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At least 721 records · Page 40Linked to original sources
PubMed2003

1,2-dimethyl-4-nitro-5-morpholinoimidazole and its hydrate: a case of a centrosymmetric-noncentrosymmetric ambiguity.

The compound studied is 1,2-dimethyl-4-nitro-5-morpholinoimidazole (1) in its anhydrous (1) and hydrated [(1).H(2)O] crystal forms. In spite of the strong electron-withdrawing effect of the nitro group, the unsubstituted N atom of the imidazole moiety retains its basic character and acts as an acceptor for intermolecular hydrogen bonds: either weak C-H....N bonds in (1) or strong O-H...N bonds, with the water molecules, in (1).H(2)O. The packing in (1) is determined by weak C-H.N and C-H.O hydrogen bonds, van der Waals interactions and the stacking of imidazole fragments. The crystal structure of (1).H(2)O is determined by strong O-H(water)...N3(imidazole) and O-H(water)...O(water) hydrogen bonds. This structure consists of a centrosymmetric 'matrix' of imidazole derivative molecules and locally noncentrosymmetric arrays of hydrogen-bonded water molecules. Each of these arrays is strictly homodromic, i.e. it runs only in one direction:.H-O....H-O....H-O.... or....O-H....O-H....O-H.... These homodromic domains are statistically distributed within the crystal.

Journal Article↗
PubMed1995

Application of linear and nonlinear time series modeling to heart rate dynamics analysis.

The linear autoregressive (AR) model is often used to investigate the pathophysiologic mechanisms controlling heart rate (HR) dynamics. This study implemented parametric models new to this field to determine if a more appropriate HR dynamics modeling structure exists. The linear AR and autoregressive-moving average (ARMA) models, and the nonlinear polynomial autoregressive (PAR) and bilinear (BL) models were fit to instantaneous HR time series obtained from nine subjects in the supine position. Model orders were determined by the Akaike Information Criteria (AIC). Model residual variance was used as the primary intermodel comparison criterion, with significance evaluated by a chi 2 distributed statistic. The BL model best represented the HR dynamics, as its residual variance was significantly (p < 0.05) smaller than that of the corresponding AR model for nine out of nine data sets. In all cases, the BL model had a smaller residual variance than either the ARMA or PAR models. The bilinear model was ineffective at data forecasting, however, we show that this cannot reflect BL model validity because poor prediction is inherent to the BL model structure. The apparent superiority of the nonlinear bilinear model suggests that future heart rate dynamics studies should put greater emphasis on nonlinear analyses.

Adult↗
PubMed1997

A spark counter as a control unit of a radio frequency surgery device.

A new approach in radio frequency (rf) electrosurgery, used for tissue treatment, is achieved by using a new process control method. An external control unit allows a commonly available rf-generator to automatically supply the appropriate power for differing tissue types, thus ensuring best cutting quality. The sparks, generated during the scalpel electrode interaction with the tissue, appear statistically distributed. The spark rate depends on various factors and is monotonic with the supplied electrical power. This allows it to be used as the controlled variable in the cutting process. The ac current passing through the tissue is evaluated by an external control unit using an analyzation algorithm to determine the number of sparks. The external unit is comprised of a system, which measures the spark rate, and a subsequent proportional integral (P.I.) controller. The functionality of the control method as well as the electrical circuitry is verified through cuts with different degrees of carbonization and cuts through tissue heterojunctions.

Adipose Tissue↗
PubMed2000

Road loads acting on manual wheelchairs.

A barrier to performing more in-depth analyzes during the wheelchair design process is a lack of dynamic reaction force and moment data, and the instrumentation to collect this data. Instrumentation was developed to collect the dynamic force and moment data. New data collections methodologies and analysis techniques were implemented to facilitate computer-aided-engineering for wheelchair designs. Data were collected during standardized wheelchair fatigue tests, while driving over a simulated road course within a laboratory, and while driving in the community. Seventeen subjects participated in this study. Based upon the three test conditions, a pseudo-statistical distribution of the force and moment data at both a caster and rear wheel was developed. The key parameters describing the distribution and the extremums of the data (minima and maxima) were compared using analysis of variance. The results showed that the force and moment distributions and extreme values were similar for the both sets of human trials (i.e., simulated road course and field trials). However, the standardized testing (i.e., wheelchair fatigue testing) differed from both human trials. The force/moment data gathered during this study are suitable for inputs in finite element analysis and dynamic modeling. Our results suggest that the fatigue tests should be modified to change the magnitude and increase the frequency of the forces and moments imparted on the wheelchair. The data reported from this study can be used to improve wheelchair standards and to facilitate computer-aided-engineer in wheelchair design.

Acceleration↗
PubMed2003

Noncharged amino acid residues at the solvent-exposed positions in the middle and at the C terminus of the alpha-helix have the same helical propensity.

