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At least 559 records · Page 31Linked to original sources
PubMed1997

Kinetic Isotope Effects and Transition State Geometries. A Theoretical Investigation of E2 Model Systems.

Ab initio calculations at the MP2/6-31+G level have been performed on E2 model systems to investigate whether differences in kinetic isotope effects correlate with changes in transition state geometries. By combining various nucleophiles (NH(2)(-), OH(-), F(-), PH(2)(-), SH(-), Cl(-)) and leaving groups (NH(3), Br(-), Cl(-), F(-), SH(-)) for reactions of the type Nu(-) + CH(3)CH(2)X, a large diversity of transition structures from reactant-like to product-like are generated. For each reaction one primary and two different alpha-secondary kinetic isotope effects are calculated. The primary kinetic isotope effects depend strongly on the nucleophilic placement in the periodic system, which mainly is due to differences in equilibrium isotope effects. When this effect is subtracted, the primary kinetic isotope effects display the expected maximum for symmetric transition structures, although the maximum is broad. The secondary kinetic isotope effects associated with the leaving group provide a qualitative correlation with the hybridization at the carbon, but the corresponding effects at the carbon where the hydrogen abstraction takes place is uncorrelated with the transition state geometry.

Journal Article↗
PubMed2004

Combined electronic and nuclear dynamics in a simple model system. II. Spectroscopic transitions.

We investigate the correlated electronic and nuclear motion in a model system as proposed by Shin and Metiu [J. Chem. Phys. 102, 9285 (1995)]. The quantum dynamics is studied during laser induced electronic transitions. Here, the influence of nonadiabatic coupling on the absorption spectrum is investigated and the Franck-Condon principle is illustrated in terms of the temporal changes of electronic and nuclear densities. In the case of intense field excitation, multiphoton processes become important, and electronic as well as vibrational wave packets are prepared.

Journal Article↗
PubMed2002

Multiple glassy states in a simple model system.

Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction ("sticky hard spheres"). The experiments, using well-characterized colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering.

Journal Article↗
PubMed2005

Colloids as model systems for problems in statistical physics.

Owing to their mesoscopic length scales, colloidal suspensions provide ideal model systems suitable for addressing many problems in the field of statistical physics. Exemplarily, we highlight the versatile nature of such systems by discussing experiments with stochastic resonance and a practical realization of a recently proposed ratchet cellular automaton.

Algorithms↗
PubMed1976

[Analysis of the applicability of model systems to measuring the activity of lipid-soluble antioxidants by the electrochemiluminescent method].

A comparative study of two most common model systems (methanol-sodium citrate and chloroform-aceton-maleic acid (10(-3) M)) was carried out to measure lipid-soluble antioxidants by the method of electrochemiluminescence. alpha-naftole, pyrogalole, phenole, hydroquinone, acrylamide, cholesterine and alkohol lipid extract from rat liver were used as inhibitors. The analysis has shown that it is worthwhile to apply only the system chloroform-aceton-maleic acid as a model of electroluminescent studies.

Acetone↗
PubMed2001

Evaluation of antioxidant activity of epigallocatechin gallate in biphasic model systems in vitro.

The antioxidant activity of epigallocatechin gallate (EGCG) was studied in different in vitro model systems, which enabled evaluation of both chemical and physical factors involved in assessing the role of EGCG in oxidative reactions. EGCG suppressed the initiation rate and prolonged the lag phase duration of peroxyl radical-induced oxidation in a phospholipid liposome model to a greater extent (p < 0.01) compared to both Trolox and alpha-tocopherol. Effectiveness of these antioxidants to prolong the peroxyl radical-induced lag phase was inversely related to lipophilic character. EGCG also protected against both peroxyl radical and hydroxyl radical-induced supercoiled DNA nicking. The rate constant describing EGCG reaction against hydroxyl radical was 4.22+/-0.07 x 10(10) M(-1) x sec(-1), which was comparable to those of Trolox and alpha-tocopherol, respectively. EGCG exhibited a synergistic effect with alpha-tocopherol in scavenging 1,1-diphenyl-2-picylhydrazyl (DPPH) radical, thus displaying a direct free radical scavenging capacity. In vitro Cu2+-induced-human LDL oxidation was accelerated in the presence of EGCG and attributed to the conversion of Cu2+ to Cu+. We conclude that the particularly effective antioxidant properties of EGCG noted in both chemical and biological biphasic systems were related to a unique hydrophilic and lipophilic balance which enabled effective free radical scavenging. The same chemical-physical properties of EGCG also enabled prooxidant activity, only when in contact with unbound transition metal ions in a multiphasic system.

2,2'-Dipyridyl↗
PubMed1997

Artificial dental plaque biofilm model systems.

