Biomedical subjects

W G Jang

Publications and source records attributed to W G Jang.

4 recordsLinked to original sources

PubMed2003

Polarization-modulated smectic liquid crystal phases.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Journal Article↗

PubMed2001

Evidence from infrared dichroism, x-ray diffraction, and atomistic computer simulation for a "zigzag" molecular shape in tilted smectic liquid crystal phases.

Infrared dichroism (IR) and atomistic computer simulation are employed to probe molecular shape in smectic liquid crystal phases where the optic axis is tilted relative to the layer normal. Polar plots of absorption profiles due to core (phenyl, C-C) and tail (alkyl or methylene, CH2) vibrations in the tilted synclinic (smectic-C) phase of a variety of materials show the phenyl (core) IR absorbance symmetry axes to be consistently tilted at larger angle from the layer normal than the alkyl or methylene (tail). This suggests that, on average, the tails are less tilted than the cores. Furthermore, we find that optic axis tilt angle is close to the core tilt angle measured by IR dichroism, as expected, since liquid crystal birefringence arises primarily from the cores. These results are in accord with the "zigzag" model of Bartolino, Doucet, and Durand. However, we find that only a small fraction of the tail, the part nearest the core, is tilted, and only this part contributes significantly to layer contraction upon molecular tilt.

Journal Article↗

PubMed2001

Time-resolved infrared spectroscopy of nematic-liquid-crystal electro-optic switching.

Time resolved infrared (IR) vibrational absorption spectroscopy is used to probe the dynamics of electric field-induced reorientation of the biphenyl molecular core and alkyl tail subfragments of the nematic liquid crystal 5CB (4-pentyl-4-cyanobiphenyl). The planar to homeotropic transition, induced by application of an electric field step to high pre-tilt planar aligned cells, is studied for switching times ranging from 200 micros down to 8 micros, the latter a factor of 1000 times faster than any previous nematic IR study. The reorientation rates of the core and tail are found to be the same to within experimental uncertainty, and scale inversely with applied field squared, as expected for the balance of field and viscous torques. Thus any molecular conformation change during switching must relax on a shorter time scale. A simple model shows that these data eliminate the possibility of there being substantial differences between the reorientational dynamics of the tails and cores on the time scales longer than on the order of 10 micros.

Journal Article↗

PubMed1999

Identification of retinoic acid receptor element in human cholesteryl ester transfer protein gene.

The transcriptional regulation of the human cholesteryl ester transfer protein (CETP) gene by retinoic acid was investigated by a transient transfection assay. A series of deleted vectors generated from the 5'-upstream region (3434 bp) of the CETP gene linked to the luciferase reporter gene was individually transfected to HepG2 cells. Promoter analyses revealed essential regulatory machinery in the -110/-40 region of the upstream sequence of the human CETP gene. When the cells transfected with the reporter vectors were stimulated with all-trans retinoic acid (tRA), the hormone response was drastically reduced when the -165/-110 region was deleted, thereby suggesting that there may be a retinoic acid receptor element (RARE) in the region. A footprinting analysis showed that the DNA segment at the -165/-134 is protected by the HepG2 nuclear extract. A competition analysis on the gel mobility shift assay using consensus RARE and a purified retinoic acid receptor confirmed the -165/-134 region as being RARE.

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