Chiral nematic ordering of polysaccharides

In this paper, evidence for the chiral nematic (cholesteric) self-ordering of cellulose-derived materials is reviewed. A wide range of cellulose derivatives, and some other polysaccharides, form chiral nematic phases, both in concentrated solution and in the melt. Solid films and gels retaining the chiral nematic ordering of the polymer chains may be prepared from these liquid crystalline phases. Optical and electron microscopic techniques may be used to elucidate the helicoidal structure of chiral nematic cellulosics in the liquid crystalline phase, and of the films, gels and solids prepared from such phases. Remarkably, dilute aqueous suspensions of cellulose crystallites prepared by acid degradation also show chiral nematic order; the order is preserved in dry films prepared from the suspensions. The structure of some of these samples prepared in vitro shows a marked resemblance to structures observed in vivo.     

Enantioselective separations using chiral supported liquid crystalline membranes

Porous and nonporous supported liquid crystalline membranes were produced by impregnating porous cellulose nitrate supports with cholesteric liquid crystal (LC) materials consisting of 4-cyano-4'-pentylbiphenyl (5CB) mixed with a cholesterol-based dopant (cholesteryl oleyl carbonate [COC], cholesteryl nonanoate [CN], or cholesteryl chloride [CC]). The membranes exhibit selectivity for R-phenylglycine and R-1-phenylethanol because of increased interactions between the S enantiomers and the left-handed cholesteric phase. The selectivity of both phenylglycine and 1-phenylethanol in 5CB/CN membranes decreases with effective pore diameter while the permeabilities increase, as expected. Phenylglycine, which is insoluble in the LC phase, exhibits no transport in the nonporous (completely filled) membranes; however, 1-phenylethanol, which is soluble in the LC phase, exhibits transport but negligible enantioselectivity. The enantioselectivity for 1-phenylethanol was higher (1.20 in 5CB/COC and 5CB/CN membranes) and the permeability was lower in the cholesteric phase than in the isotropic phase. Enantioselectivity was also higher in the 5CB/COC cholesteric phase than in the nematic phase of undoped 5CB (1.03). Enantioselectivity in the cholesteric phase of 5CB doped with CC (1.1), a dopant lacking hydrogen bonding groups, was lower than in the 5CB/COC phases. Finally, enantioselectivity increases with the dopant concentration up to a plateau value at approximately 17 mol%.     

Supramolecular enantiodifferentiating photoisomerization of (Z)-cyclooctene in lyotropic and thermotropic liquid crystals

In order to understand the roles of moderately organized media and the factors controlling the chirality transfer in supramolecular photochirogenesis, enantiodifferentiating photoisomerization of (Z)-cyclooctene to the chiral (E)-isomer (1E) has been performed for the first time in liquid crystal (LC) systems such as lyotropic LCs of poly(gamma-benzyl-L-glutamate) (PBLG), difluorobenzene derivatives mixture, and thermotropic cholesteryl oleyl carbonate LCs. Basically, the as-employed LCs provided small enantiomer excess (<5%). It is interesting that lyotropic PBLG LCs give contrasting results in cholesteric and nematic mesophases, revealing the importance of the relevant mesophase structure of LC. Selective excitation in achiral difluorobenzene LC doped with a chiral sensitizer facilitates us to conclude that the LC's chiral spatial arrangement is not sufficient or suitable to induce appreciable enantiomeric excess (ee) in the product, but the existence of molecular chirality (of a chiral sensitizer) is essential to afford an optically active (nonracemic) product at least in the present photosensitization system. The photosensitizations in thermotropic LCs further reveal that the product's ee can be manipulated by the LC mesophase not directly but through the sensitizer's conformational changes induced by the supramolecular interactions with the surrounding LC structure.     

Helicoidal self-ordering of cellulose microfibrils in aqueous suspension.

In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.     

Intrinsic chiral domains enantioselectively segregated from twisted nematic cells of bent-core mesogens

Enantioselective segregation has been attained in the Bx phase of a novel substituted oxadiazole achiral banana-shaped liquid crystal (LC) without introducing any chiral species. This bent-core molecule exhibits LC polymorphism; the higher temperature nematic (N) phase and the lower temperature banana smectic phase (Bx phase), in which spontaneous chiral segregation with (+) and (-) chiral domains occurs with equal probabilities. In twisted cell geometries, extrinsically induced twisted N structures are formed and result in intrinsically chiral conglomerate when the temperature is decreased from N to Bx. The observed optical activity in homochiral Bx phase is comparable to those theoretically predicted and is proportional to the cell thickness.     

