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PubMed2002

Rayleigh-Bénard convection with rotation at small Prandtl numbers.

We present experimental and theoretical results near the onset of the Rayleigh-Bénard convection with rotation about a vertical axis in a fluid with a Prandtl number sigma close to 0.18. In the experiment we used a H2-Xe gas mixture with a separation ratio Psi=0.22 and a Lewis number L=1.22 at various pressures and dimensionless rotation rates Omega up to 400. On the basis of a standard weakly nonlinear stability analysis, we found a supercritical, stationary bifurcation for Omega < than approximately 13, which became subcritical over the range 13 < than approximately Omega < or approximately 160. For Omega > than approximately 160 a supercritical Hopf bifurcation precedes the stationary instability of the uniform state. Following the unstable straight-roll fixed point in the subcritical regime by Galerkin methods we determined the location of the saddlenode and the stability of the nonlinear two-dimensional straight-roll state. The rolls were found to be unstable to three-dimensional Küppers-Lortz perturbations for 3.8 < than approximately Omega < than approximately 160. Theoretical results for a pure fluid with the same sigma were qualitatively similar. Measurements using shadowgraph flow visualization yielded a bifurcation line and an Omega range of subcriticality, which agreed with the stability analysis. In the subcritical range the experiment revealed a discontinuity of the pattern amplitude at onset, but was unable to find any hysteresis. Patterns at onset fluctuated irregularly between the ground state and the finite-amplitude state. In this parameter range the convection pattern further above onset was chaotically time dependent. Investigation of the Hopf bifurcation line was difficult because of a wall mode that, for large Omega, preceded the bulk instability. For Omega approximately equal to 400, patterns were found in the sample interior only when the expected Hopf bifurcation was exceeded by about 10%. This is consistent with the convective nature of the bifurcation. However, the observed structure, although time periodic, was spatially disordered and had a frequency that was considerably larger than the expected Hopf frequency. In a separate sample cell with a radial ramp in the spacing no structure was observed at all in the cell interior until the expected stationary instability was reached.

Journal Article↗
PubMed1999

Effects of protein-polyphenol interactions on beverage haze, stabilization, and analysis.

The haze-forming activity of a polypeptide depends greatly on its proline content. Haze-forming polyphenols have at least two binding groups, each of which has at least two hydroxy groups on an aromatic ring. The protein/polyphenol ratio has a strong influence on the amount of haze formed; the largest amount occurs when the numbers of polyphenol binding ends and protein binding sites are nearly equal. This has important consequences for turbidimetric methods used to measure haze-active proteins and polyphenols in beverages. The ratio also influences the effectiveness of a number of stabilization procedures.

Beverages↗
PubMed1998

[Alterations in postural control: the use of spectral analysis in stability measurement].

UNLABELLED: Static posturography studies the frequency at which the center of gravity in the human body oscillates. There is no universal agreement as to the importance of this index or whether it is specific in identifying alterations in the postural control systems (vestibular, proprioceptive and visual). Nevertheless, some hypotheses appear generally accepted: a) visual information dominates; b) proprioception affects anterior-posterior oscillation; c) relatively low frequency body oscillation is affected by vestibular input while higher frequency oscillations are controlled by proprioceptive information. In order to check these hypotheses, four groups of patients underwent spectral analysis of bodily oscillations recorded with static posturography. The four groups of patients were similar in terms of age and broken down as follows: Group A-peripheral diabetic neuropathy (10 cases); Group B-diabetic neuropathy-free patients (10 cases); Group C-pure peripheral vestibulopathy (10 cases); Group D-normal subjects (20 cases). Analysis was performed dividing the frequency spectrum (FFT) into three groups: gr. I) 0.01-0.7 Hz; gr. II) 0.7-1 Hz; gr. III) > 1 Hz. Analysis of the spectra for Group A showed a significant increase in high frequency bodily oscillation (gr. III) with the eyes open in the anterior-posterior direction (M = 8.6 +/- 5.5; p < 0.05) and in the lateral direction (M = 4.0 +/- 2.0; p < 0.05); an increase was also found in both directions with the eyes closed (M = OCX II: 7.4 +/- 1.5; p < 0.05) (M = OCX III: 5.6 +/- 1.6; p < 0.01) (M = OCY II: 16.8 +/- 5.0; p < 0.05) (M = OCY II: 9.2 +/- 4.4; p < 0.05). In Group B the increase was limited to high frequency anterior-posterior oscillations (gr. III) at open eyes (M = 3.6 +/- 1.6; p < 0.05) and at closed eyes (M = 4.4 +/- 1.5; p < 0.05). In Group C there was an increase in high frequency lateral oscillations (gr. III) at open eyes (M = 4.2 +/- 0.5; p < 0.01) and panfrequency oscillations at closed eyes (M = OCX I: 47.7 +/- 5.6; p < 0.01) (M = OCX II: 23.2 +/- 4.9; p < 0.01) (M = OCX III: 7.2 +/- 2.3; p < 0.05). CONCLUSIONS: The diabetic proprioceptive deficit appears to lead to multiplanar destabilization at the medium-to-high frequency range. The labyrinthine deficit, on the other hand, appears to cause lateral destabilization at all frequencies. The method of analysis used appears to provide useful parameters with which to evaluate static posture since they can specifically identify alterations in orthostatic bodily control of various origins.

