Simultaneous calorimetric and polarization microscopy investigations of light induced changes over phase transitions in a liquid crystal-napthopyran mixture.

We have studied the specific heat and the thermal conductivity in a 4-(n-octyl)-4'-cyanobiphenyl liquid crystal (LC)-photochromic molecules mixture, before, during, and after the photo-activation of the dispersed photochromic molecules, over both the smectic A-nematic and the nematic-isotropic phase transitions. The evaluation of the specific heat has enabled the determination of the changes of the phase transition characteristics induced by the photochromic molecules photoisomerization, while that of the thermal conductivity could be used to monitor the modifications induced in the average LC molecular orientation. The polarization microscopy imaging of the sample texture constituted a valuable support for the interpretation of the obtained thermal conductivity results.

Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell gap.

Despite numerous efforts, continuous wave (CW) lasing in dye doped, one-dimensional (1D) photonic bandgap cholesteric liquid crystal (CLC) structures has not been previously reported, to our knowledge. Here we report on the observation of lasing in such structures under both coherent (laser) and incoherent (LED) CW light excitation. To achieve this effect, we used a 1D-photonic bandgap structure made of a polymer stabilized CLC with a pitch gradient across the cell thickness. A spectral reflectivity profile of such a CLC structure reveals local minima in the area within a photonic stopband and close to it. The realization of lasing pumped by low power CW light sources opens the possibility of all-organic, compact, tunable CW lasers for display and medical applications.

Electric-field-induced nematic-cholesteric transition and three-dimensional director structures in homeotropic cells.

We study the phase diagram of director structures in cholesteric liquid crystals of negative dielectric anisotropy in homeotropic cells of thickness d which is smaller than the cholesteric pitch p. The basic control parameters are the frustration ratio d/p and the applied voltage U. Upon increasing U, the direct transition from completely unwound homeotropic structure to the translationally invariant configuration (TIC) with uniform in-plane twist is observed at small d/p < or = 0.5. Cholesteric fingers that can be either isolated or arranged periodically occur at 0.5 < or = d/p<1 and at the intermediate U between the homeotropic unwound and TIC structures. The phase boundaries are also shifted by (1) rubbing of homeotropic substrates that produces small deviations from the vertical alignment; (2) particles that become nucleation centers for cholesteric fingers; (3) voltage driving schemes. A novel reentrant behavior of TIC is observed in the rubbed cells with frustration ratios 0.6 < or = d/p < or = 0.75, which disappears with adding nucleation sites or using modulated voltages. In addition, fluorescence confocal polarizing microscopy (FCPM) allows us to directly and unambiguously determine the three-dimensional director structures. For the cells with strictly vertical alignment, FCPM confirms the director models of the vertical cross sections of four types of fingers previously either obtained by computer simulations or proposed using symmetry considerations. For rubbed homeotropic substrates, only two types of fingers are observed, which tend to align along the rubbing direction. Finally, the new means of control are of importance for potential applications of the cholesteric structures, such as switchable gratings based on periodically arranged fingers and eyewear with tunable transparency based on TIC.

Gliding of liquid crystals on soft polymer surfaces.

Magnetic-field-induced reorientation of nematic liquid crystals on polymer layers is studied near the glass transition temperature of the polymer. Kinetic curves for different field strengths and temperatures are presented. A model is developed which takes into account the structural rearrangements in the polymer induced by its interaction with the anisotropic potential of the liquid crystal. Simulations based on the model are in good quantitative agreement with the experimental data.

Optically induced periodic structures in smectic-C liquid crystals.

We explore periodic structures of smectic-C (SmC) liquid crystals, induced optically by a polarization grating. The studied cells contain a passive surface of rubbed polyimide and an active photosensitive substrate of azo-dye doped polyimide. In a nematic phase the director field can be periodic independent of the angle between the grating vector and the rubbing direction. In the SmA phase periodic structure can be induced only by layer undulations. The SmC behaves similarly to the nematic phase, but the director can rotate only on a cone, which results in a more complex geometry. The periodic pattern is superimposed with four different director and layer structures. In spite of the coexistence of the nonuniform structures the diffraction efficiency is better in the SmC, than in the nematic phase.

Species differences of serum albumins: III. Analysis of structural characteristics and ligand binding properties during N-B transitions.