Difficulties with in vivo studies of natural plaque and its complex, heterogeneous structure have led to development of laboratory biofilm plaque model systems. Technologies for their culture are outlined, and the rationale, strengths, and relative uses of two complementary approaches to microbial models with a focus on plaque biodiversity are analyzed. Construction of synthetic consortia biofilms of major plaque species has established a variety of bacterial interactions important in plaque development. In particular, the 'Marsh' nine-species biofilm consortia systems are powerful quasi steady-state models which can be closely specified, modified, and analyzed. In the second approach, microcosm plaque biofilms are evolved in vitro from the natural oral microflora to the laboratory model most closely related to plaque in vivo. Functionally reproducible microcosm plaques are attainable with a biodiverse microbiota, heterogeneous structure, and pH behavior consistent with those of natural plaque. The resting pH can be controlled by urea supply. Their growth patterns, pH gradient formation, control of urease levels by environmental effectors, and plaque mineralization have been investigated. Microcosm biofilms may be the only useful in vitro systems where the identity of the microbes and processes involved is uncertain. Together, these two approaches begin to capture the complexity of plaque biofilm development, ecology, behavior, and pathology. They facilitate hypothesis testing across almost the whole range of plaque biology and the investigation of antiplaque procedures yielding accurate predictions of plaque behavior in vivo.

Bacteriological Techniques↗
PubMed2002

Size-distribution analysis of proteins by analytical ultracentrifugation: strategies and application to model systems.

Strategies for the deconvolution of diffusion in the determination of size-distributions from sedimentation velocity experiments were examined and developed. On the basis of four different model systems, we studied the differential apparent sedimentation coefficient distributions by the time-derivative method, g(s*), and by least-squares direct boundary modeling, ls-g*(s), the integral sedimentation coefficient distribution by the van Holde-Weischet method, G(s), and the previously introduced differential distribution of Lamm equation solutions, c(s). It is shown that the least-squares approach ls-g*(s) can be extrapolated to infinite time by considering area divisions analogous to boundary divisions in the van Holde-Weischet method, thus allowing the transformation of interference optical data into an integral sedimentation coefficient distribution G(s). However, despite the model-free approach of G(s), for the systems considered, the direct boundary modeling with a distribution of Lamm equation solutions c(s) exhibited the highest resolution and sensitivity. The c(s) approach requires an estimate for the size-dependent diffusion coefficients D(s), which is usually incorporated in the form of a weight-average frictional ratio of all species, or in the form of prior knowledge of the molar mass of the main species. We studied the influence of the weight-average frictional ratio on the quality of the fit, and found that it is well-determined by the data. As a direct boundary model, the calculated c(s) distribution can be combined with a nonlinear regression to optimize distribution parameters, such as the exact meniscus position, and the weight-average frictional ratio. Although c(s) is computationally the most complex, it has the potential for the highest resolution and sensitivity of the methods described.

Animals↗
PubMed1990

Lineage directed HL-60 cell sublines as a model system for the study of early events in lineage determination of myeloid cells.

Current experimental models are poorly suited to study the early biochemical and molecular events of the lineage determination process in myeloid progenitor cells. Viable lineage-committed precursors cannot be identified until after they have expressed their mature phenotype and these precursors cannot be grown to large number while lineage is committed but still immature. Recently, we have identified stable sublines of the HL-60 human leukemia cell line which differ from each other in that they selectively differentiate to either neutrophils (UR-1-4), monocyte/macrophages (MRI), eosinophils (clones 2 and 15), or mixtures of two (clones 7 and 8) or of all three lineages (UR-1-2) when stimulated to mature with butyric acid under identical conditions. Characterization of these sublines provided evidence that the expression of lineage in HL-60 cells is a multistep process and that the lineage tendencies (lineage direction) the clones exhibited when cultured with butyric acid represent a step in that process earlier than irreversible lineage commitment but later than the multipotential wild type HL-60 cells. First, treatment of these sublines with compounds that induce differentiation of HL-60 cells to specific lineages (dimethylsulfoxide, neutrophil; 1,25-(OH)2 vitamin D3, monocyte), generally induced differentiation to the lineage associated with that inducer rather than the butyrate-associated lineage. Second, culture of neutrophil or monocyte-directed sublines in medium of elevated pH for two months leads to the development of eosinophils. Culturing the sublines first in butyric acid for variable lengths of time and switching to either DMSO or VD3 indicated that irreversible lineage commitment develops on a time course similar to the development of the commitment to mature. Markers of monocytic and eosinophilic differentiation could not be simultaneously demonstrated in single mature cells, consistent with the phenomenon of lineage fidelity. In addition, several assays were validated that could reliably classify mature HL-60 cells to their lineage. The collection of these sublines appears to constitute a model system with well-defined behavior with respect to the early events of lineage determination that can be grown to quantities sufficient for biochemical and molecular analysis. Exploring the differences between these clones may provide a new way to examine the early events of the lineage development process in myeloid cells.

