Liquid-ordered state

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11-15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained the bilayer is paracrystalline (Fig. ll-15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11—15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid,... 

This micromembrane was also used to investigate the spontaneous formation of microdomains, when the distal lipid monolayer is made up of a lipid mixture. Microdomains are in the gel state when they consist primarily of glycolipids and sphingohpids, in a liquid-ordered state (so-called hpid rafts ) when they also contain cholesterol, and in a hquid-disordered state when they consist primarily... 

In another study Milehev and Landau [27] investigated in detail the transition from a disordered state of a polydisperse polymer melt to an ordered (liquid erystalline) state, whieh oeeurs in systems of GM when the ehains are eonsidered as semiflexible. It turns out that in two dimensions this order-disorder transition is a eontinuous seeond-order transformation whereas in 3d the simulational results show a diseontinuous first-order transformation. Comprehensive finite-size analysis [27] has established... 

The liquid-ordered gel-like state of liposomes is stabilized by S-layer coating [123]... 

Being bordered by the solid and liquid states, the liquid crystal state has some of the order of a solid, combined with the fluidity of a liquid. As such, it is an anisotropic fluid and it is this anisotropy that has led to the widespread application of liquid crystals. 

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

It has been shown frequently that without the presence of strong intermolecular interactions, discotic molecules are highly mobile in the liquid crystalline state.1 They undergo both lateral as well as rotational translations, resulting in the absence of positional order. Similarly, such discotics also freely rotate in the columnar aggregates they form in solution. This lack of positional order in the columns accounts for the absence of chiral or helical supramolecular order. We will demonstrate this characteristic using results obtained for triphenylenes. 

Incompatible elements can achieve very large enrichment in the liquid. Steady-state is achieved over a characteristic length in proportion with (ktL/ktR)L. For small porosities, this length is in the order of (4>/)L for incompatible elements, in the order of L for compatible elements (Figure 9.12). A very small fraction

limit concentration ( C0 / >) and the characteristic length of incompatible elements. 

Polyarylates prepared from cyclohexyl-HQ (Ch-HQ) and PEC (Ch-HQ/PEC) did not show liquid crystallinity due to the more bulky substituent on the HQ unit compared to those on f Bu-HQ and Ph-HQ. As-spun fibers of Ch-HQ/PEC exhibited lower moduli than those of fBu-HQ/PEC and Ph-HQ/PEC. Therefore, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state (7j — 7j,) is an influential factor, as shown in Table 19.1. 

FIG. 1. Two-dimensional representation of short-range (a,b) and long-range order (A,B) in the crystalline (a), liquid (b), and liquid crystalline state (c). From Miiller-Goymann, C.C., Flussigkristalline Systeme in der Pharmazeutischen Technologie, PZ Prisma, 5 129-140 (1998). 

To this purpose, Fossheim et al. reported temperature sensitive liposomal Gd(III)-based probes (137-138). The composition of the liposomes was chosen in order to tune the temperature of transition between the gel-crystalline, where the liposome is water-impermeable, to the liquid-crystalline state, where water has free access to the interior of the liposome. This means that at temperatures below the transition, the relaxivity of the system is very low (paramagnetic contribution close to zero), whereas at higher temperature the Gd(III) complex ([GdDTPABMA(H20)] is the reference) is no longer silent... 

---