Biomedical subjects

B R Ratna

Publications and source records attributed to B R Ratna.

9 recordsLinked to original sources

PubMed2004

Isotropic-nematic transition in liquid-crystalline elastomers: lattice model with quenched disorder.

When liquid-crystalline elastomers pass through the isotropic-nematic transition, the orientational order parameter and the elastic strain vary rapidly but smoothly, without the expected first-order discontinuity. This broadening of the phase transition is an important issue for applications of liquid-crystalline elastomers as actuators or artificial muscles. To understand this behavior, we develop a lattice model of liquid-crystalline elastomers, with local directors coupled to a global strain variable. In this model, we can consider either random-bond disorder (representing chemical heterogeneity) or random-field disorder (representing heterogeneous local stresses). Monte Carlo simulations show that both types of disorder cause the first-order isotropic-nematic transition to broaden into a smooth crossover, consistent with the experiments. For random-field disorder, the smooth crossover into an ordered state can be attributed to the long-range elastic interaction.

Journal Article↗

PubMed2004

New addresses on an addressable virus nanoblock; uniquely reactive Lys residues on cowpea mosaic virus.

Cowpea mosaic virus (CPMV) is a robust, icosahedrally symmetric platform successfully used for attaching a variety of molecular substrates including proteins, fluorescent labels, and metals. The symmetric distribution and high local concentration of the attached molecules generates novel properties for the 30 nm particles. We report new CPMV reagent particles generated by systematic replacement of surface lysines with arginine residues. The relative reactivity of each lysine on the native particle was determined, and the two most reactive lysine residues were then created as single attachment sites by replacing all other lysines with arginine residues. Structural analysis of gold derivatization not only corroborated the specific reactivity of these unique lysine residues but also demonstrated their dramatically different presentation environment. Combined with site-directed cystine mutations, it is now possible to uniquely double label CPMV, expanding its use as an addressable nanoblock.

Arginine↗

PubMed2003

Order parameter measurements of dichroic dyes dissolved in smectic liquid crystals that tilt without layer contraction.

Measurements of the orientational order parameter of dissolved dichroic dyes are reported for two smectic-A liquid crystals that tilt in the presence of an in-plane electric field without any decrease in the layer spacing. The dye order parameter is determined by measuring the anisotropic absorption of linearly polarized light. Different dyes are used and measurements are also performed on a smectic liquid crystal that tilts with the expected layer contraction to check how closely the measurements reflect the order parameter of the liquid crystal. The variation of the dye order parameter with electric field is in accordance with the recently proposed model of azimuthal disorder of the tilt angle direction, but the surprising finding is that the local dye orientational order parameter is significantly lower than for the smectic liquid crystal that tilts with the expected layer contraction. This suggests that another mechanism might be contributing significantly to the smectic order, one possibility of which is the tendency for different parts of these siloxane-containing molecules to segregate within each layer. Another possible explanation is that the azimuthal disorder is due to a modulated phase with a high enough density of defects to decrease the value of the local order parameter.

Journal Article↗

PubMed2002

Isotropic-nematic transition in liquid-crystalline elastomers.

In liquid-crystalline elastomers, the nematic order parameter and the induced strain vary smoothly across the isotropic-nematic transition, without the expected first-order discontinuity. To investigate this smooth variation, we measure the strain as a function of temperature over a range of applied stress, for elastomers cross-linked in the nematic and isotropic phases, and analyze the results using a variation on Landau theory. This analysis shows that the smooth variation arises from quenched disorder in the elastomer, combined with the effects of applied stress and internal stress.

Journal Article↗

PubMed2001

Anomalous softening of order parameter fluctuations at the smectic-A to -C* transition in a siloxane-substituted chiral liquid crystal.

Unconventional softening of order-parameter fluctuations is observed at the smectic-A to smectic-C* phase transition in a chiral liquid crystal possessing multiple siloxane substituents on its hydrocarbon chains. Together with an optical "stripe" texture detected above the transition, the atypical dynamics can be explained by the pretransitional development within the smectic layers of a modulated state of the order parameter. Conventional soft-mode behavior is restored when the degree of chain substitution is reduced.

Journal Article↗

PubMed1999

Monte Carlo simulation of smectic liquid crystals and the electroclinic effect: the role of molecular shape.

