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.     

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.     

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.     

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.     

Chiral doping of nematic phases and its application to the determination of absolute configuration

Doping nematic liquid crystals with nonracemic chiral compounds induces a twisted nematic (cholesteric) phase. The ability of solutes to twist the nematic phase may be related to the overall shape of the chiral dopant and consequently to its absolute configuration. The cholesteric induction is therefore a powerful tool complementary to chiroptical techniques to obtain stereochemical information on chiral molecules.     

HPRT and the Lesch—Nyhan syndrome

The view is presented that extracellular architecture in plant cell walls results from an interplay between molecular self-assembly and mechanical reorientation due to growth forces. A key initial self-assembly step may involve hemicelluloses. It is suggested that hemicelluloses may self-assemble into a helicoid via a cholesteric liquid crystalline phase; the detailed molecular structure of hemicelluloses (stiff backbone, bulky side chains, and the presence of asymmetric carbon atoms) is shown to be consistent with cholesteric requirements for such self-assembly. Since hemicelluloses are hydrogen-bonded to the periphery of cellulose microfibrils, the cellulose could then itself become helicoidally arranged. Such ‘universal plywood’ structure is found in the walls of a wide variety of plants, and in several types of cell (including wood). The permanent effects of growth stresses on patterns seen in sections of helicoids are displayed by computer graphics plots, and the expected changes in stiffness are calculated.     

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.     

Enantioselective potential of polysaccharide‐based chiral stationary phases in supercritical fluid chromatography

The enantioselective potential of two polysaccharide‐based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5 μm silica particles were tris‐(3,5‐dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose‐based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose‐based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose‐based chiral stationary phase were achieved particularly with propane‐2‐ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO₂, respectively. Methanol and basic additive isopropylamine were preferred on amylose‐based chiral stationary phase. The complementary enantioselectivity of the cellulose‐ and amylose‐based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.     

The effect of PH on fibrillogenesis of collagen in the egg capsule of the dogfish, Scyliorhinus canicula

The collagen of the egg capsule of the dogfish, Scyliorhinus canicula is stored and secreted by the secretory cells of the D-zone of the nidamental gland (Rusaou?n-Innocent, 1990b). The collagen appears to pass through several morphologically distinct textures during storage, secretion and fibril formation which may represent different lyotropic liquid crystalline phases (Knight et al., 1993). In the present communication we report evidence that a fall in hydrogen ion concentration induces fibrillogenesis during the secretion of the dogfish egg capsule. In an attempt to understand the factors involved in collagen assembly, we investigated the effects of subjecting isolated collagen storage granules in vitro to solutions ranging in pH from 2-11 and Na(+), K(+), Ca(++), Mg(++), Zn(++) and Cu(++) ions at concentrations varying from 0.01-0.5 M. From pH 2 to pH 4 most granules appeared completely amorphous; from pH 5 to pH 7 granules showed the following previously reported liquid crystalline textures: isotropic, lamellar, micellar, hexagonal columnar, transversely banded twisted nematic, and unbanded twisted nematic. At pH 8 granules showed both the hexagonal columnar phase (phase IV) and small quantities of the final fibrillar phase together with a previously undescribed texture. The latter texture, which we refer to as phase VII, had a D period (17.5 nm) half that of the lamellar texture (phase II) and the final egg capsule fibrils (phase VI). From pH 9 to pH 11, only the final fibrillar texture (phase VI) together with small quantities of the new texture (phase VII) were present. Na(+), K(+), Ca(++), Mg(++), Zn(++) and Cu(++) ions did not appear to have an observable effect on the phases found in isolated granules at pH 7.0. The role of pH in collagen storage and fibrillogenesis was confirmed by direct estimation of the pH in vivo using vital staining with neutral red, a range of pH indicators applied to unfixed cryostat sections and direct measurements of the pH of the jelly within the egg capsule. The implications of these findings for the mechanism of collagen storage and fibrillogenesis in the dogfish egg capsule and other collagenous systems are discussed.     

Reversed-phase liquid chromatographic enantioseparation by cycloalkylcarboxylates of cellulose and amylose

Three novel cycloalkylcarboxylates, cyclopentyl, cyclohexyl, and 1-adamantylcarboxylates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated using a methanol-water mobile phase. Among these esters, cellulose tris(cyclohexylcarboxylate) showed a relatively high chiral recognition ability. The 1-adamantylcarboxylates of cellulose and amylose showed dissimilar chiral recognition abilities from the other two, probably due to the low degree of substitution and the high hydrophobicity of this group.     

Chromatographic enantioseparation by cycloalkylcarbamate derivatives of cellulose and amylose

Cyclopentyl and (+/-)-exo-2-norbornylcarbamates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography (HPLC) were evaluated. Among these carbamates, cellulose tris(cyclopentylcarbamate) and amylose tris((+/-)-exo-2-norbornylcarbamate) showed particularly high chiral recognition, which is comparable to that of several well-known phenylcarbamate derivatives. The chiral recognition mechanism of cellulose tris(cyclohexylcarbamate), which was previously found to be an effective chiral stationary phase for HPLC, was investigated using NMR spectroscopy. The derivative dissolved in chloroform exhibited the chiral discrimination of several enantiomers in NMR as well as in HPLC. For example, the 1,1'-bi-2-naphthol enantiomers were distinctly discriminated in the (1)H, (13)C, and 2D-NOESY spectra.     

