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A C Ribeiro

Publications and source records attributed to A C Ribeiro.

At least 19 recordsLinked to original sources
PubMed2006

Frustrated structure of an anticlinic-like smectic-C phase.

We present a polarising optical microscopy study of the low-temperature anticlinic-like tilted mesophase of the liquid-crystal compound octylphenyl-2-chloro-4-(p-cyano-benzoyloxy) (DB8Cl). This mesophase has been described as a bilayer smectic structure in which the molecules within each layer are organised in an anticlinic way. The optical textures observed in samples with planar orientation show a double stripe pattern, with the lines aligned parallel to the rubbing direction, characteristic of a double periodic modulation of the refractive index of the material. The long-period modulation is temperature dependent and disappears for thin sample cells (< 5 microm). The short-period modulation is nearly independent of the thickness of the cells. The experimental results are analysed in terms of a model which considers that there is a special distribution of the principal optical axis which may be in or out of the polariser-analyser plane. The observed periodic variation of the principal optical axis could not be interpreted in terms of the original structure proposed for this phase. DB8Cl presents a structure formed by dimers that can be viewed as flexible bent-core-like molecules, showing similarities with phases found in banana-like systems, but exhibiting a much more complex structure.

Crystallization↗
PubMed2005

Field-cycling NMR relaxometry of a liquid crystal above in mesoscopic confinement.

We measured the proton spin-lattice relaxation times in the isotropic phase of liquid crystal 4-n-pentyl-4-cyanobiphenyl (5CB) confined into porous glass (CPG) with the average pore diameter approximately 72 nm. The analysis of T1(-1) frequency dispersions, spanning over four decades, shows that the main relaxation mechanism induced by the ordered surface layer are molecular reorientations mediated by translational displacements (RMTD). The RMTD contribution to T1(-1) is proportional to the inverse square root of Larmor frequency, a consequence of the equipartition of diffusion modes along the surface. Low and high frequency cutoffs of the RMTD mechanism clearly reveal that the surface alignment of liquid crystal is random planar with the size of uniformly oriented patches approximately 5 nm, depending on the treatment of the CPG matrix. According to the size of the uniformly oriented patches varies also the thickness of the ordered surface layer and its temperature behavior. The surface-induced order parameter is found to be temperature independent and determined by the local short range surface interactions.

Journal Article↗
PubMed2005

NMR relaxation study of molecular dynamics in columnar and smectic phases of a PAMAM liquid-crystalline co-dendrimer.

We present the first results obtained by proton ((1)H) nuclear magnetic relaxation studies of molecular dynamics in a supermolecular liquid-crystal dendrimer exhibiting columnar rectangular and smectic-A phases. The (1)H spin-lattice relaxation time (T(1)) dispersions are interpreted using two relaxation mechanisms associated with collective motions and local molecular reorientations of the dendritic segments in the low- and high-frequency ranges, respectively. The T(1) values show a drop around 2.3 MHz that is attributed to a contribution coming from cross-relaxation between (1)H and nitrogen nuclear spins. In the high-frequency range the motions appear to be of similar nature in both mesophases and are ascribed to reorientations of dendritic segments (belonging to the core and/or to the mesogenic units) characterized by two correlation times. Notable differences in the dynamics between the columnar and layered phases are observed in the low-frequency range. Depending on the mesophase they are discussed in terms of elastic deformations of the columns and layer undulations. In this study we find that the dendritic core influences the dynamics of the mesogenic units both for local and collective motions. These results can be understood in terms of spatial constraints imposed by the dendritic architecture and by the supermolecular arrangement in the mesophases.

Journal Article↗
PubMed2005

Deuterium NMR investigation of the biaxial nematic phase in an organosiloxane tetrapode.

Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.

Journal Article↗
PubMed2004

Novel pulsed switched power supply for a fast field cycling NMR spectrometer.

In this paper, we outline the operating principles of a pulsed switched power supply for a fast field-cycling nuclear magnetic resonance spectrometer. The power supply uses a variant of a four-quadrant chopper with a duty cycle that defines the average output current. With this topology only two semiconductors are necessary to drive hundreds of amperes with an output power of several kilowatts. The output current ripple has a well-defined shape that can be reduced to acceptable values by a careful design of the semiconductors' controlling circuits and drivers. A power supply prototype was tested with a home build air-core magnet operating with fields between 0 and 0.21 T. The system is computer controlled using pulse generator and data acquisition PC cards, and specific user-friendly home-developed software. A comparative proton relaxometry study in two well-known liquid crystal compounds 5CB and MBBA was performed to check the reproducibility of the T1 measurements.

