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A Mahendrasingam

Publications and source records attributed to A Mahendrasingam.

14 recordsLinked to original sources
PubMed2000

Micro-SAXS and force/strain measurements during the tensile deformation of single struts of an elastomeric polyurethane foam.

A microdeformation stage based on a piezoelectric crystal actuator capable of measuring the force applied to micrometre-sized polymeric samples is described. Laboratory force/strain measurements on a single strut of an elastomeric polyurethane foam have been conducted for the first time. The device has also been used on the microfocus beamline at the European Synchrotron Radiation Facility to collect microbeam small-angle X-ray scattering data simultaneously with strain and force measurements during the time-resolved tensile deformation of a single strut of elastomeric polyurethane foam.

Journal Article↗
PubMed1997

Investigation of the variation in orientation and crystallinity in poly(ethylene terephthalate) containers using microfocus X-ray diffraction.

The microfocus X-ray beamline at the European Synchrotron Radiation Facility has been used to investigate the variation in molecular orientation and crystallinity in the wall of a container fabricated from poly(ethylene terephthalate). Two-dimensional wide-angle X-ray scattering patterns were recorded and displayed in real time as the specimen was tracked across the incident X-ray beam enabling the measurement of textural changes to be made with a spatial resolution of ~2 mum.

Journal Article↗
PubMed1996

Time-Resolved Simultaneous SAXS/WAXS of the Drawing of Polyethylene at the Daresbury SRS.

A system has been developed which represents a significant advance in the quality and extent of small- and wide-angle X-ray scattering data (SAXS and WAXS) that can be recorded simultaneously with strain data during the drawing and annealing of polymer materials. WAXS data are recorded using a Photonic Science charge-coupled-device area detector and SAXS data using a gas-filled multiwire area detector. Strain data, for the region of the specimen from which the SAXS/WAXS data are collected, are calculated from an accurately synchronized continuously recorded video image of the specimen. The system allows X-ray and video image data to be collected as a series of frames with essentially no ;dead-time' between frames. The data are fully two-dimensional and can be collected for a wide range of d spacings. The use of this system to investigate the stress-induced orientation and phase changes during the drawing of a range of grades of commercially available polyethylene is described.

Journal Article↗
PubMed1996

DNA hydration studied by neutron fiber diffraction.

The development of neutron high angle fiber diffraction to investigate the location of water around the deoxyribonucleic acid (DNA) double-helix is described. The power of the technique is illustrated by its application to the D and A conformations of DNA using the single crystal diffractometer, D19, at the Institut Laue-Langevin. Grenoble and the time of flight diffractometer, SXD, at the Rutherford Appleton ISIS Spallation Neutron Source. These studies show the existence of bound water closely associated with the DNA. The patterns of hydration in these two DNA conformations are quite distinct and are compared to those observed in X-ray single crystal studies of two-stranded oligodeoxynucleotides. Information on the location of water around the DNA double-helix from the neutron fiber diffraction studies is combined with that on the location of alkali metal cations from complementary X-ray high angle fiber diffraction studies at the Daresbury Laboratory SRS using synchrotron radiation. These analyses emphasize the importance of viewing DNA, water and ions as a single system with specific interactions between the three components and provide a basis for understanding the effect of changes in the concentration of water and ions in inducing conformational transitions in the DNA double-helix.

Crystallography↗
PubMed1996

Time-of-flight Laue fiber diffraction studies of perdeuterated DNA.

The diffractometer SXD at the Rutherford Appleton Laboratory ISIS pulsed neutron source has been used to record high resolution time-of-flight Laue fiber diffraction data from DNA. These experiments, which are the first of their kind, were undertaken using fibers of DNA in the A conformation and prepared using deuterated DNA in order to minimise incoherent background scattering. These studies complement previous experiments on instrument D19 at the Institut Laue Langevin using monochromatic neutrons. Sample preparation involved drawing large numbers of these deuterated DNA fibers and mounting them in a parallel array. The strategy of data collection is discussed in terms of camera design, sample environment and data collection. The methods used to correct the recorded time-of-flight data and map it into the final reciprocal space fiber diffraction dataset are also discussed. Difference Fourier maps showing the distribution of water around A-DNA calculated on the basis of these data are compared with results obtained using data recorded from hydrogenated A-DNA on D19. Since the methods used for sample preparation, data collection and data processing are fundamentally different for the monochromatic and Laue techniques, the results of these experiments also afford a valuable opportunity to independently test the data reduction and analysis techniques used in the two methods.

Crystallography↗
PubMed1995

Microfocus X-ray Diffraction of Spherulites of Poly-3-hydroxybutyrate.

The microfocus X-ray beamline at the European Synchrotron Radiation Facility has been used to investigate the variation in molecular orientation and crystallinity in spherulites of the organic polymer poly-3-hydroxybutyrate (PHB). This is the first report of the correlation of optical and X-ray measurements on spherulitic polymer films where X-ray diffraction patterns have been recorded and displayed continuously in real time while the specimen was tracked in steps of 10 mum across an incident X-ray beam with a diameter as small as 10 mum.

