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At least 271 records · Page 15Linked to original sources
PubMed1977

Photochemical transformation of the DDT and methoxychlor degradation products, DDE and DMDE, by sunlight.

DDE and DMDE, degradation products of the pesticides DDT and methoxychlor, rapidly undergo an unusual photoisomerization in solution when exposed to sunlight. The isomerization involves the exchange of a vinyl chlorine and an ortho aromatic hydrogen. Other photoproducts identified were corresponding benzophenones and 1,1-diaryl-2-chloroethylenes. Quantum yields for the reactions were measured and then used to compute sunlight photolysis half-lives for DMDE and DDE. Although both compounds absorb only the short-wavelength ultraviolet component of sunlight, their photolysis was found to be surprisingly rapid. During summer at latitude 40 degrees N, the photolysis half-lives near the surface of a water body are one hour and one day for dissolved DMDE and DDE, respectively. Photolysis of the DDE photoisomers is about an order of magnitude slower than that of DDE, suggesting that they may accumulate under environmental conditions. The DDE photoisomers photocyclize to form chlorinated dibenzofulvene and dichlorofluorenone. Neither DDE nor its photoisomers photoreact in solution to form PCB's. The environmental significance of these results is discussed, and its is suggested that the persistence of DDE in inland surface waters may be related to its tendency to sorb onto sediments and biota where not light is present.

Dichlorodiphenyl Dichloroethylene↗
PubMed1995

Discriminating D1 and D2 agonists with a hydrophobic similarity index.

Currently, methods for calculating molecular similarity indices have been developed for comparing steric, charge density, and molecular electrostatic potential (MEP) properties. Much of the existing technology may, however, be applied to the quantitative comparison of molecular hydrophobicities. In this article we present an empirical hydrophobic similarity index. We utilize atomic hydrophobic parameters derived from a quantum mechanical semiempirical wavefunction. Hydrophobicity at points on a grid is computed with a recently introduced "molecular lipophilicity potential." The overlap of pairs of molecules is calculated with the metric introduced by Carbó. This approach is applied to a case in which steric and electrostatic criteria have already been shown to be inadequate in rationalizing selectivity, namely, requirements for recognition at the dopamine D1 and D2 receptors. We demonstrate that, for a set of dopamine agonists, D1 ligands show higher similarity in this property that D2 analogs. This indicator of similarity is more successful at accounting for D1 selectivity than previous methods.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed2025

Enzymatic Anti-Baldwin Ring-Closure Cascade for Fused Bicyclic Ether Formation.

Pyrenulic acids are cytotoxic polyketides isolated from the ascomycete Pyrenula sp. derived from Vietnamese lichen that are characterized by complex fused cyclic core structures. Genome sequencing, in silico sequence analysis, and RT-PCR studies identified the pyrenulic acid (pya) biosynthetic gene cluster. Based on a functional analysis of the enzymes by expression of each gene in a heterologous host using Aspergillus nidulans, we discovered two cytochrome P450s PyaJ and PyaG that effect epoxidation and hydroxylation of the alkyl chain terminal, respectively, and an α/β hydrolase PyaF that constructs a 6- and 7-membered fused bicyclic diether skeleton by catalyzing successive epoxide ring-opening 6-endo and 7-endo cyclization reactions. To elucidate the detailed mechanism of pyrenulic acid formation, we obtained PyaF as a recombinant enzyme and performed an in vitro experiment, which confirmed catalysis by PyaF of the cyclization reaction. In addition, we performed alignment analysis of PyaF with α/β hydrolases with known functions, as well as an in-depth computational study. In-depth computational analyses of the cyclization reaction pathways with density functional theory quantum mechanics and detailed characterization of PyaF by Chai-1-based protein structure modeling with molecular dynamics simulations and site-specific mutagenesis predicted the active amino acid residues of this serine α/β hydrolase to be an unusual catalytic serine tetrad involving Ser170, Asn342, Asp314, and His169, with Tyr255 and His284 acting as general bases to facilitate opening of the epoxides. Our study provides insight into how regioselectivity of enzymatic anti-Baldwin epoxide ring-opening cascades for the formation of a fused cyclic ether structure is controlled.