Butyrates↗
PubMed1998

In vitro model system for the identification and characterization of proteins involved in inflammatory processes.

An in vitro model featuring important inflammatory cellular states was established, based on the murine monocyte/macrophage cell line RAW 264.7. Macrophages are key players in chronic inflammation, and major parts of the biochemical reactions taking place in vivo, e.g., the production of proinflammatory cytokines, can be triggered in vitro by stimulation of the cells with bacterial lipopolysaccharide (LPS). A mastergel, representing a synthetic image of the expressed basic set of cellular proteins, was designed by a computer-assisted overlay of a statistically significant number of two-dimensional electrophoresis (2-DE) gels of unstimulated RAW 264.7 cells. This image served as a reference for qualitative and quantitative changes in the protein pattern induced by stimulation of the macrophages with LPS. The optimal conditions for LPS stimulation were evaluated by monitoring the expression and secretion of the proinflammatory cytokine tumor necrosis factor-alpha(TNF-alpha). The comparison of the mastergel with the 2-DE gels of LPS-stimulated cells revealed several changes in the protein pattern. In order to prove the relevance of the presented model system, we focused on two low molecular weight proteins, which showed significant changes in the apparent concentration in a 2-DE pattern. These proteins were further characterized by microsequencing of internal peptides. A comparison of the obtained sequences with protein databases identified them as cofilin and keratinocyte lipid-binding protein.

Amino Acid Sequence↗
PubMed1988

A model system for the in vivo determination of the thrombolytic effect of plasminogen activators.

An in vivo model system for measuring the thrombolytic efficiency of plasminogen activators was used. The formation of radiolabelled microthrombi was induced by infusion with I-125 labelled fibrinogen and thrombin. Reactive fibrinolysis was inhibited by administration of suboptimal levels of e-aminocaproic acid. The thrombolytic and subsequent fibrinolytic events were followed in the capillary bed of the lungs of anesthetized rats by external monitoring of the I-125 activity over the lung field. The model was successfully employed to demonstrate the thrombolytic effect of plasminogen activator produced by a transplanted spontaneous rat prostate adenocarcinoma cell line (PA III). The system proved to be reproducible with detection limits of 6000 I.U. using the PA-III cell line derived activator.

Animals↗
PubMed1980

An animal model system for the immunotherapy of lung carcinoma using sensitized lymphocytes: a preliminary study.

An animal model system was established for immunotherapy of lung cancer using line-1 lung carcinoma in Balb/cR mice and xenogeneic lymphocytes from New Zealand white (N2W) rabbits. The immunization schedule consisted of footpad and subcutaneous injections of 1 X 10(8) viable line-1 cells into two rabbits, twice at a weekly interval. Two other rabbits were maintained as source of unsensitized lymphocytes. All rabbits were bled on day 14 at which time 45 mice were divided into three groups of 15 for both transplantation of 1 X 10(4) viable line-1 cell (subcutaneously) and treatment with 1) control saline, 2) 2.5 X 10(6) unsensitized lymphocytes, and 3) 2.5 X 10(6) sensitized lymphocytes. These treatments were repeated twice at weekly intervals. Results were evaluated four weeks after transplant and initial treatment. Tumor developed in 13/15 control mice, 15/15 treated with unsensitized lymphocytes, and 7/15 with sensitized-lymphocyte injection. The mean tumor weight for group 1 (control mice) was 1.38 +/- 0.22 gm (SEM), group 2 was 2.09 +/- 0.25 gm, and 0.56 +/- 0.28 gm for group 3. There was a statistically significant difference between control and group 3 for both tumor appearance (P = 0.014) and tumor weight (P = 0.026).

Animals↗
PubMed2000

Guidance for the performance evaluation of three-dimensional air quality modeling systems for particulate matter and visibility.

Guidance for the performance evaluation of three-dimensional air quality modeling systems for particulate matter and visibility is presented. Four levels are considered: operational, diagnostic, mechanistic, and probabilistic evaluations. First, a comprehensive model evaluation should be conducted in at least two distinct geographical locations and for several meteorological episodes. Next, streamlined evaluations can be conducted for other similar applications if the comprehensive evaluation is deemed satisfactory. In all cases, the operational evaluation alone is insufficient, and some diagnostic evaluation must always be carried out. Recommendations are provided for designing field measurement programs that can provide the data needed for such model performance evaluations.

Air Pollution↗
PubMed2000

Effects of ozone and oxygen on the degradation of carotenoids in an aqueous model system.

