The molecular electrostatic potential of the B-DNA helix

The overlap multipole expansion procedure is utilized for the evaluation of the component of the electrostatic molecular potential of the B-DNA helix due to its sugar-phosphate backbone. The overall shape of the potential, its extension in space, the location of the minima and the differences in the values of the potential in particularly significant regions (minor and major grooves, vicinity of the phosphates) are indicated. The isopotential surfaces are practically cylindrical at distances larger than 15 Å from the helix axis but exhibit a more complex structure at shorter distances.This paper is dedicated to Professor H. Hartmann on the occasion of his 65th birthday.     

The molecular electrostatic potential of the B-DNA helix

The influence of a single and double helical DNA environment on the molecular electrostatic potentials at the C₈ and amino sites of guanine and adenine are studied. The results are employed in a tentative explanation of the variation in binding sites of N-acetoxy-N-2-acetylaminofluorene with DNA, following whether the nucleic acid is in a native or denatured state.     

Solution of a molecular statistical model for chiral nematic liquid crystals

A molecular statistical model for the chiral nematic phase of liquid crystals is investigated. The model is treated in the molecular field approximation. The resulting set of coupled integral equations for the order parameters and the pitch determining equation are solved numerically. The model hamiltonian consists of a nematic and a twist producing term. If only the nematic term is present, the model is known to have a first order phase transition. The model containing only the twist producing term is shown to exhibit a second order phase transition. The order parameters and wave length are presented for three ratios of the coupling constants in the case that both interactions are present.     

Induction of a chiral nematic phase in smectic polymers

A number of new photosensitive copolyacrylates of different composition were obtained by the copolymerization of chiral photochromic benzilidene-p-menthane- 3-one acrylic monomers with a smectogenic monomer containing a hexyloxyphenylbenzoate mesogenic group. The chiral, photochromic monomers differ by the length of the aliphatic spacer and the aromatic fragment. It was found that the introduction of a small number of chiral units into the copolymers (5 mol%) leads to the “degeneration” of the smectic C phase, which characterizes the hexyloxyphenylbenzoate homopolymer, and to the formation of the smectic A phase. An unusual effect of chiral nematic phase induction was observed for copolymers containing chiral side groups with two ring aromatic fragments. It should be pointed out that the chiral nematic phase does not occur in the case of the homopolymers of both initial comonomers. An explanation of this effect, based on the consideration of the chemical structure of the chiral and hexyloxyphenylbenzoate units, was suggested. The optical properties of cholesteric copolymers were investigated; the helical twisting power of the chiral groups of different structures was calculated. The possibility of using such copolymers as new photosensitive materials was demonstrated. Received: 16 December 1999/Accepted: 1 February 2000     

Ferrielectric and antiferroelectric chiral twin liquid crystals showing a stable chiral nematic phase

A homologous series of chiral twin liquid crystals possessing identical chiral moieties at both peripheral ends, i.e. optically active α,ω -bis{4-[(4′-(1-methylheptyloxycarbonyl)-4-biphenylyl)oxycarbony]phenoxy}alkanes, has been prepared and their liquid crystalline properties investigated. The homologues preferred to show the ferrielectric and/or antiferroelectric phase rather than the ferroelectric phase. With ascending central spacer length, the temperature range of the ferrielectric phase became narrow and eventually disappeared for the dodecyl homologue, suggesting that the coupling in motion and/or direction between two mesogenic parts of each twin molecule has an important effect on the stabilization of the ferrielectric phase. The octyl and dodecyl homologues showed a wide temperature range chiral nematic phase (ca. 10°C or more), so that these compounds were found to be the first examples showing antiferroelectric and/or ferrielectric phases with a broad temperature range of the chiral nematic phase. X-ray diffraction studies showed that the homologues tend to form a mono-layered structure. The formation of a relatively well defined layered structure was also indicated, which is considered to be important for generating anticlinic ordering in the antiferroelectric and ferrielectric phases. An isotropic-isotropic transition characterized by the emergence of a broad diffuse DSC peak was observed for the even-membered homologues.     