It was established previously that helical propensities of different amino acid residues in the middle of alpha-helix in peptides and in proteins are very similar. The statistical analysis of the protein helices from the known three-dimensional structures shows no difference in the frequency of noncharged residues in the middle and at the C terminus. Yet, experimental studies show distinctive differences for the helical propensities of noncharged residues in the middle and in the C terminus in model peptides. Is this a general effect, and is it applicable to protein helices or is it specific to the model alanine-based peptides? To answer this question, the effects of substitutions at positions 28 (middle residue) and 32 (C2 position at the C terminus) of the alpha-helix of ubiquitin on the stability of this protein are measured by using differential scanning calorimetry. The two data sets produce similar values for intrinsic helix propensity, leading to a conclusion that noncharged amino acid residues at the solvent-exposed positions in the middle and at the C terminus of the alpha-helix have the same helical propensity. This conclusion is further supported with an excellent correlation between the helix propensity scale obtained for the two positions in ubiquitin with the experimental helix propensity scale established previously and with the statistical distribution of the residues in protein helices.

Amino Acid Substitution↗
PubMed2003

Revisiting the Ramachandran plot: hard-sphere repulsion, electrostatics, and H-bonding in the alpha-helix.

What determines the shape of the allowed regions in the Ramachandran plot? Although Ramachandran explained these regions in terms of 1-4 hard-sphere repulsions, there are discrepancies with the data where, in particular, the alphaR, alphaL, and beta-strand regions are diagonal. The alphaR-region also varies along the alpha-helix where it is constrained at the center and the amino terminus but diffuse at the carboxyl terminus. By analyzing a high-resolution database of protein structures, we find that certain 1-4 hard-sphere repulsions in the standard steric map of Ramachandran do not affect the statistical distributions. By ignoring these steric clashes (NH(i+1) and O(i-1)C), we identify a revised set of steric clashes (CbetaO, O(i-1)N(i+1), CbetaN(i+1), O(i-1)Cbeta, and O(i-1)O) that produce a better match with the data. We also find that the strictly forbidden region in the Ramachandran plot is excluded by multiple steric clashes, whereas the outlier region is excluded by only one significant steric clash. However, steric clashes alone do not account for the diagonal regions. Using electrostatics to analyze the conformational dependence of specific interatomic interactions, we find that the diagonal shape of the alphaR and alphaL-regions also depends on the optimization of the NH(i+1) and O(i-1)C interactions, and the diagonal beta-strand region is due to the alignment of the CO and NH dipoles. Finally, we reproduce the variation of the Ramachandran plot along the alpha-helix in a simple model that uses only H-bonding constraints. This allows us to rationalize the difference between the amino terminus and the carboxyl terminus of the alpha-helix in terms of backbone entropy.

Amino Acids↗
PubMed1983

The analysis of the ultraviolet radiation doses required to produce erythemal responses in normal skin.

When the abnormality of a skin response to sunlight is expressed in terms of a decreased minimum erythemal dose (MED) on photo-testing, it is essential to know the range of response in normal skin. In any homogeneous population, the statistical distribution of the MED is skewed and is not Gaussian (normal), but a Gaussian distribution can be fitted to the logarithm of the MED. Using established methods of probit analysis, such 'lognormal' distributions have been fitted to the observed proportions of erythemal responses to a series of test exposure doses of UV radiation (250-365 nm), thus providing estimates of the average log MED for normal skin, the standard deviations of the distributions and associated confidence limits. These estimates, in arithmetical terms, are available for quantitative comparison with the results of other test exposures on skin which may be abnormal.

Dose-Response Relationship, Radiation↗
PubMed1986

Multiple-transition cell cycle models that exhibit transition probability kinetics.

Cell cycle models that allow multiple random transitions and asymmetric cell division may exhibit a property that has been used to support the transition probability model of the cell cycle: that the absolute value of the difference between sibling cell inter-mitotic times varies from one sibling pair to another and is described by an exponential statistical distribution. Three models that show this property are described, each of which postulates the existence of objects that are partitioned between daughter cells during cell division and whose number influences the duration of the subsequent cell cycle, e.g., surface receptors for growth factors or transcriptional complexes that are carried by sister chromatids. In the first model, sibling cells receive identical numbers of the objects, which are used to perform multiple random transitions that constitute part of the cell cycle. The second model is like the first, except that the partitioning of objects between the newly-formed sibling cells is random. The third model is also like the first, except that all of the objects are passed to one of the sibling cells. These results show that the general mechanisms that are responsible for the dispersion of inter-mitotic times, the correlation between sibling generation times and the apparently exponentially distributed difference between sibling generation times, could be a combination of unequal cell division, multiple random cell cycle transitions and heterogeneity of mitotic cells.

Cell Cycle↗
PubMed2006

No evidence of West Nile virus infection in Dutch blood donors.