Cyclization↗
PubMed2000

Leading ruelle resonances of chaotic maps

The leading Ruelle resonances of typical chaotic maps, the perturbed cat map and the standard map, are calculated by variation. It is found that, excluding the resonance associated with the invariant density, the next subleading resonances are, approximately, the roots of the equation z(4)=gamma, where gamma is a positive number that characterizes the amount of stochasticity of the map. The results are verified by numerical computations, and the implications to the form factor of the corresponding quantum maps are discussed.

Journal Article↗
PubMed1996

Lipoprotein changes in relation to body mass index in Native American adolescents.

In view of their known high incidence of noninsulin dependent diabetes (NIDDM), we sought to determine whether Native American (Plains Indian) children and adolescents show evidence of risk factors for both NIDDM and cardiovascular disease. Children and adolescents between the ages of 4 and 19 y were recruited, and field days were organized for data collection, which included height, weight [to compute body mass index (BMI)], waist and hip circumference, family histories, quantum of Native American ancestry, and blood sampling for fasting lipids, apolipoproteins, insulin, and glucose. BMI increased with age in boys and girls and tended to be higher than in Caucasian children. The difference was significant in 5-9-y-old (p < 0.05) and 10-14-y-old (p < 0.05) boys and 10-14-y-old girls (p < 0.001). Ten- to 14-y-old girls in the highest quartile for BMI had higher triglyceride levels (p < 0.05) and lower HDL cholesterol (p < 0.001) when compared with those in the lower quartiles. In contrast, 15-19 y olds in the highest quartile for BMI had higher cholesterol, LDL cholesterol, and apolipoprotein B (p < 0.001). The mean fasting insulin levels were not related to BMI. The data suggest that, within this Plains Indian population, obesity associated with elevated lipid levels tends to begin at an early age in Native American children. Insulin levels do not appear to be related to BMI, a putative index of adiposity, in this population of children known to be prone to NIDDM in adult life.

Adolescent↗
PubMedDate not supplied

Information partnerships--shared data, shared scale.

How can one company gain access to another's resources or customers without merging ownership, management, or plotting a takeover? The answer is found in new information partnerships, enabling diverse companies to develop strategic coalitions through the sharing of data. The key to cooperation is a quantum improvement in the hardware and software supporting relational databases: new computer speeds, cheaper mass-storage devices, the proliferation of fiber-optic networks, and networking architectures. Information partnerships mean that companies can distribute the technological and financial exposure that comes with huge investments. For the customer's part, partnerships inevitably lead to greater simplification on the desktop and more common standards around which vendors have to compete. The most common types of partnership are: joint marketing partnerships, such as American Airline's award of frequent flyer miles to customers who use Citibank's credit card; intraindustry partnerships, such as the insurance value-added network service (which links insurance and casualty companies to independent agents); customer-supplier partnerships, such as Baxter Healthcare's electronic channel to hospitals for medical and other equipment; and IT vendor-driven partnerships, exemplified by ESAB (a European welding supplies and equipment company), whose expansion strategy was premised on a technology platform offered by an IT vendor. Partnerships that succeed have shared vision at the top, reciprocal skills in information technology, concrete plans for an early success, persistence in the development of usable information for all partners, coordination on business policy, and a new and imaginative business architecture.

Economic Competition↗
PubMed1996

Glycine and GABA receptors: molecular mechanisms controlling chloride ion flux.

We have been able to show that the three clearly identified atoms common to the inhibitory neurotransmitters glycine and GABA, that we previously hypothesized to serve as attachment points at the glycinergic and gabanergic receptor, can indeed interact through both electrostatic and hydrogen bonding to several amino acids, which have been identified in molecular biological investigations as both present and critical in the physiological functioning of key polypeptides common to these inhibitory receptors. In addition, amino acids also involved in stabilizing the interaction between the antagonists strychnine and R5135 at the glycinergic and gabanergic receptors, respectively, have been shown to fit our complex model. We identify in detail molecular mechanisms to explain how glycine and GABA initiate chloride ion movement from extraneuronal fluid in the synaptic cleft to intraneuronal volume. In addition, we also identify the molecular mechanisms involved in the blocking of chloride ion movement by strychnine at the glycinergic receptor and by R5135 at the gabanergic receptor. We also present two computer-generated color prints, one for the glycine receptor and one for the GABA receptor, which show the quantum mechanically geometry optimized complex formed between receptor side chains, i.e., the part of the amino acids in the polypeptide that interacts with the zwitterionic inhibitory neurotransmitters. These computer-generated color figures also show a) the important electrostatic and hydrogen bonding in these interactions, b) a van der Waals model of this complex to illustrate that no steric repulsions exist, and c) the molecular electrostatic potential energy map showing the electrostatic potentials of neurotransmitter bound to the receptor model. Finally, we show with computer calculations that the pseudo-rings, formed between the positive quanidinium group in arginine and one of the oxygen atoms in the carboxyl group in both glycine or GABA, result in a positive planar region which appears to be involved in a charge-transfer complex with aromatic benzene groups in amino acids such as phenylalanine and tryosine.

