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P Pasini

Publications and source records attributed to P Pasini.

At least 19 recordsLinked to original sources
PubMed2006

Memory effects in nematics with quenched disorder.

We present a combined experimental and Monte Carlo study of a nematic phase in the presence of quenched disorder. The turbidity of a nematic liquid crystal embedded in a porous polymer membrane is measured under different applied field conditions for field-cooled and zero-field-cooled samples. We find that a significant permanent alignment of the nematic can be induced by fields as low as 0.1 V/microm applied during the isotropic to nematic transition. An analogous effect and dependence on sample history is found by studying the order parameter of a sprinkled disorder Lebwohl-Lasher spin model, indicating that dilute quenched randomness is sufficient to produce memory effects in nematics. The large memory induced by field cooling appears to be written in the system during the transition as a result of the field action on freely oriented nematic nuclei. At lower temperature the nuclei consolidate into permanent nematic textures developed from the interaction with quenched disorder.

Journal Article↗
PubMed2005

Nematics with quenched disorder: pinning out the origin of memory.

Memory effects and glassy behavior have been repeatedly observed in disordered nematic liquid crystals but the connection between these effects and the system topology remained unrevealed. We present an analysis of the local and global topology of the nematic ordering in the presence of quenched disorder and we show that nematics with quenched disorder can be mapped into a system of pinned defect lines and that the memory of the system stems from the pinning of these strings.

Journal Article↗
PubMed2003

Structures and transitions in thin hybrid nematic films: a Monte Carlo study.

We confirm by Monte Carlo simulations of a Lebwohl-Lasher lattice spin model the existence of a biaxially ordered nonbent structure in a liquid-crystalline cell subject to opposing boundary conditions. We report on the observation of the bending transition from the biaxial to the bent-director structure when the temperature of the system is lowered. The structural transition is monitored both by the change of the order parameters and by heat capacity. We discuss the thickness dependence of the transition temperature by means of wetting-induced phenomena and elastic deformations. We propose the correspondence to the phenomenological description, which agrees well without any fitting parameters.

Journal Article↗
PubMed2003

Nematics with dispersed polymer fibrils: a Monte Carlo study of the external-field-induced switching.

We present a Monte Carlo study of molecular ordering in nematics with dispersed regular and random arrays of straight and distorted polymer fibrils. We focus on the collective molecular reorientation--the switching--resulting from the competing aligning effects of fibrils and of a progressively applied transversal external field, and for straight fibrils identify structural Fréedericksz and saturation transitions. The role of fibril topography in the switching is monitored by simulating electric capacitance Slightly distorted fibrils are shown to give a sharper switching at a lower threshold.

Journal Article↗
PubMed2002

Topological defects in schlieren textures of biaxial and uniaxial nematics.

Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/-1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/-1 defects are true singular bulk disclinations that split into pairs of k=+/-1/2 lines; the latter disappear by annihilation processes of the type +1/2-1/2=0. These observed differences are of relevance for the current debate on the existence of biaxial phases.

Journal Article↗
PubMed2002

Nematics with quenched disorder: how long will it take to heal?

Nematics with quenched disorder have been repeatedly predicted to form glass phases. Here we present turbidity experiments and computer simulations aimed at studying glass key features such as dynamics and history dependence in randomly perturbed nematics. Electric field-cooling alignment has been employed to prepare samples in suitably oriented starting states. Remarkable remnant order and slow dynamics are found both by experiment and simulations, indicating that random disorder can, by itself, induce a nematic glass state even without perturber restructuring.

Journal Article↗
PubMed2002

Polymer network-induced ordering in a nematogenic liquid: a Monte Carlo study.

In this Monte Carlo study we investigate molecular ordering in a nematogenic liquid with dispersed polymer networks. The polymer network fibers are assumed to have rough surface morphology resulting in a partial randomness in anchoring conditions, while the fiber direction is assumed to be well defined. In particular, we focus on the loss of long-range aligning capability of the network when the degree of disorder in anchoring is increased. This process is monitored by calculating relevant order parameters and the corresponding 2H nuclear magnetic resonance spectra, showing that the aligning ability of the network is lost only for completely disordering anchoring conditions. Moreover, above the nematic-isotropic transition temperature surface-induced paranematic order is detected. In addition, for perfectly smooth fiber surfaces with homeotropic anchoring conditions topological line defects can be observed.

Journal Article↗
PubMed2001

Increased generation of reactive oxygen species in isolated rat fatty liver during postischemic reoxygenation.

