S. L. Gómez

Thermal-Lens Model Compared with the Sheik-Bahae Formalism in Interpreting Z-Scan Experiments on Lyotropic Liquid Crystals

We carried out Z-scan measurements on lyotropic liquid crystals and on lyotropic liquid crystals doped with ferrofluid. In these experiments, the sample is translated through the focal region of a focused Gaussian laser beam. The dependence of the far-field intensity on sample position due to intensity-dependent optical nonlinearities has been analyzed on the basis of the thermal-lens model of Gordon et al. and the Gaussian decomposition analysis of Sheik-Bahae et al. The thermal-lens model is nonlocal in space and time, whereas the Gaussian decomposition is predicated on a strictly local response. We compare the goodness of fit of the predictions of these models to experimental data, and we discuss limitations of these models in describing Z-scan experiments on systems with nonlocal response.

Tricritical-like behavior of the nonlinear optical refraction at the nematic-isotropic transition in the E7 thermotropic liquid crystal

We use Z-scan technique to investigate the nonlinear optical response of the thermotropic liquid crystal E7 in the neighborhood of the nematic-isotropic phase transition. The analysis of the data for the nonlinear optical birefringence is compatible with an effective critical exponent of the order parameter, β = 0.28 ± 0.03, which is close to the classical value, β = 0.25 , for a tricritical point. The nonlinear optical absorption in the nematic range depends on the geometrical configuration of the nematic director with respect to the polarization beam, and vanishes in the isotropic phase.

Cu and Fe metallic ions-mediated oxidation of low-density lipoproteins studied by NMR, TEM and Z-scan technique.

In this work we report on a study of the morphological changes of LDL induced in vitro by metallic ions (Cu(2+) and Fe(3+)). These modifications were characterized by transmission electron microscopy, nuclear magnetic resonance and the Z-scan technique. The degree of oxidative modification of LDL was determined by the TBARS and lipid hydroperoxides assays. It is shown that distinct pathways for modifying lipoproteins lead to different morphological transformations of the particles characterized by changes in size and/or shape of the resulting particles, and by the tendency to induce aggregation of the particles. There were no evidence of melting of particles promoted by oxidative processes with Cu and Fe.

Associaçäo entre polimorfismo Gln27Glu do receptor Beta 2-adrenérgico e hipertensäo arterial sistêmica em obesos mórbidos

b2-adrenergic receptors (b2AR) are membrane-bound receptors, which upon binding the endogenous cathecolamines epinephrine and nore-pinephrine signal to the interior of cells via stimulatory guanine nucleotide-binding protein Gs. The sympathetic nervous system activation stimulates energy mobilization and utilization in the adipose tissue that is a favored target for high-energy substrate storage, mobilization and utilization. Adrenergic responsiveness may be altered in obesity and could be an important factor in the pathogenesis and maintenance of obesity state. In the hypertensive state there is physiological and biochemical evidence that b-adrenergic responsiveness is diminished in the face of increased sympathetic tone. Recently, several different polymorphic forms of the human b2AR have been identified in general population, including N-terminal substitutions of glutamine (Gln) for glutamic acid (Glu) at position 27. The aim of this study was to investigate the potential interaction between the b2AR (Gln27Glu) polymorphism and obesity accumulation and hypertension in morbidly obese subjects. The Ita I genotypes of b2AR were established using RFLP methods in 135 individuals with BMI 48 ± 8.02kg/m2. The frequency of Gln/Glu was 31.9% and in the homozygous Glu/Glu was 12.6%. No association was found between BMI, weight gain during the past years and the Ita I genotypes and neither was associated with levels of triglycerides, cholesterol, insulin and glucose. Positive association was found between blood pressure (systolic and diastolic) and presence of polymorphism. The results indicate at the first time that presence of polymorphism 27Glu may provide a mechanism for enhanced vascular reactivity and identify a candidate gene for hypertension in this obesity group.

Thermal-lens effect of low-density lipoprotein lyotropic-like aggregates investigated by using the Z-scan technique

The thermal nonlinear optical response of human normal and copper‐oxidized low‐density lipoproteins (LDLs) were investigated by using the Z‐scan technique, as a function of temperature and concentration of LDL particles. The results show that the Z‐scan signals from normal LDL are characteristic of a sample with negative thermo‐optical coefficient. The amplitude of the Z‐scan signal increases linearly with the LDL concentration, following the usual Beer–Lambert law. On the other hand, oxidized LDLs do not show thermal nonlinear optical response. This behaviour can be attributed to an absorbing element that is modified by the oxidative process. On the other hand, changes in the physical state of the cores and conformation of the ApoB100 protein, due to an increase in temperature, seems to enhance the native LDL thermal nonlinear optical response.