Magnetic alignment of the chiral nematic phase of a cellulose microfibril suspension

Stable suspensions of tunicate cellulose microfibrils were prepared by acid hydrolysis of the cellulosic mantles of tunicin. They formed a chiral nematic phase above a critical concentration. External magnetic fields were applied to the chiral nematic phase in two different manners to control its phase structure. (i) Static magnetic fields ranging 1-28 T were used to align the chiral nematic axis (helical axis) in the field direction. (ii) A rotating magnetic field (5 T, 10 rpm) was applied to unwind the helices and to form a nematic phase. These phenomena were interpreted in terms of the anisotropic diamagnetic susceptibility of the cellulose microfibril. The diamagnetic susceptibility of the microfibril is smaller in the direction parallel (chi( parallel)) to the fiber axis than in the direction perpendicular (chi( perpendicular)) to the fiber axis, that is, chi( parallel) < chi( perpendicular) < 0. Because the helical axis coincides with the direction normal ( perpendicular) to the fiber axis, the helical axis aligned parallel to the applied field. On the other hand, the rotating magnetic field induced the uniaxial alignment of the smallest susceptibility axis, that is, chi( parallel) in the present case, and brought about unwinding of the helices.     

New polymer liquid-crystalline CdS nanocomposites forming a chiral nematic phase

A new approach has been developed for the design of liquid-crystalline polymer nanocomposites combining the unique properties of polymer cholesterics and quantum dots of CdS.     

Towards the biaxial nematic phase through molecular design

In this article, the search for the elusive biaxial nematic phase (NB) in liquid crystals is considered. The structure of the phase is described along with theoretical and computational work which suggests how it might be realised. An overview of the work of the Exeter group in this area is then given showing the different approaches adopted and illustrating how one of these has led to a new type of amphiphilicity based on shape.     

A simple molecular theory of a nematic–nematic phase transition in highly polar compounds

Abstract

We have extended an earlier molecular model which was developed to explain the double reentrant sequence in highly polar compounds to predict the possibility of a nematic (N₁)–nematic (N₂) phase transition in such compounds. At moderate densities the dipolar interactions would give rise to an antiparallel near neighbour arrangement of the polar molecules while at higher densities, the dipole–induced dipole and chain–chain dispersion interactions give rise to a parallel configuration. The N₁–N₂ transition corresponds to a jump in the relative concentration of the two species. Using the mean field approximation we have calculated the phase diagram. The weak first order transition disappears above a critical point as a function of an appropriate parameter. We have also calculated the specific heat anomaly around the transition region.     

Polymer-dispersed liquid crystals From the nematic curvilinear aligned phase to ferroelectric films

Polymer-dispersed liquid crystals (PDLC) are widely used for electro-optic applications such as flexible displays, privacy windows or projection displays. Besides these applications, the confinement of a liquid crystal to small cavities is of fundamental interest. The present paper contains a review of the work on nematic and cholesteric PDLCs. Moreover, some very recent developments are summarized such as the use of ferroelectric liquid crystals for PDLC applications.     

Solvation of the Whelk-O1 chiral stationary phase: a molecular dynamics study

Density functional theory calculations and molecular dynamics simulations are employed to explore the solvation of the Whelk-O1 chiral stationary phase. First, a semi-flexible representation of the Whelk-O1 selective molecule is extracted from an extensive series of B3LYP/6-311+ G(2d,p) calculations. The resulting model is used to build a chiral surface, including end-caps, for molecular dynamics study of the interface between solvent and Whelk-O1. Three solvent environments in common use for Whelk-O1 HPLC have been examined: a normal-phase solvent of n-hexane/2-propanol; a reversed-phase solvent of water/methanol; and a supercritical solvent of CO(2) and methanol. In each case, we analyze the interface with an emphasis on solvent composition and solvent hydrogen bonding to the Whelk-O1 selector.     

Blue phase stability of n-OCB homologue chiral nematic liquid crystal mixtures

Blue phase (BP) stability of a chiral nematic liquid crystal (LC) mixture is dependent upon chemical structure as well as physical properties. In this study, the blue phase temperature range dependent on alkyl chain length was investigated in order to evaluate the relationship between blue phase stability and the molecular structures of four kinds of 4-n-alkyloxy-4'-cyanobiphenyl (n-OCB) homologue chiral nematic LC mixtures composed of rod-like nematic LCs. It was confirmed that the blue phase temperature range was strongly dependent upon the molecular parity, K ₃₃/K ₁₁ and the helical twist power of the n-OCB homologues chiral nematic LC mixtures.     