BACKGROUND AND OBJECTIVES: West Nile virus (WNV) can be transmitted by transfusion through infected blood components. This study aimed to determine the prevalence of WNV infection among Dutch blood donors to assess whether WNV is a possible threat for the Dutch blood supply. MATERIALS AND METHODS: Plasma samples from 61 992 blood donations were pooled in 7,749 test pools of eight donations using a Tecan robot. These samples were collected between April and October 2004. The pools were tested for the presence of WNV RNA by using the Procleix WNV assay. RESULTS: No WNV RNA-positive pools were detected. Based on Poisson distribution statistics, extrapolation of our data to all the Dutch donations in 2004 revealed that between 0 and 55 cases of WNV infection could be expected. CONCLUSIONS: No evidence of the presence of WNV RNA in Dutch blood donor samples from 2004 was found. However, surveillance of this emerging infection is of importance to safeguard the blood supply in the future because the transmission cycle of WNV is complex and hard to predict.

Blood Banks↗
PubMed2006

The Aquatic Conservation Strategy of the Northwest Forest Plan.

Implemented in 1994, the Aquatic Conservation Strategy of the Northwest Forest Plan was designed to restore and maintain ecological processes for aquatic and riparian area conservation on federal lands in the western portion of the Pacific Northwest. We used decision support models to quantitatively evaluate changes in the condition of selected watersheds. In the approximately 10 years since strategy implementation, watershed condition scores changed modestly, but conditions improved in 64% of 250 sampled watersheds, declined in 28%, and remained relatively the same in 7%. Watersheds that had the largest declines included some where wildfires burned 30-60% of their area. The overall statistical distribution of the condition scores did not change significantly, however Much of the increase in watershed condition was related to improved riparian conditions. The number of large trees (>51 cm diameter at breast height) increased 2-4%, and there were substantial reductions in tree harvest and other disturbances along streams. Whether such changes will translate into longer-term improvements in aquatic ecosystems across broader landscapes remains to be seen.

Animals↗
PubMed1991

Fluorescence energy transfer between dimethyldiazaperopyrenium dication and ethidium intercalated in poly d(A-T).

Dimethyldiazaperopyrenium is one of the largest known DNA intercalators. Fluorescence energy transfer occurred between dimethyldiazaperopyrenium (donor) and ethidium (acceptor) when these dyes were bound to a double-stranded polynucleotide such as poly d(A-T). The addition of increasing amounts of ethidium bromide led to a marked shortening of the fluorescence lifetime of the donor, whereas the excited state of the acceptor was progressively populated via energy transfer from the donor. Critical Förster distance between these two chromophores was calculated to be 3.8 nm. The observed transfer efficiency was lower than that calculated on the basis of this critical distance and a statistical distribution of bound drugs. These results are discussed taking into account the conformational change induced by intercalation of dimethyldiazaperopyrenium in the double-stranded polynucleotide.

Energy Transfer↗
PubMed1988

Variation of population release kinetics in polydisperse multiparticulate systems (microcapsules, microspheres, droplets, cells) with heterogeneity of one, two or three parameters in the population of individuals.

Release kinetics of active substances from ensembles of microparticles such as microcapsules, cells, droplets and liposomes constituted of individual entities releasing their contents at constant rates may follow zero order, first order, sigmoid or biphasic equations. The release equation observed depends upon the statistical distribution of release-determining parameters among the population. Typical cases are presented in terms of the distribution of two parameters, payload (m infinity) and time for complete payload release (t infinity) which also define the release rate constant (k). Heterogeneity of the two parameters generally leads to first order ensemble behaviour, whereas heterogeneity of one parameter only may lead to different ensemble release equations, viz. zero order (m infinity heterogeneous) or first order (t infinity heterogeneous). Biphasic distribution can lead to an apparent 'burst' effect, with apparent change of kinetics. The presence of a third heterogeneous parameter, lag time, yields a sigmoid ensemble release curve. These conclusions are demonstrated by simulations or experimentally, and are also valid for linearized curves of microparticles following matrix kinetics.

Capsules↗
PubMed2003

Incorporation of experimentally-derived fiber orientation into a structural constitutive model for planar collagenous tissues.

Structural constitutive models integrate information on tissue composition and structure, avoiding ambiguities in material characterization. However, critical structural information (such as fiber orientation) must be modeled using assumed statistical distributions, with the distribution parameters estimated from fits to the mechanical test data. Thus, full realization of structural approaches continues to be limited without direct quantitative structural information for direct implementation or to validate model predictions. In the present study, fiber orientation information obtained using small angle light scattering (SALS) was directly incorporated into a structural constitutive model based on work by Lanir (J. Biomech., v. 16, pp. 1-12, 1983). Demonstration of the model was performed using existing biaxial mechanical and fiber orientation data for native bovine pericardium (Sacks and Chuong, ABME, v.26, pp. 892-902, 1998). The structural constitutive model accurately predicted the complete measured biaxial mechanical response. An important aspect of this approach is that only a single equibiaxial test to determine the effective fiber stress-strain response and the SALS-derived fiber orientation distribution were required to determine the complete planar biaxial mechanical response. Changes in collagen fiber crimp under equibiaxial strain suggest that, at the meso-scale, fiber deformations follow the global tissue strains. This result supports the assumption of affine strain to estimate the fiber strains. However, future evaluations will have to be performed for tissue subjected to a wider range of strain to more fully validate the current approach.