Two-dimensional ultrastructural elements lead to three-dimensional reconstruction of protuberances on the cocoon membrane of the leech, Theromyzon tessulatum

Protuberances on the cocoon surface of the leech, Theromyzon tessulatum, are roughly parallel rows of triangular prisms arranged equidistantly to each other on the outer surface of the cocoon membrane. The distance between neighboring protuberances is approximately 1.6 microm, the height approximately 0.5 microm and the semi-width approximately 0.3 microm. The fibrillar arrangement within the protuberance maintains some elements of the helicoids found within the cocoon membrane but a high proportion of large holes disrupt the symmetry of the protuberance ultrastructure. A procedure for 3D reconstruction of the protuberance using the complementarity between the paratangential and normal sections through the cocoon is presented. Our results demonstrate that the ultrastructure of protuberances show elements of a twisted fibrillar arrangement, but the demands of filling a narrow space ruled by acute angles appears to cause a high degree of ultrastructural disorganization.     

Significance of mobile phase composition in enantioseparation of chiral drugs by HPLC on a cellulose-based chiral stationary phase

Eight randomly selected pharmaceuticals, which included ibuprofen, ketoprofen, albuterol, acebutolol, propafenone, betaxolol, methylphenidate, and homatropine, were directly separated on a cellulose tris(4-methylbenzoate) chiral stationary phase (CSP) without derivatization via normal phase mode HPLC. Enantioresolution was achieved by the optimization of the type and the ratio of mobile phase modifiers and additives. The modifiers included alcohols; the mobile phase additives were trifluoroacetic acid (TFA) and triethylamine (TEA). It was found that methanol and ethanol were superior to isopropanol as mobile phase modifiers for enhancing chiral separation of some of the chiral drugs. The results also demonstrated that TFA has a dominant effect on chiral separations for both acidic and basic chiral drugs, although for some basic drug such as homatropine, TEA was more beneficial at improving enantioseparation. The separation of acebutolol enantiomers was achieved for the first time by adding both TFA and TEA to the mobile phase. The purpose of this paper is to demonstrate that the applicability of cellulose based CSPs can be expanded by controlling the mobile phase compositions through the addition of trace amounts of achiral additives and the selection of the appropriate alcoholic modifier. © 1996 Wiley-Liss, Inc.     

Study on the HPLC‐based separation of some ezetimibe stereoisomers and the underlying stereorecognition process

The enantioseparation of ezetimibe stereoisomers by high‐performance liquid chromatography on different chiral stationary phases, ie, 3 polysaccharide‐based chiral columns, was studied. It was observed that cellulose‐based Chiralpak IC column exhibited the best resolving ability. After the optimization of mobile phase compositions in both normal and reversed phase modes, satisfactory separation could be obtained on Chiralpak IC column, especially in normal phase mode. The use of prohibited solvents as nonstandard mobile phase gave rise to better resolution than that of standard mobile phases (n‐hexane/alcohol system). In addition, the presence of ethanol in nonstandard mobile phase has played an important role in enhancing chromatographic efficiency and resolution between ezetimibe stereoisomers. Various attempts were made to comprehensively compare the chiral recognition capabilities of immobilized versus coated polysaccharide‐based chiral columns, amylose‐based versus cellulose‐based chiral stationary phases, reversed versus normal phase modes, and standard versus nonstandard mobile phases. Moreover, possible solute‐mobile phase‐stationary phase interactions were derived to explain how stationary and mobile phases affected the separation. Then the method validation with respect to selectivity, linearity, precision, accuracy, and robustness was carried out, which was demonstrated to be suitable and accurate for the quantitative determination of (RRS)‐ezetimibe impurity in ezetimibe bulk drug.     

The growing outer epidermal wall: design and physiological role of a composite structure

Background: The cells of growing plant organs secrete an extracellular fibrouscomposite (the primary wall) that allows the turgid protoplaststo expand irreversibly via wall-yielding events, which are regulatedby processes within the cytoplasm. The role of the epidermisin the control of stem elongation is described with specialreference to the outer epidermal wall (OEW), which forms a ‘tensileskin’. Novel Facts: The OEW is much thicker and less extensible than the walls ofthe inner tissues. Moreover, in the OEW the amount of celluloseper unit wall mass is considerably greater than in the innertissues. Ultrastructural studies have shown that the expandingOEW is composed of a highly ordered internal and a diffuse outerhalf, with helicoidally organized cellulose microfibrils inthe inner (load-bearing) region of this tension-stressed organwall. The structural and mechanical backbone of the wall consistsof helicoids, i.e. layers of parallel, inextensible cellulosemicrofibrils. These ‘plywood laminates’ containcrystalline ‘cables’ orientated in all directionswith respect to the axis of elongation (isotropic material).Cessation of cell elongation is accompanied by a loss of order,i.e. the OEW is a dynamic structure. Helicoidally arranged extracellularpolymers have also been found in certain bacteria, algae, fungiand animals. In the insect cuticle crystalline cutin nanofibrilsform characteristic ‘OEW-like’ herringbone patterns. Conclusions: Theoretical considerations, in vitro studies and computer simulationssuggest that extracellular biological helicoids form by directedself-assembly of the crystalline biopolymers. This spontaneousgeneration of complex design ‘without an intelligent designer’evolved independently in the protective ‘skin’ ofplants, animals and many other organisms.     