Adult↗
PubMed2001

High-performance liquid chromatographic analysis and stability of anti-tumor agent temozolomide in human plasma.

Temozolomide (SCH 52365; TEMODAL) is an antineoplastic agent with activity against a broad spectrum of murine tumors. This compound is currently marketed in the European Union for the treatment of patients with glioblastoma multiforme and anaplastic astrocytoma, which are serious and aggressive types of brain cancers. It has been postulated that temozolomide exerts its in vivo activity via the decomposition product MTIC, which is believed to alkylate nucleophiles, and in the process is converted to AIC. A high-performance liquid chromatographic (HPLC) method was developed and validated for the analysis of temozolomide in human plasma. The determination of temozolomide involved extraction with ethyl acetate followed by separation on a reversed phase C-18 column and quantification by UV absorbance at 316 nm. The calibration curve was linear over a concentration range of 0.1-20 microg/ml. The limit of quantitation was 0.1 microg/ml, where the coefficient of variation (CV) was 0% and the bias was 10.0%. The method was precise with a coefficient of variation ranging from 2.5 to 6.9% and accurate with a bias ranging from 5.0 to 10.0%. Temozolomide was unstable at 37 degrees C in human plasma with a degradation t1/2 of 15 min; however, it was stable at 4 degrees C for at least 30 min. Temozolomide was stable in acidified human plasma (pH < 4) for at least 24 h at 25 degrees C, and for at least 30 days at -20 degrees C. Moreover, temozolomide was stable in acidified human plasma after being subjected to three freeze thaw cycles. The assay was shown to be specific, accurate, precise, and reliable for use in pharmacokinetic studies.

Antineoplastic Agents↗
PubMed1993

[The correlation between left ventricular segmental kinesis and a Selvester QRS analysis in stabilized myocardial infarct].

We have studied the relationship between left ventricular segmental wall motion evaluated by means of biplane left ventricular cineangiography and the myocardial infarct size estimated using Selvester's scoring system based on the QRS of the electrocardiogram. Seventy consecutive patients with old myocardial infarction were recruited. All patients underwent diagnostic retrograde left heart catheterization including left ventriculography and selective coronary arteriography. Nineteen patients were excluded from the study due to intraventricular conduction disturbances and left ventricular hypertrophy. Of the remaining 51 patients, aged 33-76 years (mean 56 years), 31 had inferior or postero-inferior myocardial infarction (Group 1); 7 patients had combined anterior and inferior myocardial infarction (Group 2); 13 patients had anterior or antero-septal myocardial infarction (Group 3). The analysis of left ventricular segmental wall motion was performed according to the Collaborative Studies in Coronary Artery Surgery assigning at hypokinesia, akinesia and dyskinesia 1, 2 and 3 points, respectively. The values of statistical correlations between QRS scoring system and ventricular scoring were: r = 0.85 (Group 1); r = 0.78 (Group 2); r = 0.70 (Group 3). Our data indicate that Selvester's scoring system may provide useful information on functional status and on the extent of left ventricular wall motion abnormalities in patients with previous myocardial infarction.

Adult↗
PubMed2001

Spatio-temporal pattern formation on spherical surfaces: numerical simulation and application to solid tumour growth.