BACKGROUND: Whether fatty infiltration of the liver influences the generation of reactive oxygen species (ROS) during reperfusion is unclear. Thus, this study aimed to compare the ROS formation that occurs during postanoxic reoxygenation in isolated normal and fatty livers. METHODS: Isolated livers from fed Sprague-Dawley rats with normal or fatty livers induced by a choline-deficient diet were reperfused at 37 degrees C for 60 min with an oxygenated medium containing 10 microM of lucigenin after 1 hr of warm ischemia. Superoxide anion generation was assessed by the chemiluminescence (CLS) signal emitted from the organ surface. The hepatic content of malondialdehyde (MDA) and glutathione was determined at the end of reperfusion. Tissue injury was evaluated by the liver histology and the alanine aminotransferase (ALT) release in the perfusate. RESULTS: CLS started rapidly with reoxygenation and it diffused to the whole organ in both groups. However, CLS emission was significantly higher in fatty liver (after 10 min: 812.425+/-39.898 vs. 294.525+/-21.068 photons/cm2/sec; P<0.01). A greater concentration of MDA was measured at the end of reoxygenation in fatty liver. Finally, the liver histology and the ALT release indicated a greater injury in steatotic than normal liver. CONCLUSIONS: The CLS technique allows a direct visualization and comparison of ROS generation from the organ surface. Fatty infiltration increases ROS generation in the liver during postischemic reoxygenation, likely leading to the greater lipid peroxidation observed in these experiments. The increased oxidative stress may contribute to the reduced tolerance of steatotic livers to ischemia-reperfusion injury.

Alanine Transaminase↗
PubMed2001

Oxygen free radical production in rat liver: dose-related effect of ethanol on reperfusion injury.

Ethanol is known to have a deleterious effect on liver ischemia-reperfusion injury, but recent reports suggest that light ethanol consumption may produce a protective effect in several organs. We aimed to investigate effects of different doses of ethanol on liver oxidative injury. Rats were fed with ethanol-containing diets (24, 30, 36, 40% for groups A, B, C, D, respectively). After four weeks, livers were exposed to ischemia-reperfusion. Chemiluminescence was recorded; total lipids, adenosine triphosphate, malondialdehyde, reduced glutathione and lactic dehydrogenase were assessed. In all groups, ischemia resulted in the disappearance of O2*-, a decrease in glutathione and adenosine triphosphate, and stable malondialdehyde values. During the reperfusion phase, O2*- production, malondialdehyde and lactic dehydrogenase increased, reaching significantly higher values in groups C and D and significantly lower values in group B. The effect of ethanol on ischemia-reperfusion injury seems to be a dose-related response, with an additional toxic effect only at high doses of ethanol.

Adenosine Triphosphate↗
PubMed2001

Short daily hemodialysis: blood pressure control and left ventricular mass reduction in hypertensive hemodialysis patients.

Several retrospective and uncontrolled prospective studies reported blood pressure (BP) normalization and left ventricular mass (LVM) reduction during daily hemodialysis (DHD). Conversely, the burden of these major independent risk factors is only marginally reduced by the initiation of standard thrice-weekly dialysis (SHD), and cardiovascular events still represent the most common cause of death in hemodialysis patients. Therefore, we performed a randomized two-period crossover study to compare the effect of short DHD versus SHD on BP and LVM in hypertensive patients with end-stage renal disease. We studied 12 hypertensive patients who had been stable on SHD treatment for more than 6 months. At the end of 6 months of SHD and 6 months of DHD in a sequence of randomly assigned 24-hour ambulatory BP monitoring, echocardiography and bioimpedance were performed. Throughout the study, patients maintained the same Kt/V. A significant reduction in 24-hour BP during DHD was reported (systolic BP [SBP]: DHD, 128 +/- 11.6 mm Hg; SHD, 148 +/- 19.2 mm Hg; P < 0.01; diastolic BP: DHD, 67 +/- 8.3 mm Hg; SHD, 73 +/- 5.4 mm Hg; P = 0.01). The decrease in BP was accompanied by the withdrawal of antihypertensive therapy in 7 of 8 patients during DHD (P < 0.01). LVM index (LVMI) decreased significantly during DHD (DHD, 120.1 +/- 60.4 g/m(2); SHD, 148.7 +/- 59.7 g/m(2); P = 0.01). Extracellular water (ECW) content decreased from 52.7% +/- 11.4% to 47.6% +/- 7.5% (P = 0.02) and correlated with 24-hour SBP (r = 0.63; P < 0.01) and LVMI (r = 0.66; P < 0.01). In conclusion, this prospective crossover study confirms that DHD allows optimal control of BP, reduction in LVMI, and withdrawal of antihypertensive treatment. These effects seem to be related to reduction in ECW content.

Blood Pressure↗
PubMed2000

Comparison between chinese medical herb Pueraria lobata crude extract and its main isoflavone puerarin antioxidant properties and effects on rat liver CYP-catalysed drug metabolism.

