Reaction field method for the molecular orientation potential in an anisotropic medium

The molecular orientation potential in an anisotropic medium was studied using the reaction field method. The interaction energy between a molecule and an anisotropic medium was calculated using the dielectric anisotropy as a perturbation parameter. We have investigated the influence of molecular geometry on the molecular orientation potential using biphenyl and terphenyl, which constitute cores of liquid crystal molecules. We have calculated the barrier height for the molecular rotation changing the twist angle between phenyl rings, using anab initio molecular orbital method. We have found that the torsional motion of the phenyl rings affects the orientation potential, and that deviation from the molecular cylindrical symmetry lowers the saddle point for the rotation.     

Elongated films of polymer-dispersed liquid crystals as scattering polarizers

Elongated films of polymer dispersed liquid crystals have been experimentally investigated. The dependence of transmittance light polarization on elongation coefficient and temperature has been studied. The possibilities of practical applications of elongated films of polymer dispersed liquid crystals are discussed.     

Direct measurement of interaction forces in free thin liquid films stabilized with phosphatidylcholine

Microscopic foam films from suspensions of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles have been obtained. The film formation is facilitated at temperatures above the gel-liquid crystalline transition of DMPC. A detailed study of the dependence of equilibrium thickness of DMPC foam films on electrolyte concentration at constant capillary pressure and a direct measurement of the disjoining pressure isotherm has been carried out. Formation of thick equilibrium horizontal microscopic films stabilized with DMPC at low external pressures and low electrolyte concentrations was found and interpreted as being due to the existence of long-range electrostatic interactions in these films. A diffuse electric layer potential of 36 mV has been calculated. The DMPC films have been compared to films obtained from non-ionic surfactant solutions where the long range electrostatic repulsion is explained as being due to specific adsorption of OH^– at the film interfaces. However, unlike the results obtained for surfactant films, in this study formation of common black films and thinning of the DMPC Newton film with pressure have been observed.Offprint requests to: D. Exerowa     

Thermotropic and lyotropic properties of long chain alkyl glycopyranosides: part III: pH-sensitive headgroups

As part of a series of papers, the influence of carbohydrate headgroups and aliphatic chains on the mesogenic properties of glycolipids was investigated. Alkyl glycosides with different types of aliphatic chains were synthesised. Neutral glycolipids were oxidized to their uronic acid derivatives, using the well established TEMPO-oxidation. For comparison a 6-deoxy-6-amino alkylglucopyranoside was synthesised. In addition, the thermotropic and lyotropic phase behaviour of the synthesised compounds were investigated. The thermotropism was characterised by polarising microscopy, the lyotropism by the contact preparation method.     

Presence of anisotropic (birefringent) crystalline structures in embryonic and juvenile monogonont rotifers

A systematic survey for the presence of birefringent (anisotropic) structures in rotifers was undertaken. Several common features of rotifers exhibit anisotropism (e.g. trophi & muscles). However, unusual anisotropic crystalline structures (ACS) were found in late stage embryos (i.e. possessing eyespots and trophi, and showing movement). ACS were found in 18 of 26 species of monogonont rotifers (comprising 11 genera of 5 families). In Sinantherina socialis, ACS were present in the lower gut as compact, spherical masses of minute crystals that slowly broke apart and disappeared within 20 hours of hatching. Although several authors have described the existence of refractive bodies in rotifers, to my knowledge this is the first report of their birefringent properties.     

Computer Simulation of Liquid Crystal Films

Abstract

We present the results of extensive Monte Carlo simulations of liquid crystal films of various thicknesses. A simple nearest-neighbour lattice model, the Lebwohl-Lasher model, is employed, with periodic boundaries in two directions and free, planar, surfaces in the third. Particular attention is devoted to locating the temperature of the order-disorder (nematic-isotropic) phase transition. Weak first-order behaviour apparently persists in systems as thin as 8 layers across, but below this the transition cannot be detected. The shift of the transition temperature from its bulk value approaches the expected asymptotic linear dependence on inverse thickness, but significant deviations from this are seen for films of 10 layers thickness and less. These results enable an accurate estimate to be made of the bulk phase transition temperature in the thermodynamic limit, and the result is consistent with that extrapolated from systems with full periodic boundaries.     

