Viruses liquid crystal phases

More than half a century ago, Bawden and Pirie [77] found that aqueous solutions of tobacco mosaic virus (TMV), a charged rodlike virus, formed a liquid crystal phase at as very low a concentration as 2%. To explain such remarkable liquid crystallinity was one of the central themes in the famous 1949 paper of Onsager [2], However, systematic experimental studies on the phase behavior in stiff polyelectrolyte solutions have begun only recently. At present, phase equilibrium data on aqueous solutions qualified for quantitative discussion are available for four stiff polyelectrolytes, TMV, DNA, xanthan (a double helical polysaccharide), and fd-virus. 

It fits nicely into the picture of dual structure-phase views of biomesogenic organizations that objects of rod-like appearance , for instance the little world of the tobacco mosaic virus (which - its overall design reduced to a simple rod-like entity - became the starting point of Onsager s theory [37]), as well as much simpler protein and nucleic acid helices are typical mesophase formers in the classical liquid-crystal phase... 

Solutions of rod-like entities in a normally isotropic solvent often form liquid-crystal phases for sufficiently high solute concentration. These anisotropic solution mesophases are called lyotropic liquid crystals". Although the rod-like entities are usually quite large compared with typical thermotropic liquid-crystal mesogens, their axial ratios are seldom greater than 15. Deoxyribonucleic acid (DNA), certain viruses (e.g., tobacco mosaic virus (TMV)), and many synthetic poly-... 

This capability of self-assembly to make ordered arrays of nanostructures is, in essence, nothing new. Crystallization of molecular or atomic species (whether it is the phase transition of liquid water into solid ice, or of liquid silicon into semiconductor-grade silicon crystal) is an example of self-assembly, as are the formation of surfactants in soap bubbles126, the crystallization of viruses for x-ray structure determination127, and the ordering of liquid crystals in displays128. The novelty of self-assembly is in the focus on the formation of matter structured rationally at scales less than 100 nm, and the realization that the only practical method of achieving these structure is to have the components assemble themselves spontaneously. 

Whether quasicrystalline structures are limited to alloys remains an open question. It is possible that their occurrence is much more widespread than had been previously thought. Indeed there is evidence for quasicrystallinity in both thermotropic and lyotropic liquid crystals. Diffraction patterns of decagonal symmetry have been recorded in lyotropic liquid crystals [K. Fontell, private communication], (Fig. 2.19), and there is theoretical evidence for the existence of a quasicrystalline structure within the blue phase of cholesterol (Chapters 4, 5). (The decagonal structure has quasisymmetry perpendicular to the tenfold axes, and translation symmetry along them.) Viruses crystallise in icosahedral clusters and the list continues to grow. In addition to five-fold symmetry, it has been shown that eight and ten- fold quasisymmetry is possible. ... 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

As early as 1938, Langmuir observed the phase separation of clay suspensions into an isotropic phase and a birefringent gel at the macroscopic level in test-tubes [9]. However, in the same report, he noted that this property of phase separation was gradually lost with time, which he tentatively explained by the incorporation of impurities diffusing from the glass tubes. He also compared this system to normal liquid crystals. Later, in 1956, Emerson observed a banded texture similar to that displayed by the Tobacco Mosaic Virus [48]. The investigation of clay suspensions from the structural point of view has been recently resumed. However, the study of the nematic order of suspensions of montmorillonite clays is in fact complicated by their gel properties. In spite of sustained efforts to understand its nature, the gelation mechanism has not yet been fully elucidated [49]. 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

The phase state of lyotropic liquid crystals is mostly determined by a composition of a mixture [27]. Building elements of lyotropics are either rod-like macromolecules (like a tobacco mosaic virus or poly-7-benzyl-glutamate), or micelles formed by amphiphilic molecules in solutions. The shape of a micelle is responsible for the structure of a mesophase [28]. Figure 1.12 shows five possible shapes of building elements (a), responsible for the formation of the corresponding five mesophases (b). 

Ritonavir (Norvir Abbott) is a drug for treating patients infected with human immunodeficiency virus-1 (HIV-1). In solid state the compound consists exclusively of one kind of monoclinic crystals. As this form, now called form I, was not sufficiently bioavailable by the oral route, a liquid formulation containing dissolved ritonavir in a hydroalcoholic solution was developed. It gave satisfactory results during the development and early manufacturing phases. However, in 1998, 2 years after the launch, a second form (form II) precipitated during shelf life. The new form is thermodynamically more stable and about 50% less water-soluble than form I. Within weeks-days, this new form was produced in all the production lines and Abbott had to stop the production of ritonavir. Finally, after considerable effort and expense, a new formulation of ritonavir was developed. 

The two central features of sample preparation inTEM are (1) to produce specimens thin enough (100-200 nm) to transmit a substantial portion of the illuminating beam and (2) in the case of low temperature microscopy, to vitrify any aqueous phases in order to prevent ice crystal damage. In the case of bulk samples, it is necessary to cut frozen sections that are either dried before being examined at low temperatures or examined in the frozen hydrated state. If the objects are small enough—e.g., macromolecules, membrane fragments, viruses, particles, polymers—they may be incorporated into a thin liquid suspension that is rapidly frozen prior to being examined by TEM. 

Just as also in the world of the micro-units phases are known which in their degree of order occupy a place in between the crystalline and the liquid, so one also finds them with some corpuscular proteins with very extended molecules. Under various conditions tobacco mosaic virus protein can be precipitated from its solution in the form of fibrous aggregates which were first looked upon as crystals but on closer investigation proved. to be paracrystals. 

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