Journal Article↗
PubMed2003

Rectangular to hexagonal columnar phase transition exhibited by a biforked mesogen.

We present the structural study of a biforked compound exhibiting a Colr-Colh phase transition. This study was performed by means of DSC, optical microscopy, X-ray diffraction, dilatometry and molecular modelling. The molecular packing within the two columnar mesophases of this compound is discussed. For the first time, it is shown that the clusters of adjacent molecules which form the elementary section of a column are tilted with respect to the columnar axis in the rectangular mesophase. From the whole set of experimental data, the number of molecules in a columnar section is found to be about 5 in the rectangular mesophase, and 4 in the hexagonal one. The transition between the two columnar mesophases is discussed with respect to its slow kinetics.

Journal Article↗
PubMed2002

Polymorphous low-grade adenocarcinoma of the salivary glands with transformation to high-grade carcinoma.

AIMS: Polymorphous low-grade adenocarcinoma of the minor salivary glands is an infiltrative neoplasm characterized by bland-looking tumour cells arranged in diverse architectural patterns. It is considered to be of low-grade malignant potential in that nodal metastases are seen in only a minority, and distant spread is rare. Even more unusual is the transformation of polymorphous low-grade adenocarcinoma to a histologically high-grade carcinoma, i.e. dedifferentiation. In this paper, we describe the clinicopathological and immunohistochemical findings in two further examples. METHODS AND RESULTS: Two patients presented each with a tumour of the palate. Histopathological examination showed the typical morphological, cytological and immunohistochemical features of a polymorphous low-grade adenocarcinoma. In one case there was a second component of high-grade carcinoma showing nuclear atypia, markedly increased mitotic activity and MIB1 index, as well as prominent zones of necrosis. It expressed epithelial markers and androgen receptors, and thus resembled salivary duct carcinoma. Similar tumour tissue was observed in one of the cervical nodal metastases, which was biopsied at the same time as the palate. In the second patient, a high-grade component was discovered when the tumour recurred in the palate 13 years after the initial biopsy. Whilst morphologically similar to that in first case, there were significant immunohistochemical differences such as retention of some of the polymorphous low-grade adenocarcinoma profile and absence of androgen receptor expression. CONCLUSIONS: Polymorphous low-grade adenocarcinoma was first described relatively recently, and as experience with it continues to accumulate, it is becoming clear that late recurrences and metastases, whilst still infrequent, may not be quite as rare as previously thought. Reports of histological transformation are even scarcer, and most occurred at least 13 years after the polymorphous low-grade adenocarcinoma was initially recognized. It is a real possibility that this phenomenon, like clinical progression, may also be encountered more often as time passes. Therefore, we believe that, whilst polymorphous low-grade adenocarcinoma is certainly far less aggressive than, for example, adenoid cystic carcinoma, it nevertheless remains a true malignancy with a potential to prove fatal in a minority of patients.

Adenocarcinoma↗
PubMed2001

Inflammatory myofibroblastic tumor involving the pterygopalatine fossa.

SUMMARY: Inflammatory myofibroblastic tumors (IMT) comprise a rare group of lesions characterized histologically by acute and chronic inflammatory cells with a variable degree of fibrous stroma. Occurrence in the extracranial head and neck in children is unusual, and involvement in the pterygopalatine fossa has not, to our knowledge, been reported as occurring in this age group. We present the CT findings of an IMT of the pterygopalatine fossa in a 6-year-old female patient with a 2-week history of fever and a painless swelling of the left cheek. The diagnosis of IMT should be included in the differential diagnosis of a child presenting with an aggressive mass associated with systemic features such as fever, elevated sedimentation rate, and leukocytosis.

Child↗
PubMed2000

[Clinical findings of sensitivity to analgesics and nonsteroidal antiinflammatory drugs].