PURPOSE: The aim of this study was to investigate the characteristics of the structural transitions and changes in ligand binding properties of different albumins during the pH-dependent structural transition, often referred to as the N-B transition. METHODS: Structural transitions were evaluated by means of spectrometry, differential scanning calorimetry and chemical modification. In addition, ligand binding properties were investigated using typical site-specific bound drugs (warfarin, phenylbutazone, ibuprofen and diazepam). RESULTS: Conformational changes, including N-B transition, clearly occurred in albumins from all species used in this study. The conformational stabilities of all the albumins were clearly lost in the weakly alkaline pH range. This was probably the result of the destruction of salt bridges between domain I and domain III in the albumin molecule. In addition, the profiles of the ANS-induced fluorescence were different and could be classified into two patterns, suggesting that hydrophobic pockets in the albumin molecules were different for the different species. The data suggest that the amino acid residues responsible for the transitions were some of the His residues located in domain I. Further, the ligand binding properties of the albumins were slightly different but statistically significant. CONCLUSIONS: The overall mechanisms of the N-B transition may be similar for all the albumins, but its impact is considerably different among the species in terms of both structural characteristics and ligand binding properties. Furthermore, the transitions appear to be multi-step transitions.

Species differences of serum albumins: II. Chemical and thermal stability.

PURPOSE: The chemical and thermal stability of five species of mammalian serum albumins (human, bovine, dog, rabbit, and rat) were investigated, and conformational stabilities were compared to obtain structural information about the different albumins. METHODS: The chemical stability was estimated by using guanidine hydrochloride (GdnCl), and monitored by fluorometry and circular dichroism (CD). Thermal stability was evaluated by differential scanning calorimetry (DSC). RESULTS: In human, bovine, and rat albumin, two transitions were observed when GdnCl-induced denaturation was monitored fluorometrically, indicating at least one stable intermediate, although, in dog and rabbit albumin, only one transition was observed. However, GdnCl denaturation, as monitored by the ellipticity, showed a two-state transition in all species used in this study. Since these proteins, showing two transitions, contained a conserved tryptophan residue within domain II, these structural changes might have occurred in domain II during intermediate formation. DSC measurements showed that human, bovine, and rat albumin exhibited single sharp endotherms and these were clearly consistent with a two-state transition, while the deconvolution analysis of broad thermograms observed for dog and rabbit albumin showed that the absorption peaks could be approximated by a two-component composition, and were consistent with independent transitions of two different cooperative blocks. CONCLUSIONS: These experimental results demonstrate that species differences exist with respect to the conformational stability and the mechanism of the unfolding pathway for mammalian albumin.

Species differences of serum albumins: I. Drug binding sites.

PURPOSE: The purpose of this study was the classification and identification of drug binding sites on albumins from several species in order to understand species differences of both drug binding properties and drug interaction on protein binding. METHODS: Binding properties and types of drug-drug interaction on the different albumins were examined using typical site I binding drugs, warfarin (WF) and phenylbutazone (PBZ), and site II binding drugs, ibuprofen (IP) and diazepam (DZ) on human albumin. Equilibrium dialysis was carried out for two drugs and the free concentrations of drugs were then treated using the methods of Kragh-Hansen (Mol. Pharmacol. 34. 160-171, (1988)). RESULTS: Binding affinities of site I drugs to bovine, rabbit and rat albumins were reasonably similar to human albumin. However, interestingly, those to dog albumin were considerably smaller than human albumin. On the other hand, binding parameters of DZ to bovine, rabbit and rat albumins were apparently different from those of human albumin. These differences are best explained by microenvironmental changes in the binding sites resulting from change of size and/or hydrophobicity of the binding pocket, rather than a variation in amino acid residues. CONCLUSIONS. We will propose herein that mammalian serum albumins used in this study contain specific drug binding sites: Rabbit and rat albumins contain a drug binding site, corresponding to site I on human albumin, and dog albumin contains a specific drug binding site corresponding to site II on the human albumin molecule.

Anomalous wavelength dependence of the dye-induced optical reorientation in nematic liquid crystals.

We report on Z-scan measurements of the spectral dependence of dye-induced enhancement of optical torque in nematic liquid crystals. It is found that the enhancement factor changes sign from positive to negative as photon energy increases from 2.4 to 2.7 eV. Modification of our earlier model is proposed to describe the observed spectral dependence.

Influence of anthraquinone dyes on optical reorientation of nematic liquid crystals.

Optical reorientation measurements in dye-doped nematic liquid crystals are reported. The dye-induced optical torque exceeds significantly the normal optical torque for all anthraquinone dyes investigated. With one of the dyes, optical Freedericksz transition in a planar layer is demonstrated.