DNA cholesteric pitch as a function of density and ionic strength

The nature of chiral interactions among chiral biopolymers, such as DNA, protein alpha-helices, and rodlike virus particles, remains elusive. In particular, a satisfactory model connecting molecular chiral interactions and the pitch of the resulting chiral mesophases is lacking. We report the measurement of short-fragment (146-bp) DNA cholesteric spherulite pitch as a function of osmotic pressure, average DNA interaxial spacing, and salt concentration. We determined cholesteric pitch and interaxial spacing by polarizing optical microscopy and x-ray scattering, respectively, from which the twist-angle between DNA molecules can be calculated. Surprisingly, we found that decreasing ionic strength resulted in weaker chiral interactions between DNA chains, as evidenced by the decrease in the twist-angle, and consequent increase in the cholesteric pitch, for a fixed interaxial spacing. We propose that this behavior can be explained by increased smearing-out of the helical charge pattern along DNA as the Debye screening length is increased.     

Imaging of anisotropic cellulose suspensions using environmental scanning electron microscopy

The effect of concentration on anisotropic phase behavior of acid-hydrolyzed cellulose suspensions has been examined using conventional polarizing microscopy and the novel technique of environmental scanning electron microscopy (ESEM). Microcrystalline cellulose dispersed in water formed biphasic suspensions in a narrow concentration range, 4-12 wt % for a suspension pH of 4, where the upper and lower phases were isotropic and anisotropic (chiral nematic), respectively. It is known from previous work that within the biphasic regime total suspension concentration affects only the volume fractions of the two phases, not phase concentration or interfacial packing. As the total suspension concentration surpassed the upper critical limit (c), however, a single anisotropic phase of increasing concentration was observed. It was evident from polarizing microscopy that the chiral nematic pitch of the anisotropic phase decreased with increasing concentration, which has been attributed to a reduction in the electrostatic double layer thickness of the individual rods, thus increasing intermolecular interactions. Chiral nematic textures were also visible using ESEM. This technique has the advantage of studying individual rod orientation within the liquid crystalline phase as it permits the high resolution of electron microscopy to be applied to hydrated samples in their natural state. To our knowledge this is the first time such lyotropic systems have been observed using electron microscopy.     

Synthesis of racemic and optically active glycidic ethers,precursors of liquid crystalline polyethers

A series of racemic and optically active oxiranes, bearing mesogenic groups, precursors of liquid crystalline polyethers, has been synthesized from epichlorohydrin or glycidol. The enantiomeric excess of the optically active oxiranes has been determined by chiral stationary phase HPLC. Compounds bearing 4-cyanobiphenyl mesogenic group exhibit monotropic liquid crystalline behavior. A transfer of chirality to the mesophase has been observed for the optically active oxiranes, which present a cholesteric phase. Chirality 10:779–785, 1998. © 1998 Wiley-Liss, Inc.     

Liquid crystalline ordering of procollagen as a determinant of three-dimensional extracellular matrix architecture

The precise molecular mechanisms that determine the three-dimensional architectures of tissues remain largely unknown. Within tissues rich in extracellular matrix, collagen fibrils are frequently arranged in a tissue-specific manner, as in certain liquid crystals. For example, the continuous twist between fibrils in compact bone osteons resembles a cholesteric mesophase, while in tendon, the regular, planar undulation, or "crimp", is akin to a precholesteric mesophase. Such analogies suggest that liquid crystalline organisation plays a role in the determination of tissue form, but it is hard to see how insoluble fibrils could spontaneously and specifically rearrange in this way. Collagen molecules, in dilute acid solution, are known to form nematic, precholesteric and cholesteric phases, but the relevance to physiological assembly mechanisms is unclear. In vivo, fibrillar collagens are synthesised in soluble precursor form, procollagens, with terminal propeptide extensions. Here, we show, by polarized light microscopy of highly concentrated (5-30 mg/ml) viscous drops, that procollagen molecules in physiological buffer conditions can also develop long-range nematic and precholesteric liquid crystalline ordering extending over 100 microm(2) domains, while remaining in true solution. These observations suggest the novel concept that supra-fibrillar tissue architecture is determined by the ability of soluble precursor molecules to form liquid crystalline arrays, prior to fibril assembly.     

Cholesteric organization of DNA in vivo and in vitro

In concentrated solutions DNA organizes spontaneously to form the "cholesteric" phase which is one type of liquid crystal. We have reproducibly obtained both continuous cholesteric phases and isolated cholesteric globules in equilibrium with the isotropic phase. A comparison is made between this in vitro cholesteric organization and dinoflagellate chromosomes which present the same organization in vivo. The observed defects are analyzed in the two cases. It appears that the cholesteric organization is due to self-assembly phenomena and that the shape of globules and chromosomes is due both to surface tensions and to the presence of defects.     