BACKGROUND: Analgesics (ANA) and nonsteroidal antiinflammatory drugs (NSAID) may simulate an allergic reaction or aggravate asthma and urticaria. OBJECTIVE: The aim of this study was the analysis of the clinical findings in patients sensitive to ANA/NSAID. METHODS AND SUBJECTS: The charts of 183 patients with history of reacting to these drugs were retrospectively analysed. RESULTS: There were 93 (51%) females and 90 (49%) males; 63 (34%) were aged 15 years or less. Females predominate in the age group older than 15 years (p = 0,02). The age by the time of the first reaction varied from 7 months to 65 years (median 15 years). Skin tests to inhalant allergens were positive to at least one antigen in 100/138 patients (72%). Clinical presentation was angioedema (86%); urticaria (39%); systemic reaction (30%); nasal and ocular (15%); and asthma (14%). The frequency of symptoms was not related to age. Family history of ANA/NSAID sensitivity was present in 3.8% of patients. Associated diseases were rhinitis (55%), chronic urticaria (47%), asthma (37%) and conjuntivitis (18%). Among asthmatics 16 (23,5%) had had attacks with ANA/NSAID. Nine non asthmatics (7.7%) had had bronchospasm as clinical presentation. This difference was statistically significant. Repeated reactions occurred in 107 (58%) patients. CONCLUSIONS: Sensitivity to ANA/NSAID was frequent in atopics; children and adults responded similarly; reactions were more common in adult females; palpebral angioedema was the most frequent clinical presentation; bronchosphasm was usually seen in asthmatics and most patients reacted repeatedly to different drugs.

Acetaminophen↗
PubMed2000

Human leukocyte antigen DR markers as predictors of progression to liver transplantation in patients with chronic hepatitis C.

OBJECTIVE: Because many patients with chronic viral hepatitis do not progress to end-stage liver disease, it is possible that host factors such as human leukocyte antigen (HLA) differences are important. Our aims were to determine HLA marker-specific rates of progression to liver transplantation among patients with chronic hepatitis C; and to determine if polymerase chain reaction (PCR)-based HLA DRB1 typing can be performed on stored serum samples. METHODS: Forty-two hepatitis C virus RNA-positive liver transplant patients and 87 untransplanted patients were included in a Cox proportional hazards model to test whether the occurrence of certain HLA DRB1 markers were associated with progression to liver transplantation. HLA DRB1 typing was performed on stored serum samples using a PCR method. RESULTS: There were no differences among the HLA DRB1 markers with regard to the HLA marker-specific rate of progression to transplantation among patients with chronic hepatitis C. CONCLUSIONS: HLA DRB1 markers do not appear to be associated with progression of disease in chronic viral hepatitis C. It is possible to perform PCR-based HLA DRB1 typing on stored frozen serum samples.

Adult↗
PubMed2000

Systemic injection of a nitric oxide synthase inhibitor suppresses sleep responses to sleep deprivation in rats.

We hypothesized that nitric oxide (NO) may play a role in homeostatic sleep regulation. To test this hypothesis, we studied the sleep deprivation (SD)-induced homeostatic sleep responses after intraperitoneal administration of an NO synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME, a cumulative dose of 100 mg/kg). Amounts and intensity of sleep were increased in response to 8 h of SD in control rats (n = 8). Sleep amounts remained above baseline for 16 h after SD followed by a negative rebound. Rapid eye movement sleep (REMS) and non-REMS (NREMS) intensities were elevated for 16 and 4 h, respectively. L-NAME treatment (n = 8) suppressed the rebound increases in NREMS amount and intensity. REMS rebound was attenuated by L-NAME in the first dark period after SD; however, a second rebound appeared in the subsequent dark period. REMS intensity did not increase after SD in L-NAME-injected rats. The finding that the NO synthase inhibitor suppressed rebound increases in NREMS suggests that NO may play a role as a signaling molecule in homeostatic regulation of NREMS.

Animals↗
PubMed1998

The controlled release of antibiotic by hydroxyapatite: anionic collagen composites.