In this paper we examine spatio-temporal pattern formation in reaction-diffusion systems on the surface of the unit sphere in 3D. We first generalise the usual linear stability analysis for a two-chemical system to this geometrical context. Noting the limitations of this approach (in terms of rigorous prediction of spatially heterogeneous steady-states) leads us to develop, as an alternative, a novel numerical method which can be applied to systems of any dimension with any reaction kinetics. This numerical method is based on the method of lines with spherical harmonics and uses fast Fourier transforms to expedite the computation of the reaction kinetics. Numerical experiments show that this method efficiently computes the evolution of spatial patterns and yields numerical results which coincide with those predicted by linear stability analysis when the latter is known. Using these tools, we then investigate the rĵle that pre-pattern (Turing) theory may play in the growth and development of solid tumours. The theoretical steady-state distributions of two chemicals (one a growth activating factor, the other a growth inhibitory factor) are compared with the experimentally and clinically observed spatial heterogeneity of cancer cells in small, solid spherical tumours such as multicell spheroids and carcinomas. Moreover, we suggest a number of chemicals which are known to be produced by tumour cells (autocrine growth factors), and are also known to interact with one another, as possible growth promoting and growth inhibiting factors respectively. In order to connect more concretely the numerical method to this application, we compute spatially heterogeneous patterns on the surface of a growing spherical tumour, modelled as a moving-boundary problem. The numerical results strongly support the theoretical expectations in this case. Finally in an appendix we give a brief analysis of the numerical method.

Animals↗
PubMed2001

Role of polarization mode dispersion on modulational instability in optical fibers.

We introduce the theory of modulational instability (MI) of electromagnetic waves in fibers with random polarization mode dispersion. Applying a linear stability analysis and stochastic calculus, we show that the MI gain spectrum reads as the maximal eigenvalue of a constant effective matrix. In the limiting cases of small or large fluctuations, we give explicit expressions for the MI gain spectra. In the general configurations, we give the explicit form of the effective matrix and numerically compute the maximal eigenvalue. In the anomalous dispersion regime, polarization dispersion widens the unstable bandwidth. Depending on the type of variations of the birefringence parameters, polarization dispersion reduces or enhances the MI gain peak. In the normal dispersion regime, random effects may extend the instability domain to the whole spectrum of modulations. The linear stability analysis is confirmed by numerical simulation of the full stochastic coupled nonlinear Schrödinger equations.

Journal Article↗
PubMed1997

Prediction bounds for random shelf-lives.

Every drug product requires indication of a shelf-life on the immediate container label. The labelled shelf-life is usually determined by a stability analysis with several batches of the drug product. Because of the existence of batch-to-batch variation, the true shelf-lives of different batches differ and can be treated as random variables. To obtain a single labelled shelf-life applicable to all future batches of the drug product, we propose to use a prediction bound of the random shelf-life of a future batch. Under some assumptions, the proposed labelled shelf-life (prediction bound) is lower than the true random shelf-life with a probability approximately equal to a pre-assigned level. For illustration, we provide an example concerning a new drug application stability analysis conducted in a pharmaceutical company. We also discuss an application of our technique to the problem of clean-up of contaminated soils.

Decontamination↗
PubMed1979

Stability-indicating analysis of injectable estrogen-androgen combinations in formulations containing corn oil.

A method was developed for the analysis of testosterone 17-enanthate 3-benzilic acid hydrazone, 17-beta-estradiol 3,17-dienanthate, and estradiol benzoate combinations dissolved in an oily vehicle. Testosterone 17-enanthate 3-benzilic acid hydrazone was separated from the other drugs and vehicle components by chromatography on an acetonitrile-infusorial earth column followed by quantitation using UV spectroscopy. The estradiol esters were separated from the oil by an additional chromatographic step using a heptane-silanized infusorial earth column prior to quantitation by GLC. Subjecting formulations to elevated temperatures resulted in detectable losses for testosterone 17-enanthate 3-benzilic acid hydrazone and 17-beta-estradiol 3,17-dienanthate. For both drugs, degradation was due to hydroxysis; the degradation products, testosterone 17-enanthate and 17-beta-estradiol 17-enanthate, did not interfere with the intact drug determination. Methods also were developed to estimate degradation product levels in the formulation.

Chromatography, Gas↗
PubMed2006

Carbonic anhydrase in Tectona grandis: kinetics, stability, isozyme analysis and relationship with photosynthesis.