Luckhurst-Romano model of thermotropic biaxial nematic phase

Recent experiments show that the long looked for thermotropic biaxial nematic phase is finally stabilized in a low mass liquid crystalline system. Inspired by this experimental observation we concentrate on some theoretical issues concerned with this phase. In particular we show that the simplest Lebwohl-Lasher biaxial model, as introduced by Luckhurst and Romano, is consistent with the minimal coupling Landau-de Gennes phenomenological approach. The model shows a rich spectrum of possibilities, in particular a direct isotropic-biaxial nematic phase transition. A possible bridge between molecular and phenomenological approaches, in particular an interpretation of the alignment tensor, is discussed.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

Investigation of the adsorption behaviour of a chiral model compound on a tartardiamide-based network-polymeric chiral stationary phase

The adsorption behaviour of the enantiomers of 2-phenylbutyric acid on the chiral stationary phase (CSP) Kromasil CHI-TBB was studied using hexane/MTBE (90/10) as eluent. Adsorption isotherms were acquired at 40 different enantiomer concentrations in the interval between 7.6 microM and 305 mM, an approximately 40,000-fold dynamic range. The adsorption data fitted well to the bi-Langmuir model, indicating a heterogeneous surface with two different types of adsorption sites having different equilibrium constants and capacities; namely one chiral site and one non-chiral site. A comparison with earlier adsorption studies on modern CSPs revealed that the capacity value of the "true" chiral site of Kromasil CHI-TBB is the largest reported so far. The elution profiles simulated with these parameters show excellent agreement with the corresponding experimental profiles. Guidelines for comparisons of loading capacities of CSPs are presented.     

Specific molecular interactions in Pd(II) complexes identify a new approach to the biaxial nematic phase

Liquid-crystalline complexes of Pd(II) allow a new approach to the realisation of the biaxial nematic phase.     

The phase behaviour and optical properties of a nematic/chiral dopant liquid crystalline mixture system

The phase behaviour and aggregation states of a binary mixture of a nematic liquid crystal and a chiral dopant have been investigated. The nematic liquid crystal E7 was miscible with the chiral dopant S811 over their entire concentration range. Binary E7/S811 mixtures formed the N* phase for S811 contents under 20%, and the SmA* phase for S811 contents between 40% and 90%. BP and TGBA* frustrated phases were found during cooling, for S811 contents between 25% and 35%. The helical pitches of the binary mixtures decreased with increasing chiral dopant content. From XRD profiles, the orientational ordering of the binary composites was found to increase with increasing chiral dopant content.     

The docking of chiral epoxides on the Whelk-O1 stationary phase: a molecular dynamics study

The docking of analytes on the Whelk-O1 chiral stationary phase is explored for two chiral epoxides in a hexane solvent. Density functional theory calculations are employed to develop flexible models for R/S-styrene oxide (phenyl oxirane) and (R,R/S,S)-stilbene oxide (2,3-diphenyl oxirane). Molecular dynamics simulations of the racemates in the presence of the Whelk-O1 chiral stationary phase reveal the distribution of the enantiomers at the interface. The importance of hydrogen bonding and ring-ring interactions is explored along with an examination of the major docking arrangements. The interactions between the Whelk-O1 molecules and the chiral epoxide enantiomers are quite distinct and consistent with the experimental elution orders [S.E. Schaus, B.D. Brandes, J.F. Larrow, M. Tokunage, K.B. Hansen, A.E. Gould, M.E. Furrow, E.N. Jacobsen, J. Am. Chem. Soc. 124 (2001) 1307] and separation factors [W.H. Pirkle, C.J. Welch, Tetrahedron: Asymm. 5 (1994) 777]. The impact of a polar solvent modifier is examined for R/S-styrene oxide where selectivity in 80:20 n-hexane:2-propanol is assessed.