Change of handedness in cholesteric liquid crystalline phase for N-phthaloylchitosan solutions in organic solvents

The influence of concentration on the helicoidal change of N-phthaloylchitosan (PhCh) solutions in Me2SO, DMAc and DMF was investigated by means of circular dichroism (CD). The critical concentrations to form liquid crystal phase in these three solvents were 43, 45 and 48 wt.%, respectively as measured with polarized optical microscope. There were two kinds of CD peaks, sharp peaks with absorption maximum at about 330 nm induced by the helical conformation of molecular chain, and very broad peaks covering almost whole visible region induced by the cholesteric helix of mesophase. The later only appeared in concentrated solutions with the concentration higher than the critical concentration. The handedness of both levels of helicoidal structures changed from left- to right-handed with the increase of concentration for PhCh/Me2SO solutions. The chirality transfer occurred between these two chiral levels. For PhCh/DMAc and PhCh/DMF systems, only the handedness of helical conformation reversed, but the cholesteric helix did not change. As a method to measure critical concentration, CD is more sensitive than polarized optical microscopy (POM).     

Structural organization of guanosine derivatives in dilute solutions: small angle neutron scattering analysis

Guanosine derivatives, dissolved in water, can form cholesteric and hexagonal mesophases. The common structural unit is a chiral rod-shaped aggregate consisting of a stack of Hoogsten-bonded guanosine tetrameric disks. In order to elucidate the self-association process, we decided to investigate, by small-angle neutron scattering, the structural properties of d(pG), d(GpG), d(GpGpG), d(GpGpGpG) and d(GpGpGpGpGpG) derivatives in very dilute solutions. Under our experimental conditions only d(pG) seems not to form detectable particles. On the other hand, the results for the other derivatives indicate that cylindrical aggregates, having a 10 cross-section gyration radius and a length of about 70 Å, exist in the isotropic phase. According to the structure of the hexagonal and cholesteric phases, we fitted the experimental data by using a model of rod-shaped aggregates formed by stacking about 18 to 20 guanosine tetramers. Moreover, from the measurement of the concentration of scattering particles, we deduced that guanosine derivatives are only partially aggregated, depending on their ability to form mesophases. Correspondence to: P. Mariani     

Cholesteric order in gels and films of regenerated cellulose

Congo red bound to regenerated cellulose in highly swollen gel films formed by slow precipitation from LiCl/N,N-dimethylacetamide solution exhibits induced optical activity. The induced CD band of the dye vanishes when these films are dried under uniaxial stress, indicating that the effect is structural in origin and not simply due to association of dye with chiral centers on the cellulose chain. Cellulose was also regenerated from cellulose acetate films, cast both from isotropic and cholesteric solution, by deacetylation in aqueous ammonia. Congo red bound to cellulose regenerated from cholesteric cellulose acetate exhibits an induced CD band similar to that obtained for films precipitated from LiCl/DMAC solution. The CD spectrum of Congo red in cellulose films regenerated from isotropic cellulose acetate is featureless. These observations indicate that cellulose adopts cholesteric order on slow precipitation from solution.     

In vitro chiral nematic ordering of chitin crystallites

Microfibrillar fragments of purified crab and shrimp chitin were prepared by hydrolysis in 3 HCl at its boiling point (104°C). After removal of the acid by centrifugal washing and dialysis, an ultrasound treatment converts the residual product to a colloidal suspension stabilized by NH₃⁺ charges. When dewatered to a critical concentration, spontaneous formation of a two-phase equilibrium system occurs. The upper phase (lower concentration) is isotropic and the lower phase is anisotropic. The latter displays chiral nematic order and dries to a solid film which mimics the helicoid organization characteristic of the chitin microfibrils in the cuticle of arthropods.     

Controlling the reflection wavelength of iridescent solid films of nanocrystalline cellulose

Nanocrystalline cellulose (NCC) self-assembles in suspension to form iridescent chiral nematic films upon drying that can reflect circularly polarized light at specific wavelengths. Ultrasound treatment has now been found to increase the chiral nematic pitch in suspension and red-shift the reflection wavelength of NCC films as the applied energy increases. Sonication and electrolyte addition combined allow the reflective properties of the film to be predictably tuned. The effects of sonicating an NCC suspension are cumulative and permanent. Suspensions sonicated with different energy inputs may be mixed to give an NCC film having a reflection band intermediate between those obtained from the individual suspensions. The data suggest that the ultrasound-induced red-shift is electrostatic in nature.     