Molecular organization of type IV collagen: polymer liquid crystal-like aspects

A new X-ray diffraction pattern from type IV collagen is described, which can be interpreted on the basis of crystalline and liquid crystalline origins of the reflections. Bovine anterior lens capsules extracted with 1 NaCl and oriented by extension of 60% under constant load gave medium angle X-ray diffraction patterns showing many of the characteristics typical of liquid crystals. Prominent features, apart from those wide angle features attributable to the collagen triple helix, are (1) a four-point pattern of broad reflections at d-spacing 3.9 nm, and layer line spacing near 5 nm. (2) A broad intense equatorial peak centred at 1.24 nm, indicative of LIQUID=like lateral molecular associations (3) A set of five sharp, streaked meridional reflections (previously obscured by the broad peak near 5 nm in unextracted capsules). (4) A further six higher angle reflections of a diffuse, arced and broad appearance on the meridian. The sharp streaked meridional reflections emanate from a long-range periodicity of units 8–9 nm in diameter. These features form a self-consistent system if interpreted on the basis of staggered liquid crystal-like array of collagen molecules, in which case the first five meridionals and remaining broad reflections, sampled on the meridian, can all be indexed as orders of 21 nm.     

Plasmid analysis and antimicrobial susceptibilities of Peptostreptococcus species

Abstract There are no published methods on plasmid isolation from Peptostreptococcus spp., therefore two methods of plasmid isolation from this genera were analysed: the boiling and alkaline-SDS methods. Plasmid DNA was not recovered by the boiling method, however, with the alkaline-SDS method, cryptic plasmid DNA was detected in two P. asaccharolyticus and one P. magnus strains. To achieve optimum lysis, Peptostreptococcus cells were treated with lysozyme (2 mg/ml) for 15 min. at 37°C followed by proteinase K (0.2 mg/ml) for 1 h at 37°C. In addition we report, the occurence of clindamycin or metronidazole-resistant peptostreptococci, but these phenotypes were not correlated with plasmid carriage.     

Fracture formation in vitrified thin films of cryoprotectants

As a part of an ongoing effort to study the continuum mechanics effects associated with cryopreservation, the current report focuses on fracture formation in vitrified thin films of cryoprotective agents. The current study combines experimental observations with continuum mechanics analysis. Experimental results have been developed using a new imaging device, termed a "cryomacroscope", which has been recently presented by the current research team. A newly developed liquid nitrogen-based cooling stage is presented in this paper. The samples under investigation are 0.5 ml droplets of cryoprotective agents, having a characteristic diameter of 20 mm and a characteristic thickness of 1.5 mm. Tested samples included dimethyl sulfoxide (DMSO) in a concentration range from 6 to 8.4M, and the cryoprotectant cocktails VS55 and DP6. Some samples contained small bovine muscle segments, having a characteristic dimension of 1mm, in order to study stress concentration effects. Experimental results show that the onset of fracturing in vitrified films of cryoprotectants is very consistent, occurring over a small temperature range. Fracture pattern, however, was affected by the cooling rate. The presence of tissue segments did not affect the onset temperature of fracture, but affected the fracture pattern. The continuum mechanics analysis solidified the hypothesis that fracture is driven by thermal stress, not by temperature per se, and allowed fracture strain to be inferred from observed fracture temperature. In conjunction with the current report, additional photos of fracture formation in thin films are available at .     

Hexagonal-columnar mesophases of acylated azacrowns with enhanced stabilities

A series of discotic liquid crystals based on completely acylated macrocyclic oligoamines (azacrowns) has been synthesized. Discotic mesophases could be very highly stabilized by making use of an increased steric hindrance, caused by supplemental lateral meta-substituents which were introduced into the cinnamic acids used as the acylation agents. Employing this method yields compounds exhibiting hexagonal mesophases over a very broad range of more than 300°. Even if very large macrocycles are used as the core of such disc-shaped compounds, enantiotropic mesophases are achieved. Themeta-substituents as well as the size of the azacrowns were varied and their influence on the mesomorphic behavior was studied.     

