports the definition of a crystalline-to-mesophase thermotropic transition. 

The phase transitions of liquid crystals in solutions occur normally through two mechanisms, i.e. lyotropic and thermotropic transitions. The lyotropic liquid crystal occurs upon addition of solvent into the crystalline phase, while the thermotropic liquid crystal occurs upon heating the crystalline phase, as illustrated by the two arrows in Fig. 10.3, respectively. The phase diagrams for the transition from the homogeneous solution to the liquid crystal are formed by two almost parallel curves, reflecting the concentration gap between the two coexisting phases. 

Chapman and co-workers (1966, 1967) studied the spectra of films of pure phospholipids, including phosphatidylcholine and phosphatidylethanolamine. Phospholipid spectra were extremely variable, changing markedly with temperature and method of sample preparation. In particular, the gel-to-fluid thermotropic transition causes the resultant vibrational bands to be smeared out and to be devoid of fine structure (Fig. 9.8). Wallach etal (1978) studied multibilayer films of pure phospholipids and phospholipid mixtures as a function of fatty acid composition, temperature and degree of hydration. Table 9.3 summarizes the main band frequencies observed in these studies. 

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

Table I shows the thermotropic transitions of phospholipids and vesicle dispersions of 1 and 2, along with enthalpies of transition. NCP-1 shows a sharp chain melting transition temperature at 37.5 C and a sharper transition peak at 32.8 C upon cooling. The AH at 37.5 C was determined to be 6.07 kcal/mole. Dispersions of the NCVD-1 show a chain melting transition at 25.9 C and a cooling transition at 23.6 C. CVD-1 shows a melting transition at 29.4 °C and a cooling transition at 25.9 C.

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