Androstanes↗
PubMed1998

Steady-state and time-resolved fluorescence of Esperase: comparison with the X-ray structure in the region of the two tryptophans.

Fluorescence emission properties of the alkaline protease Esperase have been investigated using steady-state and time-resolved fluorescence spectroscopy. The local polarity and solvent accessibility of the tryptophyl chromophores is characterized. Quenching studies demonstrated that Trp 6 and Trp 113 are 'buried' to acrylamide, iodide ions and caesium ions. An abnormally low tryptophan quantum yield was calculated showing that the emission of the two indole rings is significantly quenched by nearby side chains or peptide bonds. The fluorescence decay of PMS-Esperase was well fitted by two exponentials with lifetimes of 2.7 and 0.35 ns. X-ray data for Esperase (S. Klupsch, Ph.D. Thesis, University of Hamburg, Hamburg, Germany) in the region of the two tryptophans were used to explain the observed emission properties. Gln 182 and Asn 204 as well as Asn 117 and Met 119 are the most likely quenchers, respectively, of the Trp 6 and Trp 113 fluorescence. The two tryptophans in Esperase are 'buried' in hydrophobic regions and are excellent intrinsic probes to study folding-unfolding reactions. Experiments in the presence and absence of added calcium ions demonstrated the stabilizing role of the Ca(2+)-binding sites.

Computer Graphics↗
PubMed2000

Dose efficiency and low-contrast detectability of an amorphous silicon x-ray detector for digital radiography.

The effect of dose reduction on low-contrast detectability is investigated theoretically and experimentally for a production grade amorphous silicon (a-Si) x-ray detector and compared with a standard thoracic screen-film combination. A non-prewhitening matched filter observer model modified to include a spatial response function and internal noise for the human visual system (HVS) is used to calculate a signal-to-noise ratio (SNR) related to object detectability. Other inputs to the SNR calculation are the detective quantum efficiency (DQE) and the modulation transfer function (MTF) of the imaging system. Besides threshold detectability, the model predicts the equivalent perception dose ratio (EPDR), which is the fraction of the screen film exposure for which the digital detector provides equal detectability. Images of a contrast-detail phantom are obtained with the digital detector at dose levels corresponding to 27%, 41%, 63% and 100% of the dose used for screen-film. The images are used in a four-alternative forced choice (4-AFC) observer perception study in order to measure threshold detectability. A statistically significant improvement in contrast detectability is measured with the digital detector at 100% and 63% of the screen-film dose. There is no statistical difference between screen-film and digital at 41% of the dose. On average, the experimental EPDR is 44%, which agrees well with the model prediction of 40%.

Equipment Design↗
PubMed1993

Reproduction of correct electrostatic field by charges and dipoles on a closed surface.

A general algorithm based on the Green function theorem has been developed to correctly reproduce electrostatic fields inside a closed space by point charges and point dipoles on the surface surrounding the space. For actual computations, limited numbers of point charges, including charge pairs replacing point dipoles, are enough to approximate the inner fields. As examples, reaction fields were reproduced by the current surface charges and dipoles for the dielectric models, where a monopole, dipole, or quadrupole was individually set at the center in a vacuum sphere surrounded by high dielectric continuum. The potentials due to those reaction fields agree well with the analytical ones. As an application of this method to the analysis of the electronic structure of the active site of a protein, a combination of the continuum dielectric model and ab initio molecular orbital calculation was carried out. Other applications to molecular dynamics and quantum mechanical calculations are also discussed.

Algorithms↗
PubMed1994

Integration of graph theory and quantum chemistry for structure-activity relationships.