Transverse dipoles added to DNA chains by drug binding can induce inversion of the long-range chirality of DNA condensates

The addition of poly(ethylene glycol) (PEG) to a DNA solution induces phase separation of droplets of condensed DNA. These droplets possess liquid crystalline properties and their ordering is cholesteric. It was recently proved that daunomycin, by binding to DNA chains, inverts the long-range chirality of their tertiary packing into aggregates. The present paper suggests one possible mechanism by which this inversion can take place. Daunomycin bears a cationic group in its sugar residue. Its intercalation adds a helicoidal distribution of transverse dipoles to DNA chains. By this mechanism, in favourable cases, ionic or strongly polar groups in drugs which bind DNA can induce handedness inversion of the cholesteric ordering of its condensates. This inversion mechanism was tested experimentally using several, charged and uncharged, homologues of daunomycin. All those bearing the cationic ammonium group inverted the long-range chirality of the PEG-induced DNA mesomorphic state. The effects of the uncharged desamino homologues could not be evaluated because of their lower solubility and binding affinity for DNA.     

Structure and nonlinear optical properties of copper phthalocyanine thin films

This paper describes a joint study of the structure and nonlinear optical properties of vacuum evaporated thin films of copper phthalocyanine (CuPc for brevity). Film thickness ranges from 50 to 500 nm. The anisotropic paramagnetic resonance of Cu⁺⁺ ions reveals that the Pc rings lie almost parallel to the substrate plane with however a large angular distribution (30° FWHM). Third harmonic optical generation measurements performed at 1.064 m and 1.907 m fundamental wavelengths give respectively an average value of the cubic susceptibility ⁽³⁾(-3,)=(4±0.4)·10^–12 e.s.u. and (2.1+-0.2) · 10^-12 These values, although significantly higher than for a common ionic crystal, are about one order of magnitude lower than in conjugated 1-D systems, which shows that the 2-D -electron delocalization is less profitable than the 1-D one. Besides third harmonic, we have also observed second harmonic generation. Its polarization dependence is characteristic of a quadratic susceptibility enhanced in one direction, almost perpendicular to the substrate, withd _eff comprised between 30 and 60 · 10^-9 e.s.u. The possible origins ofd _eff are discussed.     

Cobalt hexafluoroacetylacetonate polyether adducts for thin films of cobalt oxides

The novel [Co(C₅F₆HO₂)₂ · 2H₂O · CH₃(OCH₂CH₂)₂OCH₃] and [Co(C₅F₆HO₂)₂ · 2H₂O · CH₃(OCH₂CH₂)₃OCH₃] low-melting adducts have been synthesized and characterized by elemental analysis, IR spectroscopy, mass spectra and TG-DTG thermal measurements. The former adduct is liquid at room temperature, whilst very mild heating of the latter results in a thermal stable liquid compound. Both adducts can easily be evaporated. Deposition experiments, in a low-pressure horizontal hot-wall reactor, on optical transparent SiO₂ substrates, using these precursors, result in CoO or Co₃O₄ films, depending on the deposition conditions. XRD measurements provide evidence that CoO and Co₃O₄ consist of cubic, highly oriented, (2 0 0) and (3 1 1) crystals, respectively. The mean crystallite sizes were evaluated from the XRD line broadening. Both optical spectra and resistivity measurements of Co₃O₄ films show that they are semi-conducting and their band-gap was determined from the optical induced transitions. The film cross section and the surface atomic composition were investigated by SEM and XPS analyses, respectively.     

The liquid crystalline nature of the cytoskeleton in epidermal cells of the chaetognath Sagitta

The chaetognaths have a multilayered epidermis, which is not covered by cuticle, except in the head region. Two kinds of cells are found in the epidermis: the filament-rich cells, adjacent to the basement membrane, and superficial cells, which are filament poor. The filament-rich cells, which are linked by gap junctions and columnar junctions, are highly developed in the collarette region, which joins the head and the trunk. As elsewhere in the epidermis these cells are covered by the filament poor cells which are linked by zonulae adhaerentes, gap junctions and septate junctions. The filaments present in the inner cells of the collarette form a twisted fibrous arrangement, which shows parallel series of nested arcs when observed in oblique section. Such systems are well known in numerous skeletal materials and correspond to polymerized analogues of certain liquid crystals. The amount of connective tissue is extremely reduced in Sagitta. One can hypothesize that filament-rich cells are abundant in regions which undergo strong deformations. This is the case in the collarette, in contact with the basement membrane of the epidermis (which in turn is in contact with a myotendinous system), in a region where ingested prey must go through the general cavity where there is high internal pressure.     

