Liquid crystalline physical gels

In the cases of the hydrogen-bonded materials described in the previous sections, single homogeneous liquid-crystalline phases without phase separation are displayed by the formation of intermolecular hydrogen bonds. Here, liquid-crystalline physical gels, anisotropic functional materials with heterogeneous self-organized structures (Type B in Fig. 2), are discussed. 

Fig. 26. Thermoreversible phase transitions of liquid-crystalline physical gels...

Moriyama M, Mizoshita N, Yokota T, Kishimoto K, Kato T. 2003. Photoresponsive anisotropic soft solids liquid crystalline physical gels based on a chiral photochromic gelator. Adv Mater 15 1335 1338. 

Yabuuchi K, Rowan AE, Nolle RJM, Kato T. 2000. Liquid crystalline physical gels self aggregation of a gluconamide derivative in mesogenic molecules for the formation of anisotropic functional composites. Chem Mater 12 440 443. 

Kato, T., Hirai, Y., Nakaso, S., Moriyama, M., 2007. Liquid-crystalline physical gels. Chem. Soc. Rev. 36, 1857-1867. 

Verduzco R, Meng GN, Komfield JA, Meyer RB (2006) Buckling instability in liquid crystalline physical gels. Phys Rev Lett % 147802... 

Fig. 29 Photo-responsive liquid-crystalline physical gel based on azobenzene-containing bis(amide)cyclohexane gelator...

Fig. 30 Photo-responsive liquid-crystalline physical gel based on azobenzene-containing bis(amide) gelator, and the schematic illustration of the mechanism for the light-induced reorganization in the liquid crystal gels and the grating formation...

Mizoshita, N., et al. Smectic liquid-crystalline physical gels. Anisotropic self-aggregation of hydrogen-bonded molecules in layered structures. Chem. Commun. 9, 781-782 (1999)... 

Hirai, Y, et al. Enhanced hole-transporting behavior of discotic liquid-crystalline physical gels. Adv. Fund. Mater. 18(11), 1668-1675 (2008)... 

The transition from liquid-crystalline to gel phase, which results in a marked change in the physical properties of the lipid bilayer, also strongly affects the activities of membrane proteins. Some membrane proteins, such as the Ca +-ATPase, show low activity in gel-phase bilayers because of the effects of the gel phase on the protein conformation (22). An increase in the bilayer thickness during a liquid-crystalline-gel transition has been shown also to affect the activity of membrane proteins, for example, that of the diacylglycerol kinase (18). 

Free fatty acids, derived primarily from adipocyte triglycerides, are transported as a physical complex with plasma albumin. Triglycerides and cholesteryl esters are transported in the core of plasma lipoproteins [134], Deliconstantinos observed the physical state of the Na+/K+-ATPase lipid microenvironment as it changed from a liquid-crystalline form to a gel phase [135], The studies concerning the albumin-cholesterol complex, its behavior, and its role in the structure of biomembranes provided important new clues as to the role of this fascinating molecule in normal and pathological states [135]. 

The above-mentioned physicochemical properties of phospholipids lead to spontaneous formation of bilayers. Depending on the water-lipid ratio, on the type of phospholipids, and the temperature, the bilayer exists in different, defined mesomorphic physical organizations. These are the La high-temperature liquid crystalline form, the Lp gel form with restricted movement of the hydrocarbon chains, and an inverted hexagonal phase, Hn (see Sections 1.3.1 and 1.3.2). 

Physically, the membrane may exist in two states the "solid" gel crystalline and the "liquid" fluid crystalline states. For each type of membrane, there is a specific temperature at which one changes into the other. This is the transition temperature (Tc). The Tc is relatively high for membranes containing saturated fatty acids and low for those with unsaturated fatty acids. Thus, bilayers of phosphatidylcholine with two palmitate residues have a Tc = 41°C but that with two oleic acid residues has a Tc = -20°C. The hybrid has a Tc = -5°C. Sphingomyelin bilayer, on the other hand, may have a Tc of close to body temperature. In the gel crystalline state, the hydrophobic tails of phospholipids are ordered, whereas in the fluid crystalline state they are disordered. At body temperature, all eukaryotic membranes appear to be in the liquid crystalline state, and this is caused, in part, by the presence of unsaturated fatty acids and in part by cholesterol. The latter maintains the fatty acid side chains in the disordered state, even below the normal Tc. There is thus no evidence that membranes regulate cellular metabolic activity by changing their physical status from the gel to the fluid state,... 

Metallomesogens are materials related to liquid crystalline ionic hquids because some of them have ionic structures [68, 69]. Gels based on ionic hquids are also one of related materials. Physical gelation of ionic liquids is accompanied by the... 

An obvious hypothesis is that this unusual membrane lipid composition is related directly to membrane function in some way. Within the restricted area of lipid bilayers, lipid composition is known to be an important determinant of physical properties. There are several prominent examples. First, the temperature at which the hydrocarbon chains melt when assembled in bilayers (the gel-to-liquid-crystalline transition temperature, marks an abrupt change in many of the physical properties of such bilayer systems for example, water permeability through such bilayers increases by several orders of magnitude above the transition. Second, the presence of cholesterol within bilayers composed of amphipathic lipids has a profound effect on lipid motion, mechanical properties (such as resistance to shear), and permeability to water. 

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). 

Enhanced photoconductive properties were reported for physical gels of a liquid-crystalline triphenylene derivative [112],... 

The gel-to-liquid-crystalline phase transition is accompanied by physical and struaural changes in the bilayer (Figure 3d,g). The surface area increases and the bilayer thickness decreases as a result of the melting of the hydrocarbon chains. The net result is a small increase in volume accompanying the... 

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