The objective of this article is to outline both graph-theoretically based and quantum chemically based structural indices of potential use in quantitative structure activity correlations. We consider graph-theoretical indices such as the connectivity index, topological index, Wiener index and molecular ID indices. Several structural and geometry-dependent indices can be derived from semiempirical and ab initio quantum calculations based on the charge densities, overlap matrices, frontier orbitals, molecular hardness, free valence, density matrices, quantum spectral difference indices, quantum spectral indices and bond matrices. Finally, the use of electrostatic potentials and charge densities for the prediction of reactive sites will be discussed.

Butadienes↗
PubMed1998

X-ray quantum limited portal imaging using amorphous silicon flat-panel arrays.

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device (EPID) based on an amorphous silicon flat-panel array. The array has a 128 x 128-pixel matrix and each pixel is 0.75 x 0.75 mm2 in dimension so the array covers an area of 96 x 96 mm2. The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs--characteristics which suggest that the amorphous silicon EPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of "noisy" pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous silicon EPIDs are very well suited to portal imaging.

Artifacts↗
PubMedDate not supplied

Fast Fourier digital quantum mottle analysis with application to rare earth intensifying screen systems.

The advent of fast Fourier techniques has greatly facilitated the digital analysis of noise power spectra (Wiener spectra) by circumventing the need for the autocorrelation function. We are now able to Fourier analyze film data at about the same rate the microdensitometer-computer system can collect it (1000 points/sec). The new technique has been applied to the analysis of the quantum mottle of several rare earth intensifying screen systems confirming earlier estimates from our pilot studies that such screens are capable of reducing exposure by a factor of about 2 with imaging parameters comparable to those of conventional calcium tungstate systems.

Calcium↗
PubMed1998

An ab initio approach to the understanding of cytochrome P450-ligand interactions.

1. We describe the application of novel ab initio quantum mechanical methods to the study of ligand interactions with cytochrome P450cam (CYP101). 2. We find that our techniques accurately describe the transition from a low-spin state to a high-spin state of the haem Fe3+ on binding of a substrate. Furthermore, our methods correctly predict that a large fraction of low-spin character is retained on binding of an inhibitor. 3. We demonstrate the use of 'computational experiments' to elucidate key features of the mechanism of interaction. This leads us to identify a new mechanism for the suppression of the low- to high-spin transition on binding of an inhibitor, namely the shortening of the bond between the Fe atom and the coordinated S atom of the cysteine axial ligand.

Camphor 5-Monooxygenase↗
PubMed1998

Adaptive streak artifact reduction in computed tomography resulting from excessive x-ray photon noise.

The quality of a computed tomography (CT) image is often degraded by streaking artifacts resulting from excessive x-ray quantum noise. Often, a patient has to be rescanned at a higher technique or at a larger slice thickness in order to obtain an acceptable image for diagnosis. This results in a higher dose to the patient, a degraded cross plane resolution, or a reduced patient throughput. In this paper, we propose an adaptive filtering approach in Radon space based on the local statistical properties of the CT projections. We first model the noise characteristics of a projection sample undergoing important preprocessing steps. A filter is then designed such that its parameters are dynamically adjusted to adapt to the local noise characteristics. Because of the adaptive nature of the filter, a proper balance between streak artifact suppression and spatial resolution preservation is achieved. Phantom and clinical studies have been conducted to evaluate the robustness of our approach. Results demonstrate that the adaptive filtering approach is effective in reducing or eliminating quantum noise induced artifacts in CT. At the same time, the impact on the spatial resolution is kept at a low level.

Artifacts↗
PubMed1993

Modeling the anticancer action of some retinoid compounds by making use of the OASIS method.

The powerful OASIS (optimized approach based on structural indices set) approach is applied to the anticancer activity of a series of vitamin A analogs. The best three- and four-variable models obtained via the OASIS technique have correlation coefficients of 0.973 vs. 0.990 and standard deviations s2 = 0.11 and 0.05, respectively. The models incorporate the hydrophobicity factor log P, two geometric parameters (topological indices and/or 3-D steric ones), and the molecular dipole moment. For a set of 15 compounds studied here, the activity measured by ED50 was well correlated by models with approximately equal contribution of the through cell membrane transport and the geometric drug-receptor correspondence while weak nonspecific electronic interaction was also found to play some role. Comparison to previous treatments of this data is given and extension to larger sets is discussed.

Antineoplastic Agents↗