Diffusion of hydrophilic probes in bicontinuous lipidic cubic phase

The lipidic cubic phase was prepared by mixing monoolein (monooleoyl-rac-glycerol, MO) with water in 64:36% ratio and applied to the solid support-glassy carbon or platinum electrodes. Highly viscous, homogeneous and transparent cubic phase film remained stable and firmly attached to the electrode surface. In order to describe the efficiency of transport of small hydrophilic molecules within the film, we studied the diffusion of selected redox mediators along the network of aqueous channels present in the cubic phase structure. Loading times, diffusion coefficients and concentrations of the mediators in the layer were determined by voltammetry and chronocoulometry using two types of electrodes: a normal size electrode working in the linear diffusion regime and an ultramicroelectrode working under spherical diffusion conditions. In addition to the well-defined order, transparency and viscosity, the fast transport of small redox mediators through the aqueous channels of the cubic phase and along the interfacial water-lipid region is another important property of this matrix. The diffusion of the hydrophilic probes in the cubic phase was found to be more efficient than in the Nafion layers. Efficient transport of small redox mediators within the cubic phase means that not only enzymes and synthetic catalysts can be incorporated into the phase but also their fast communication with electrode surface will be enabled thanks to the simultaneous incorporation of small mobile redox mediators. This property of the cubic liquid crystalline phases based on lipids makes them especially interesting from the point of view of practical applications in biosensing and bioelectrocatalysis.     

Preliminary crystallographic analysis of class 3 rat liver aldehyde dehydrogenase

NAD-linked aldehyde dehydrogenases (A1DH) (EC 1.2.1.3) catalyze the irreversible oxidation of a wide variety of aldehydes to their respective carboxylic acids. Crystals of a class 3 AIDH (from an Escherichia coli expression system) suitable for X-ray analysis have been obtained. These crystals, which can be grown to a size of 0.8 x 0.3 x 0.2 mm, diffract to 2.5 A resolution. Analysis of the diffraction pattern indicates that the crystals belong to the monoclinic space group P21, with cell parameters a = 65.11 A, b = 170.67 A, c = 47.15 A, and beta = 110.5 degrees. Assuming one dimer per asymmetric unit, the value Vm is calculated to be 2.45 and the solvent content of the crystal is estimated to be 50%. A self-rotation function study produced significant rotation peaks (58% of the origin) on the kappa = 180 section at psi = 90 degrees and phi = 71 degrees and 341 degrees, indicating that the pseudo-dimer axis is (or is very nearly) perpendicular to the b-axis.     

Excluded volume of hard cylinders of variable aspect ratio

In this article, we report expressions for the excluded volume, v _ex and second virial coefficient, B ₂, of hard cylinders as a function of their aspect ratio (𝒜 = L/D), where L and D are the length and diameter of the cylinder, respectively. These expressions are valid for aspect ratio values within the interval 0.001 < 𝒜 < 100, that covers from thin plates to long rods and reproduce Monte Carlo (MC) simulation values. We compare these results with Onsager's predictions and with reported values of hard bodies of similar geometry, as cut spheres (CS), hard spherocylinders (HSC) and linear tangent hard sphere chains (LTHSC). Simulation values for v _ex were obtained with an overlap algorithm that is also presented in detail. The obtained results can be applied in theoretical and computer simulation studies of discotic liquid crystals.     

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.     

The importance of the explant on regeneration in thin cell layer technology

Summary The basic factor underlying the success of the tissue culture, large-scale micropropagation and genetic transformation of any plant species is regeneration. This has been achieved over the years through the use of various-sized explants ranging from protoplasts (small scale) to entire organs (large scale). Inherent problems underlie the use of either extreme, leading to both nonspecific morphogenic reactions in the latter, or to undesired necrosis in the former. This review investigates the importance of different aspects of a thin cell layer (TCL) explant, from its source to its size. TCLs, as a results of their size and origin, in combination with other controllable factors such as media and environmental conditions, have shown this system to be superior to the use of conventional explants. Numerous species that were previously unsuccessfully tissue-cultured have, with the use of TCL technology, resulted in their successful micropropagation and regeneration. These successes, based on the inherent qualities of the TCL explant—specific for a given species—are also examined.