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At least 1,153 records · Page 64Linked to original sources
PubMed1990

Two-dimensional analysis of two-file flow of red cells along capillaries.

The effect of a tank-treading motion of the red cell membrane on a staggered "zipper"-type red cell flow in capillaries is examined, using a two-dimensional theoretical model. An approximately triangular cell with a thin flexible membrane enclosing a viscous fluid is adopted as a model of the red cell. The motion of model red cells arranged periodically along a two-dimensional channel in an idealized zipper-type arrangement is analyzed numerically by a finite element method applied to the Stokes equations for the flow both inside and outside the model cells. It is shown that, if the viscosity ratio of the internal fluid to the suspending fluid is lower than a critical value, there exists a stable zipper-type arrangement of cells. In that arrangement, the cells remain stationary relative to each other with the membrane tank-treading. In contrast, inhibiting tank-treading by increasing the viscosity ratio above the critical value induces a cyclic oscillatory motion of red cells. The critical viscosity ratio increases if the channel is narrowed or if the spacing between cells is reduced. The present results suggest that the membrane tank-treading tends to stabilize zipper-type arrangements of red cells in capillaries at high hematocrit.

Capillaries↗
PubMed1990

Effectiveness of ultrasonic files in the disruption of root canal bacteria.

The physical mechanisms of ultrasound, namely cavitation and acoustic streaming, generated by the Enac-Osada ultrasonic unit were investigated for effectiveness in disrupting Streptococcus mitis. In addition, the bactericidal effect of ultrasound in the presence of 2.5% sodium hypochlorite was examined. Bacterial suspensions were irradiated directly with ultrasound in simulated root canals, and the viability of bacteria was examined after growth on a blood agar medium under anaerobic conditions at 37 degrees C for 5 days. The results indicated that ultrasound per se failed to disrupt bacteria but resulted in increases in the viable counts; the former was considered to be because of the lack of cavitation and the latter because of the dispersal effects of acoustic streaming. The 2.5% sodium hypochlorite solution demonstrated powerful bactericidal activity.

Colony Count, Microbial↗
PubMed1991

Density profiles of biological specimens from Tiff files.

A program for achieving density profiles of Tiff images is described. Densitometric analysis is performed applying a cross-lateral averaging and longitudinal interpolation along the image axis. These procedures may be adapted to a variable axis curvature, length, width and morphological resolution. Since the Tiff format is supported by all desktop scanners and a large number of frame-grabbing devices, the range of applications includes the analysis of light and electron micrographs, autoradiograms, chromatograms, and data from any kind of bidimensional medium or flat surface. A special feature is the possibility of retrieving data from published pictures taken from literature.

Biology↗
PubMed1991

A computer filing system for drug use in a pregnancy consultation service.

A computerized database for documenting drug use in pregnancy was set up on a personal computer. Summaries of the information available from the literature on more than 700 drugs taken during pregnancy have already been entered and are currently in use. This reliable, cheap, handy, and personal system (the first available on the topic) may be a potentially useful instrument not only for drug consultation services, but for individual physicians, too.

Databases, Factual↗
PubMed1991

Pharmacophoric pattern matching in files of three-dimensional chemical structures: use of smoothed bounded distances for incompletely specified query patterns.

This paper describes a technique for increasing the screen-out of pharmacophoric pattern searches in the databases of three-dimensional chemical structures when only some of the interatomic distances in the query pattern are specified. The technique involves the application of a distance bounds-smoothing procedure to the query distances; this smoothing allows the calculation of upper and lower bounds for the unspecified distances. The bounded distances can then be used to set screens additional to those that are set to describe the distances that have been specified by the searcher. Evidence is presented to suggest that use of the technique can lead to increases in the efficiency of substructure searches for partially specified query patterns.

Databases, Factual↗
PubMed1991

Pharmacophoric pattern matching in files of three-dimensional chemical structures: characterization and use of generalized valence angle screens.

This paper describes the use of generalized valence angles for the screening of pharmacophoric pattern searches in databases of three-dimensional chemical structures. A generalized valence angle is defined as the angle between two vectors, AB and BC, which have a common vertex B, and in which both vectors correspond to formal chemical bonds; one vector corresponds to a bond and the other to a non-bonded interaction; or both vectors correspond to non-bonded interactions. The screens are identified by a statistical analysis of the frequencies of occurrence of these angle-based features in the Cambridge Structural Database. The occurrence frequencies are discussed and shown to be explicable in terms of small, commonly occurring structural features. The effectiveness of the screens is demonstrated by an extensive series of searches for representative pharmacophoric patterns. The results are compared with those obtained from a similar series of searches using distance-based screens: The latter are found to give a better level of performance, and evidence is presented to suggest that this is due to a high degree of association between the assignments of the angle-based screens.

Computers↗
PubMed1992

Pharmacophoric pattern matching in files of three-dimensional chemical structures: use of bounded distance matrices for the representation and searching of conformationally flexible molecules.

This paper discusses the use of bounded distance matrices for the representation of conformationally flexible three-dimensional (3D) molecules. It is shown that pharmacophoric pattern searches of databases of flexible 3D molecules represented in this way can be carried out using screen and geometric searching algorithms that are analogous to those used for searching databases of rigid 3D structures. Molecules matching a query pattern after the geometric search must then undergo a final conformational search to determine whether they can, in fact, adopt a conformation that matches the query. An analysis of this three-stage searching procedure shows that searching databases of flexible 3D molecules is extremely demanding of computational resources.

Algorithms↗
PubMed1993

Pharmacophoric pattern matching in files of three-dimensional chemical structures: implementation of flexible searching.

The conformational space of a flexible three-dimensional (3D) molecule can be represented for searching purposes by a smoothed bounded distance matrix. Such matrices provide an effective way of carrying out flexible searching, but search times can be very long when compared with rigid searches that take no account of conformational flexibility. This paper considers two techniques for minimizing the computational requirements of flexible searching. In the first part, we compare four different indices that have been suggested for the quantification of molecular flexibility, and demonstrate that they produce comparable rankings of sets of molecules in order of decreasing flexibility. We then demonstrate that the prioritization of a set of structures in order of increasing flexibility can result in substantial reductions in the time requirements of flexible searching if some fixed number of hit structures is desired. In the second part, we report an analysis of the 3D crystal structures in the Cambridge Structural Database to generate distance ranges that are tighter than those produced by distance geometry. Experiments with a set of six query pharmacophores demonstrate that use of these tightened ranges results in substantial reductions in search times, but that this may also lead to a reduction in the number of hit molecules obtained from the final conformational search.

Databases, Factual↗
PubMed1993

Pharmacophoric pattern matching in files of three-dimensional chemical structures: characterization and use of generalized torsion angle screens.

This paper describes the use of generalized torsion angles for the screening of conformational searches in databases of three-dimensional chemical structures. A generalized torsion angle is defined as the dihedral angle between two vectors, A1-A2 and A3-A4, in which none, some, or all of the vectors A1-A2, A2-A3, and A3-A4 correspond to formal chemical bonds. The screens consist of a set of four atoms together with an associated angular range, and are identified by a statistical analysis of the frequencies of occurrence of these features in the Cambridge Structural Database. These frequencies are discussed, and the effectiveness of the screens is demonstrated by an extensive series of searches for representative pharmacophoric patterns.

Databases, Factual↗