Enantiomer separation of fungicidal triazolyl alcohols by normal phase HPLC on polysaccharide‐based chiral stationary phases

Three fungicidal triazolyl alcohols (triadimenol, hexaconazole, and cis/trans‐1‐4‐chlorophenyl‐2‐1H‐1,2,4‐triazol‐1‐yl‐cycloheptanol) were completely separated into enantiomers by chiral HPLC using polysaccharide‐based chiral stationary phases. A better separation was achieved on cellulose and amylose carbamate phases compared with a cellulose ester phase. Peak shapes were almost symmetrical except for two cases, where tailing of the first eluted enantiomer and unusual symmetric peak broadening were observed. The effect of eluents on enantioseparation was also investigated. Chirality 11:195–200, 1999. © 1999 Wiley‐Liss, Inc.     

Nanocellulose 3, 5‐Dimethylphenylcarbamate Derivative Coated Chiral Stationary Phase: Preparation and Enantioseparation Performance

Nanocrystalline cellulose (NCC) with high surface area and high ordered crystalline structure was prepared from microcrystalline cellulose (MCC) under the hydrolysis of sodium hypochlorite. NCC was further reacted with 3,5‐dimethylphenyl isocyanate to obtain the nanocellulose derivative, and then coated successfully on the surface of silica gel to a prepared NCC‐coated chiral stationary phase (CSP) as a new kind of chiral separation material. Similarly, MCC derivative‐coated CSP was also prepared as contrast. The chiral separation performance of NCC‐based CSP was evaluated and compared with MCC‐based CSP by high‐performance liquid chromatography. Moreover, the effects of the alcohol modifiers, mobile phase additives, and flow rates on chiral separations were investigated in detail. The results showed that 10 chiral compounds were separated on NCC‐based CSP with better peak shape and higher column efficiency than MCC‐based CSP, which confirmed that NCC‐based CSP was a promising packing material for the resolution of chiral compounds.Chirality 28:376–381, 2016. © 2016 Wiley Periodicals, Inc.     

First synthesis of the two enantiomers of alpha-methyldiphenylalanine [(alphaMe)Dip] by HPLC resolution.

A strategy for the preparation of enantiomerically pure (R)- and (S)-alpha-methyldiphenylalanine, constrained phenylalanine analogs, is described. A racemic precursor was prepared in high yield from easily available starting products and subjected to HPLC resolution on a noncommercial chiral stationary phase. More than 600 mg of each enantiomer was isolated in optically pure form by using a 150 x 20 mm ID column containing mixed 10-undecenoate/3,5-dimethylphenylcarbamate of cellulose covalently bonded to allylsilica gel and a mixture of n-hexane/2-propanol/acetone as the mobile phase.     

Phase diagram of nucleosome core particles

We present a phase diagram of the nucleosome core particle (NCP) as a function of the monovalent salt concentration and applied osmotic pressure. Above a critical pressure, NCPs stack on top of each other to form columns that further organize into multiple columnar phases. An isotropic (and in some cases a nematic) phase of columns is observed in the moderate pressure range. Under higher pressure conditions, a lamello-columnar phase and an inverse hexagonal phase form under low salt conditions, whereas a 2D hexagonal phase or a 3D orthorhombic phase is found at higher salt concentration. For intermediate salt concentrations, microphase separation occurs. The richness of the phase diagram originates from the heterogeneous distribution of charges at the surface of the NCP, which makes the particles extremely sensitive to small ionic variations of their environment, with consequences on their interactions and supramolecular organization. We discuss how the polymorphism of NCP supramolecular organization may be involved in chromatin changes in the cellular context.     

Benzoyl cellulose beads in the pure polymeric form as a new powerful sorbent for the chromatographic resolution of racemates

Cellulose-based stationary phases are known to be very efficient and versatile chiral sorbents for the chromatographic resolution of racemates. Except for microcrystalline cellulose triacetate (CTA I), basically all other cellulose-based phases have been prepared by coating of ca. 20% weight polymer on a wide pore silica gel used as a carrier. In this work we describe the preparation of benzoylcellulose (TBC) beads in the pure polymeric form (without inorganic carrier) from an emulsion of the organic polymer. The new material has been fully characterized and used as a chiral stationary phase for the resolution of various classes of racemic compounds such as benzylic alcohols or acetate derivatives of aliphatic alcohols and diols. The structural variety of the separated solutes as well as the irrational influence of the aromatic substituent in different classes of aryl compounds suggest that multiple interaction sites are involved in the complexation, making a prediction of the separation difficult. The benzoyl cellulose beads exhibit a very high loading capacity, which is particularly useful for preparative purposes as demonstrated for selected examples.