Journal Article↗
PubMed1994

Structure factor calculations for a side-by-side model of B-DNA.

In this paper, the side-by-side model of DNA proposed by Premilat and Albiser is investigated. The axial repeat of the model is equal to the c-axis repeat in the observed B-DNA unit cell in fibres. However, the model does not pack into the unit cell as efficiently as the B-DNA double helix does, nor is it as successful as the double helix in predicting the observed Bragg amplitudes. When the azimuthal orientations and the relative axial displacements of the two molecules in the unit cell are allowed to take general values, the best crystallographic R factor for the side-by-side model is 43.43% compared with 34.33% for the double helix. If constraints consistent with the accepted B-DNA space group, P2(1)2(1)2(1), are applied, the best R factors are 45.53% for the side-by-side model and 34.51% for the double helix. Therefore, the side-by-side model can be rejected as a possible conformation for B-DNA in crystalline fibres.

DNA↗
PubMed1992

A high angle neutron fibre diffraction study of the hydration of the A conformation of the DNA double helix.

A high angle neutron fibre diffraction study of the distribution of water around the A-form of DNA has been performed using the diffractometer D19 at the Institut Laue-Langevin, Grenoble. These experiments have exploited the ability to replace H2O surrounding the DNA by D2O so that isotopic difference Fourier maps can be computed in which peaks are identified with the distribution of water in the unit cell. All peaks of significant height have been accounted for by four families of water molecules whose positions and occupancies have been determined using least squares refinement. The coordinates of the water peaks making up each family do not deviate significantly from a regular helical arrangement with the same parameters as the DNA. Two of these families are of particular interest. The first consists of water molecules in the major groove linking successive charged phosphate oxygens along the polynucleotide chains. The second is associated with bases in the major groove and forms a central core of density along the helix axis. These two families provide a layer of hydration lining the interior wall of the major groove leaving a central channel to accommodate cations. The relationship between these observations and conformational stability is discussed.

DNA↗
PubMed1989

Neutron fibre diffraction study of DNA hydration.

Interactions with water are crucial to the conformation assumed by the DNA double helix. The location of water around the D conformation has been investigated in a neutron fibre diffraction study which shows that water is ordered in the minor groove of the DNA. The D conformation is important since its occurrence is limited to specific DNA base pair sequences which have been identified as functionally significant. This study is of particular interest because the D conformation has not been reported in single crystal studies of oligonucleotides.

DNA↗
PubMed1986

Time-resolved X-ray diffraction studies of the B in equilibrium D structural transition in the DNA double helix.

Because of the relation between topology and function, there has been much interest in the structural transitions of the various conformations of DNA polymers. The x-ray fiber diffraction analysis system at the Daresbury Synchrotron Radiation Source was used to study the reversible transition between the B and D forms of the synthetic DNA poly[d(A-T)].poly[d(A-T)]. The gradual progression of conformations between these two forms indicates that the DNA double helix does not undergo a change of handedness during this transition.

DNA↗
PubMed1983

Conformational transitions in the synthetic polynucleotide poly[d(G-C)] . poly[d(G-C)] double-helix.

Conditions are described for observing by X-ray fibre diffraction the A, B and S conformations of the poly[d(G-C)] . poly[d(G-C)] double-helix and also a new form designated as B". For fibres with an appropriate ionic content, transitions between these conformations can be induced by varying the relative humidity of the fibre environment. With increasing relative humidity the transitions B" leads to A leads to S leads to B occur. However, reducing the relative humidity does not result in a simple reversal of these transitions. If the relative humidity is reduced rapidly, a B leads to A transition is observed followed by an A leads to B" transition, but if it is reduced slowly, the transition is from B to S. Once the S form has been assumed, further reduction in the relative humidity does not result in a transition to the A form. The S form emerges as a particularly stable form of the poly[d(G-C)] . poly[d(G-C)] double-helix. From the point of view of its relationship to the classical A and B forms, the S form of poly[d(G-C)] . poly[d(G-C)] is shown to exhibit similarities to the D form of poly[d(A-T)] . poly[d(A-T)].

Humidity↗
PubMed1983

Conformational transitions in oriented fibres of the synthetic polynucleotide poly[d(AT)].poly[d(AT)] double helix.

The synthetic polynucleotide poly[d(AT)].poly[d(AT)] is of interest in studies of the relationship between nucleic acid structure and function. In particular, A + T-rich regions in DNA double helices have been invoked as centres for controlling the transcription of genetic information. Here we describe conditions for observing by X-ray fibre diffraction the A, B, C and D conformations of Na-poly[d(AT)].poly[d(AT)], and for inducing transitions between these conformations. The D form emerges as a particularly stable conformation; once assumed, it persists over a wide range of variation in the relative humidity of the fibre environment. Further, while transitions between the B and D conformations are readily reversible, transitions between A and D are much more complex.

Nucleic Acid Conformation↗