The effects of ozone and oxygen on the degradation of carotenoids in an aqueous model system were studied. All-trans beta-carotene, 9-cis beta-carotene, beta-cryptoxanthin, and lycopene were adsorbed onto a C(18) solid phase and exposed to a continuous flow of water saturated with oxygen or ozone at 30 degrees C. Carotenoids were analyzed using HPLC with a C(30) column and a photodiode array detector. Approximately 90% of all-trans beta-carotene, 9-cis beta-carotene, and beta-cryptoxanthin were lost after exposure to ozone for 7 h. A similar loss of lycopene occurred in only 1 h. When exposed to oxygen, all carotenoids, except beta-cryptoxanthin, degraded at lower rates. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates: lycopene > beta-cryptoxanthin > all-trans beta-carotene > 9-cis beta-carotene. The major degradation products of beta-carotene were tentatively identified on the basis of their elution on the HPLC column, UV-Vis spectra, and electrospray LC-MS. Predominant isomers of beta-carotene were 13-cis, 9-cis, and a di-cis isomer. Products resulting from cleavage of the molecule were beta-apo-13-carotenone and beta-apo-14'-carotenal, whereas epoxidation yielded beta-carotene 5,8-epoxide and beta-carotene 5, 8-endoperoxide.

Carotenoids↗
PubMed2003

Effect of temperature and addition of alpha-tocopherol on the oxidation of trilinolein model systems.

The effects of temperature and addition of alpha-tocopherol were evaluated in trilinolein model systems through quantification of oxidized TAG monomers, dimers, and polymers following oxidation at different temperatures. Samples of trilinolein without and with 250 and 500 mg/kg alpha-tocopherol added were stored at 25, 60, and 100 degrees C. Quantification of oxidized monomers, dimers, and polymers by a combination of adsorption and exclusion chromatography provided a useful measurement for studying the evolution of oxidation. Results showed that the amounts of primary oxidation compounds (trilinolein oxidized monomers) that accumulated during the induction period decreased as the temperature increased, indicating that the slope of the initial linear stage of oxidation depended on temperature. The end of the induction period was marked by a sharp increase in the levels of total oxidation compounds, the initiation of polymerization, and the loss of alpha-tocopherol. Addition of alpha-tocopherol did not prevent, but rather delayed, formation of trilinolein oxidized monomers and the initiation of polymerization.

Antioxidants↗
PubMed2002

Effect of lipid composition on meat-like model systems containing cysteine, ribose, and polyunsaturated fatty acids.

This paper compares the volatile constituents of model systems containing the important meat aroma precursors cysteine and ribose, with and without either methyl linoleate, an n-6 fatty acid, or methyl alpha-linolenate, an n-3 acid, both of which are present in meat. Many of the volatile compounds formed from the reaction between cysteine and ribose were not formed, or formed in lower amounts, when lipid was present. This may be due to the reaction between hydrogen sulfide, formed from the breakdown of cysteine, and lipid degradation products. In addition, cysteine and ribose modified lipid oxidation pathways, so that alcohols and alkylfurans were formed rather than saturated and unsaturated aldehydes. Several volatile compounds, which have been found at elevated levels in cooked meat from animals fed supplements high in n-3 acids, were formed when methyl alpha-linolenate reacted with cysteine and ribose. The possible effects of increasing the n-3 content of meat upon flavor formation during cooking are discussed.

Cysteine↗
PubMed2003

Lacticin 3147 favours isoleucine transamination by Lactococcus lactis IFPL359 in a cheese-model system.

The bacteriocin, lacticin 3147, increased isoleucine transamination by Lactococcus lactis IFPL359 in a cheese model system. The formation of alpha-keto-beta-methyl-n-valeric acid and 2-hydroxy-3-methyl-valeric acid increased by three times in cheese slurries at 12 degrees C and cheese aroma intensity increased as well, which corresponded with a higher 2-methylbutanal formation.

Amino Acids↗
PubMed1981

A novel immunocytochemical model system for specificity and sensitivity screening of antisera against multiple antigens.

A novel, convenient immunocytochemical model system that allows simultaneous testing of multiple peptides and peptide fragments is presented. Screening of antisera for specificity and sensitivity and ready detection of multiple antibody subpopulations is possible with this system. Using the peroxidase-antiperoxidase method, a threshold sensitivity of 100 pg of peptide was attained with the best antisera available. Routine screenings of antisera employ amounts of peptides corresponding to 1.8--180 pmol--concentrations about a thousandfold lower than those used for absorption controls. Because of the minute amounts of antigens needed, specificity screening against rare or costly antigens now becomes feasible.

Antigen-Antibody Reactions↗