Animals↗
PubMed1998

Development of wall surface tangent DPIV measurement techniques for arterial branch models.

Experimental techniques for measuring unsteady flow in a glass arterial bifurcation model have been developed to aid in quantifying three-dimensional wall shear fluctuations associated with arterial disease. The unique feature of the current technique is the use of a "curved" laser sheet, which was everywhere tangent to the inner wall of a daughter tube in an arterial bifurcation model. Surface tangent velocity vector field measurements were made to demonstrate the potential of this technique. Ensemble-averaged data showing weak secondary flows as well as statistical distributions of flow angles are presented. Measurements of this type may be used to estimate mean and instantaneous wall shear magnitude and direction, data that are necessary for understanding the importance of circumferential motions on arterial disease.

Arterioles↗
PubMed2006

Properties of preprocessed sinogram data in x-ray computed tomography.

The accurate determination of x-ray signal properties is important to several computed tomography (CT) research and development areas, notably for statistical reconstruction algorithms and dose-reduction simulation. The most commonly used model of CT signal formation, assuming monoenergetic x-ray sources with quantum counting detectors obeying simple Poisson statistics, does not reflect the actual physics of CT acquisition. This paper describes a more accurate model, taking into account the energy-integrating detection process, nonuniform flux profiles, and data-conditioning processes. Methods are developed to experimentally measure and theoretically calculate statistical distributions, as well as techniques to analyze CT signal properties. Results indicate the limitations of current models and suggest improvements for the description of CT signal properties.

Algorithms↗
PubMedDate not supplied

Image information transfer properties of x-ray fluorescent screens.

The image information transfer efficiency for five x-ray fluorescent screens (calcium tungstate, barium halide, and three rare earth screens) has been experimentally determined with monoenergetic x-ray beams at energies of 18, 22, 32, 49, 51, 58, and 69 keV. The transfer efficiency, which is defined by the ratio of the output signal-to-noise ratios, was determined from measurements of (a) the fraction of incident x rays absorbed in the screen and (b) the statistical distribution of the number of light photons emitted from the screen per absorbed x ray which was determined by light photon counting techniques. Comparisons of the information transfer efficiency, the average number of light photons emitted per absorbed x ray, and the light output energy per Roentgen are given for the above screens and x-ray energies.

Elementary Particles↗
PubMed2001

Sound propagation in a turbulent atmosphere near the ground: an approach based on the spectral representation of refractive-index fluctuations.

A new, rigorous approach is presented for the computation of the fluctuating field of a monopole source in a nonrefracting, turbulent atmosphere above a ground surface. The time-averaged sound pressure level is considered, as well as statistical distributions of the sound pressure level. The computation is based on the Rytov solution of the wave equation for a turbulent medium, evaluated for the half-space above the ground surface. The solution takes into account the ground reflection of scattered waves, which has been neglected in previous work on this subject. The present approach is based on a Fourier-Stieltjes representation of refractive-index fluctuations, and makes use of a turbulent image atmosphere to account for the ground reflection of scattered waves. This approach is rigorous only for a rigid ground surface, but it is shown that it also yields a good approximation for a finite-impedance ground surface. The accuracy of the solution is demonstrated by comparison with results of numerical computations with the parabolic equation method for a turbulent atmosphere. The assumption of a nonrefracting atmosphere implies that direct application of the solution is limited to propagation over relatively small distances. However, this study can also be considered as a basis for a generalized solution for a downward refracting atmosphere, which can be applied for larger propagation distances.

Journal Article↗
PubMed1991

Are spontaneous otoacoustic emissions generated by self-sustained cochlear oscillators?

Theoretical analyses supporting the assumption that spontaneous otoacoustic emissions (SOAEs) can be described as self-sustained oscillations (requiring a power source) are reviewed and extended. Spectral and statistical properties of spontaneous otoacoustic emissions are examined and shown to be consistent with this assumption. Several alternative models of spontaneous emissions (noise-driven saturating memoryless nonlinearity, noise-driven nonlinear-stiffness oscillator) are examined. Although some of these models are able to produce the types of statistical distributions of amplitude and displacement similar to those observed in the experimental data, this similarity is destroyed upon narrow-band filtering.

Animals↗