Using Monte Carlo simulation methods, we explore the role of molecular shape in the phase behavior of liquid crystals and the electroclinic effect. We study a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres bonded rigidly together with no intramolecular degrees of freedom. For strongly angled molecules, we find that steric repulsion alone provides the driving force for a smectic-C phase, even without intermolecular dipole-dipole interactions. For weakly angled (nearly rodlike) molecules, we find a stable smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt angle of about 19 degrees. In the SmA phase we find evidence of vortexlike point defects. We also observe a field-induced nematic-smectic phase transition.

Journal Article↗

PubMed1996

Encapsulation of hemoglobin in a bicontinuous cubic phase lipid.

The effects of encapsulating bovine hemoglobin (BHb) in the bicontinuous cubic phase formed by monooleoylglycerol and water was investigated with Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. Cubic phase was formed in the presence of 1-10 wt% BHb. Studies using X-ray diffraction reveal that at 0.5-2.5 wt% BHb, the cubic phase structure is characterized by the double diamond lattice (Pn3m). At 2.5-5 wt% BHb, coexistence of two cubic phase structures, Pn3m and the gyroid lattice (Ia3d), was observed while at BHb, concentrations higher than 5 wt% the gyroid structure persists. FTIR shows there is an increase in intensity of the free nu C = O (1745 cm-1) and a corresponding decrease in the intensity of the hydrogen bonded nu C = O (1720 cm-1) as the BHb concentration is increased. The nu C-O-CO peak shifts from 1183 cm-1 to 1181 cm-1 as the concentration of BHb raised from 2.5 to 10 wt% indicating BHb may induce subtle changes in the interfacial region of cubic phase monoolein. The bandwidth of the nu asCH2 stretch (2926 cm-1) increased in the presence of 5 wt% BHb compared to samples with 2.5 or 10 wt% BHb. The increase in frequency of the nu sCH2 stretch (2854 cm-1) induced by increasing temperature 20 to 60 degrees C was dampened when BHb was present compared to samples heated in isotonic buffer. Analysis of the amide I band at 1650 cm-1 showed that the secondary structure of BHb is not affected by encapsulation in monoolein. In vitro release studies showed that 45% of the entrapped BHb was released after 144 h at 37 degrees C. The porous nature of bulk cubic phase was further demonstrated by diffusion of K2Fe(CN)6 and conversion of 73% of the oxyhemoglobin to methemoglobin after 1 h. These results suggest that the cubic phase may be useful for encapsulation of Hb as a red cell substitute and for the encapsulation and delivery of other bioactive agents.

Animals↗

PubMed1992

Effect of alcohol chain length on tubule formation in 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine.

Aqueous dispersions of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, on cooling below the chain melting temperature, form hollow cylindrical structures known as 'tubules'. We have studied the formation of tubules in methanol/water, ethanol/water and n-propanol/water. For each alcohol, there is a defined window of alcohol/water ratios in which the lipid precipitates with the tubule morphology. As the chain length of alcohol is increased, the window shifts towards lower alcohol fraction. Light scattering studies show that at very low lipid concentrations the tubules self-assemble directly from the isotropic phase where as for lipid concentrations greater than 4 mg/ml an intermediate L alpha phase is observed. These results indicate that the mechanism of tubule formation may be dependent on lipid concentration.

Alcohols↗

PubMed1991

Self-assembling phospholipid filaments.

Aqueous dispersions of double-chain phospholipids spontaneously assemble into closed bilayers called vesicles (or liposomes). Although the vesicles are in general topologically spherical, cylindrical and helical liposomes have sometimes been observed. We present here video-enhanced microscopic studies of a diacetylenic phospholipid dispersed in ethanol/water, which reveal the existence of unusual bilayer morphologies. On cooling the dispersion from the isotropic phase, we have observed the formation of long (of the order of hundreds of micrometres), thin (0.2-2 microns) filaments, which fluctuate strongly. When the temperature is decreased further, the filaments rapidly retract into a mass of lipid. At constant temperature, on the other hand, the filaments transform into torus or ring-like vesicles. Such non-spherical structures have been predicted theoretically but not previously observed experimentally.

Phosphatidylcholines↗