Ge-gen (Radix Puerariae; RP) is used in traditional oriental medicine for various medicinal purposes. The drug is the root of a wild leguminous creeper, Pueraria lobata (Willd) Ohwi. It possesses a high content of flavonoid derivatives, the most abundant of which is puerarin (PU). Here, using the enhanced chemiluminescence technique based on horseradish peroxidase and a luminol-oxidant-enhancer reagent, we evaluated in vitro the antioxidant activity of PU and RP crude extract. Both biological samples inhibited the steady-state chemiluminescent reaction in a dose-dependent fashion. However, different inhibition mechanism were postulated, since only RP behaved like conventional antioxidants. This activity was supposed to be due the presence of compounds other than PU in the crude extract. Using each of the specific substrates to different cytochrome P450 (CYP) isoforms or the regio- and stereo-selective hydroxylation of testosterone as polyfunctional probe we found that when intragastrically administered in male Wistar rats, PU (100 or 200 mg/kg b.w.) and RP (700 or 1,400 mg/kg b.w.) significantly altered hepatic CYP-linked monooxygenases. While both CYP content and NADPH-(CYP)-c-reductase activity were significantly increased in all situations, a complex pattern of CYP modulation was observed, including both induction (PU: CYP2A1, 1A1/2, 3A1, 2C11; RP: CYP1A2, 3A1, 2B1) and inactivation (PU and RP: CYP3A, 2E1, 2B1), the latter being due to either parental agents or metabolites, as demonstrated by in vitro studies. Overall, these findings indicate that RP contains compounds with potent antioxidant activity and that both PU and RP impairs CYP-catalysed drug metabolism.

Animals↗
PubMed2000

Nematics with quenched disorder: what is left when long range order Is disrupted?

It is now generally accepted that even low amounts of quenched disorder disrupt long-range order in anisotropic systems with continuous symmetry. However, very little is known on the key item of the nature of the residual order, if any, and particularly if this has quasi-long-range or truly-short-range character. Here we address this problem both experimentally for the nematic 6CB in dilute aerosils and with computer simulations. We find that the residual order is short ranged and scales with disorder density in agreement with the Imry-Ma argument.

Journal Article↗
PubMed2000

Dynamical and field effects in polymer-dispersed liquid crystals: monte carlo simulations of NMR spectra

We analyze the dynamical aspects of molecular ordering in nematic droplets with radial and bipolar boundary conditions-as encountered in polymer-dispersed liquid crystals-by calculating and interpreting the corresponding 2H NMR spectra. In particular, we focus on effects of molecular motion such as fluctuations of molecular long axes and translational diffusion, and on external field ordering effects. As in our previous paper [Phys. Rev. E 60, 4219 (1999)], where field effects were not considered, equilibrium configurations inside nematic droplets are obtained from Monte Carlo simulations of the Lebwohl-Lasher lattice spin model.

Journal Article↗
PubMed2000

Protein microdeposition using a conventional ink-jet printer.

Many recent bioanalytical systems based on immunologic and hybridization reactions in a mono- or bidimensional microarray format require technology that can produce arrays of spots containing biospecific molecules. Some microarray deposition instruments are commercially available, and other devices have been described in recent papers. We describe a system obtained by adapting a commercial ink-jet printer and used to produce mono- and bidimensional arrays of spots containing horseradish peroxidase on cellulose paper. In a few minutes, it was possible to obtain bidimensional arrays containing several thousands of spots with a diameter as low as 0.2 mm, with each of which requiring only a few nanoliters of the enzyme deposition solution. The quantity of enzyme in each spot was evaluated with a chemiluminescent reaction and a charge-coupled device-based, low-light imaging luminograph. The chemiluminescence measurements revealed that the reproducibility of the enzyme deposition was satisfactory for analytical purposes, with the variation coefficients being lower than 10% in almost all cases.

Horseradish Peroxidase↗
PubMed1999

High anti-golgi autoantibody levels: an early sign of autoimmune disease?

IgG anti-Golgi complex antibodies were detected by means of indirect immunofluorescence in the sera of five patients during routine investigation for suspected systemic autoimmune disease. The typical picture of paranuclear fluorescence was observed on the HEp-2 cell line and in tissue sections; anti-Golgi specificity was confirmed on the HEp-2 cells using the immunoperoxidase method. The phenomenon was transient in three patients with a probable viral infection and whose sera had low titre antibodies; however, it was persistent and in high concentration in the other two who, five years later, developed an autoimmune disease. Only in the sera of these last two patients were specific bands of 123, 72, 46, 37 and 26 kilodaltons found by the immunoblotting technique on cytoplasmic extracts. Although the detection of anti-Golgi autoantibodies is rare, and may represent a transitory epiphenomenon in patients with a viral infection, their presence in high titre in the absence of a clear clinical picture may constitute an early sign of systemic autoimmune disease.

Adult↗
PubMed1999

Chemiluminescence Western blot assay for the detection of immunity against parvovirus B19 VP1 and VP2 linear epitopes using a videocamera based luminograph.

A chemiluminescent Western blot (WB) assay was developed to detect the immune status against linear epitopes of parvovirus B19 structural proteins VP1 and VP2. The chemiluminescent WB assay combined the sensitivity of chemiluminescent substrates and the objective evaluation of the luminescent signal obtained with a new system which consists of a videocamera-based, high-performance, low light level imaging luminograph connected to a personal computer for image analysis. The potential for diagnostic purposes was evaluated using reference serum samples and 75 clinical samples from patients with different clinical conditions and laboratory evaluations regarding B19 infection. The chemiluminescent Western blot assay provided reproducible results, an objective evaluation of the results and a semi-quantitative analysis of the presence of antibodies against VP1 and VP2 in human sera. The chemiluminescent Western blot assay proved more sensitive than the classic colourimetric Western blot assay.

Adolescent↗