Linear clusters of gold nanoparticles in quasinematic layers of DNA liquid-crystalline dispersion particles

The effects of small size (～2 nm) gold nanoparticles on the properties of particles of cholesteric liquid-crystalline dispersions formed by double-stranded DNA molecules were analyzed. It has been shown that gold nanoparticles induce two different processes. First, they facilitate reorganization of the spatial cholesteric structure of dispersion particles to nematic one. This process is accompanied by the fast decrease in the amplitude of abnormal band in the CD spectrum. Second, they can form ensembles consisting of gold nanoparticles. This process is accompanied by the development and displacement of surface plasmon resonance band in the visible region of the absorption spectrum. The appearance of this band is analyzed by considering two different models of the formation of ensembles consisting of gold nanoparticles. By small-angle X-ray scattering we performed structural analysis of phases formed by DNA cholesteric liquid-crystalline dispersion particles treated with gold nanoparticles. As a result of this study it was possible to prove the formation of linear clusters of gold nanoparticles in the “free space” between the adjacent DNA molecules fixed in the quasinematic layers of liquid-crystalline particles. It has been hypothesized that the formation of linear clusters of gold nanoparticles is most likely related to DNA molecules, ordered in the spatial structure of quasinematic layers, and the toxicity of these nanoparticles in biological systems hypothesized.     

Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions

Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Above a critical concentration, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. We have examined the effect of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp. Longer hydrolysis times produced shorter, less polydisperse black spruce cellulose nanocrystals and slightly increased the critical concentration for anisotropic phase formation. Increased acid-to-pulp ratio reduced the dimensions of the nanocrystals thus produced; the critical concentration was increased and the biphasic range became narrower. A suspension made from a bleached kraft eucalyptus pulp gave very similar properties to the softwood nanocrystal suspension when prepared under similar hydrolysis conditions.     

Protamine‐induced condensation of peptide nanofilaments into twisted bundles with controlled helical geometry

Chiral self‐assembly of peptides is of fundamental interest in the field of biology and material science. Protamine, an alkaline biomacromolecule which is ubiquitous in fish and mammalian, plays crucial roles in directing the helical twisting of DNA. Inspired by this, we reported a bioinspired pathway to direct the hierarchical chiral self‐assembly of a short synthetic dipeptide. The peptide could self‐assemble into negatively charged chiral micelles in water that spontaneously formed a nematic liquid crystalline phase. By incorporation with protamine, the micelles condensed with the protamine into large helical bundles with precisely controlled diameter. Furthermore, to simulate the intracellular environments, we investigated macromolecular crowding on the coassembly of peptide and protamine, which leads to the formation of much thinner helical structures. The results highlight the roles of highly charged biomacromolecules and macromolecular crowding on peptide self‐assembly, which are beneficial for the practical applications of self‐assembling peptides in biomedicine and sensing.     

Structural Origins of Chiral Second-Order Optical Nonlinearity in Collagen: Amide I Band

The molecular basis of nonlinear optical (NLO) chiral effects in the amide I region of type I collagen was investigated using sum-frequency generation vibrational spectroscopy; chiral and achiral tensor elements were separated using different input/output beam polarization conditions. Spectra were obtained from native rat tail tendon (RTT) collagen and from cholesteric liquid crystal-like (LC) type I collagen films. Although RTT and LC collagen both possess long-range order, LC collagen lacks the complex hierarchical organization of RTT collagen. Their spectra were compared to assess the role of such organization in NLO chirality. No significant differences were observed between RTT and LC with respect to chiral or achiral spectra. These findings suggest that amide I NLO chiral effects in type I collagen assemblies arise predominantly from the chiral organization of amide chromophores within individual collagen molecules, rather than from supramolecular structures. The study suggests that sum-frequency generation vibrational spectroscopy may be uniquely valuable in exploring fundamental aspects of chiral nonlinearity in complex macromolecular structures.     

Magnetic ordering of DNA liquid crystals

Sonicated calf thymus DNA with an average length of approximately 100 base pairs has been found to form a cholesteric liquid crystal at a concentration of approximately 250 mg of DNA/mL of solution. Immediately after preparation, small ordered domains of a few micrometers are formed, resulting in an opaque solution. This liquid crystal can readily be oriented in the magnetic field of an NMR magnet, resulting in a clear birefringent phase. The DNA molecules align with their helix axes perpendicular to the field so that the cholesteric pitch axis was parallel with the field. A pitch length of approximately 2.5 microns for the cholesteric phase was determined both from optical measurements (optical light rotation) and from NMR measurements (solvent diffusion). The observation that DNA molecules can be magnetically oriented opens up new possibilities for studying the structure and dynamics of the aligned DNA molecules.