Carbonic anhydrase (CA, EC: 4.2.1.1) activity in teak (Tectona grandis L.f.) was studied to determine its characteristics, kinetics and isozyme patterns. We also investigated effects of leaf age, plant age and genotype on CA activity and gas exchange parameters. Carbonic anhydrase extracted from leaves in 12 mM veronal buffer, pH 7.8, had a K(m) for CO(2) of 15.20 mM and a V(max) of 35,448 U mg(-1) chlorophyll min(-1), which values declined by 50 and 70%, respectively, after 1 week of storage at 4 degrees C. A 15% native polyacrylamide gel revealed the absence of CA isozymes in teak, with only a single CA band of 45 kD molecular mass observed across 10 segregating half-sib families and groups of trees ranging in age from 10 to 25 years. Activity remained stable during the first month in storage at 0 degrees C, but gradually declined to 25% of the initial value after 1 year in storage. During the period of active growth (February-May), maximal CA activity was observed in fully expanded and illuminated leaves. Significant variation was observed in CA activity across 10 1-year-old half-sib families and 21 5-year-old half-sib families. There was a positive correlation between CA activity and photosynthetic rate in a population of 10-year-old trees (P < 0.005). Positive correlations between CA activity and photosynthetic rate were found in 10 of 21 5-year-old half-sib families (P < 0.005 to P < 0.05), which showed greater diversity in CA activity than in photosynthetic characteristics. Thus, CA may serve as a biochemical marker for photosynthetic capacity in teak genotypes.

Carbonic Anhydrases↗
PubMed2000

Conformational analysis and stability of collagen peptides by CD and by 1H- and 13C-NMR spectroscopies.

Four small type I collagen CNBr peptides containing complete natural sequences were purified from bovine skin and investigated by CD and 1H- and 13C-nmr spectroscopies to obtain information concerning their conformation and thermal stability. CD showed that a triple helix was formed at 10 degrees C in acidic aqueous solution by peptide alpha l(I) CB2 only, and to lesser extent, by alpha 1(I) CB4, whereas peptides alpha 1(I) CB5 and alpha 2(I) CB2 remained unstructured. Analytical gel filtration confirmed that peptides alpha 1(I) CB2 and alpha 1(I) CB4 only were able to form trimeric species at temperature between 14 and 20 degrees C, and indicated that the monomer = trimer equilibrium was influenced by the chaotropic nature of the salt present in the eluent, by its concentration, and by temperature variations. CD measurements at increasing temperatures showed that alpha 1(I) CB2 was less stable than its synthetic counterpart due to incomplete prolyl hydroxylation of the preparation from the natural source. 1H- and 13C-nmr spectra acquired in the temperature range 0-47 and 0-27 degrees C, respectively, indicated that with decreasing temperature the most abundant from of alpha 1(I) CB2 was in slow exchange with an assembled form, characterized by broad lines, as expected for the triple-helical conformation. A large number of trimer cross peaks was observed both in the proton and carbon spectra, and these were most likely due to the nonequivalence of the environments of the three chains in the triple helix. This nonequivalence may have implications for the aggregation of collagen molecules and for collagen binding to other molecules. The thermal transition from trimer to monomer was also monitored by 1H-nmr following the change in area of the signal belonging to one of the two beta protons of the C-terminal homoserine. The unfolding process was found to be fully reversible with a melting temperature of 13.4 degrees C, in agreement with CD results. The qualitative superposition of the melting curves obtained by CD for the peptide bond characteristics and by nmr for a side chain suggests that triple-helical backbone and side chains constitute a single unit.

Amino Acid Sequence↗
PubMed2000

Analysis and stability of mercury speciation in petroleum hydrocarbons.

Raw petroleum and natural gas often contain high concentrations of mercury, which can be damaging to the metal components of production facilities, as well as to the environment. Various Hg species have different properties in terms of mobility, reactivity and bioavailability. Thus, for cost-effective decisions regarding plant design, Hg extraction, and pollution control, speciation information must be available at the production facility. In this paper, a simple, wet chemical speciation method, which provides data on Hg(o), dissolved and particulate total Hg, Hg(II), and methyl Hg is presented. The method incorporates species-specific extraction and separation procedures, followed by cold vapor atomic fluorescence spectrometry (CVAFS). For each species, detection limits of approximately 0.1 ng/g were obtained. Storage experiments in various containers showed that organo-mercury species were stable for at least 30 days in all containers except those made of polyethylene; and Hg(o) was stable in all containers except those made of stainless steel or polyethylene. Hg(II) was rapidly lost from all containers except those made of aluminum, which rapidly converted it to Hg(o), which was stable. In general, most of the total Hg in petroleum products was particulate Hg, followed by dissolved Hg(II) and Hg(o). Sub-ng/g concentrations of methyl-Hg were observed in most samples.

Journal Article↗