Major problems with the treatment of osteomyelitis are associated with poor antibiotic distribution at the site of infection due to limited blood circulation to the skeletal tissue. Improved treatment procedures have been used in drug delivery systems that include bioceramics and natural and synthetic polymers. This work reports the development of anionic collagen:hydroxyapatite composite paste for sustained antibiotic release. Antibiotic release by the composite was characterized by two steps. In the first, 15.0+/-4.9% was released in the first 5 h (n = 53) by a normal Fick diffusion mechanism. In the second step, only 16.8+/-2.2% was released after 7 days. In conclusion, hydroxyapatite:anionic collagen composite can be an efficient support for sustained antibiotic release in the treatment of osteomyelitis because most of the antibiotic release may be associated with composite bioresorption, thus permitting antibiotic release throughout the healing process. Hydroxyapatite:anionic collagen paste showed good biocompatibility associated with bone tissue growth with material still being observed after 60 days from the time of implants.

Animals↗
PubMed1998

Esophageal manometry: a comparison of findings in younger and older patients.

OBJECTIVE: We sought to determine the utility of esophageal manometry in an older patient population. METHODS: Consecutively performed manometry studies (470) were reviewed and two groups were chosen for the study, those > or = 75 yr of age (66 patients) and those < or = 50 years (122 patients). Symptoms, manometric findings (lower esophageal sphincter [LES], esophageal body, upper esophageal sphincter [UES]) and diagnoses were compared between the groups. RESULTS: Dysphagia was more common (60.6% vs 25.4%), and chest pain was less common (17.9 vs 26.2%) in older patients. In the entire group, there were no differences in LES parameters. Older patients with achalasia had lower LES residual pressures after deglutition (2.7 vs 12.0 mm Hg), but had similar resting pressures (31.4 vs 35.2 mm Hg) compared with younger achalasia patients. Duration and amplitude of peristalsis were similar in both groups, whereas peristaltic sequences were more likely to be simultaneous in the older group (15% vs 4%). The UES had a lower resting pressure in the older patients (49.6 vs 77.6 mm Hg) and a higher residual pressure (2.0 vs -2.7 mm Hg). The older patients were less likely to have normal motility (30.3% vs 44.3%) and were more likely to have achalasia (15.2% vs 4.1%) or diffuse esophageal spasm (16.6% vs 5.0%). When only patients with dysphagia were analyzed, achalasia was still more likely in the older group (20.0% vs 12.9%). CONCLUSION: When older patients present with dysphagia, esophageal manometry frequently yields a diagnosis to help explain their symptoms.

Aged↗
PubMed1998

Susceptibility of Nectomys rattus (Pelzen, 1883) to experimental infection with Schistosoma mansoni (Sambon, 1907): a potential reservoir in Brazil.

The aim of the present research was to evaluate the potential of Nectomys rattus, the "water rat", to develop Schistosoma mansoni infection. Comparison with N. squamipes was carried out. Both species of rodents were submitted to transcutaneous infection using different infective cercariae loads: 50, 100 or 500. N. rattus showed high susceptibility to S. mansoni, with an infection rate of 71%. Rodents were able to excrete viable eggs of S. mansoni in the faeces during all infection period. For both species, the small intestine, followed by the liver and the large intestine, presented the highest concentration of eggs among the surveyed organs. Infection caused no animal death. Moreover, N. rattus accomplished the parasite's life cycle, by infecting the snails Biomphalaria glabrata and later Mus musculus. These evidences indicate that both N. rattus, as for N. squamipes are potential reservoirs for schistosomiasis in Brazil. Considering the fact that N. rattus and N. squamipes exist in the same natural ecosystems of S. mansoni, we suggest that these rodents must be regarded as influential factors in epidemiology surveys.

Animals↗
PubMed1996

Cytogenetic findings in a patient presenting simultaneously with chronic lymphocytic leukemia and acute myeloid leukemia.

A case of simultaneous presentation of B-chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML) is described. CLL was documented by bone marrow and peripheral blood lymphocytosis with a typical B-CLL immunophenotype. The diagnosis of AML was supported by the presence of bone marrow and circulating blast cells positive for myeloperoxidase and myeloid-associated markers. Although the immunophenotyping and morphocytochemical studies indicated two different cell populations (mature B-CLL lymphocytes and myeloblasts), chromosome aberrations commonly associated with CLL and AML were found simultaneously in the same metaphases obtained from unstimulated 24-hour cultures of peripheral blood cells.

Aged↗