Showing papers by "Indian Institute of Science published in 1978"

DNA double helical fragment at atomic resolution.

Abstract: DETAILS of the molecular architecture of double helical ribonucleic acids at atomic resolution have recently become available from single crystal X-ray diffraction studies of dinucleotides composed of complementary bases1–3. No similar studies have been published for deoxynucleotides. We report here the structure of the deoxytetranucleotide d-pApTpApT (5′-P-adenylyl-(3′-5′)-thymidylyl-(3′-5′)-adenylyl-(3′-5′)-thymidine) (I) at a resolution of 1.0 A. This is the first tetranucleotide whose structure has been elucidated by X-ray diffraction. The work is part of an investigation of protein–nucleic acid interactions using single-crystal studies of small model compounds. d-pApTpApT was chosen as one of the first compounds to be examined because there is evidence that poly(dA–dT) has unusual binding properties and that A.T-rich regions in certain DNAs have specific biological roles4–7. We hope that these investigations will provide information about the influence of specific base pairs and sequences on the fine details of the DNA structures, and thus aid the understanding of the selective recognition of nucleotide sequences of the DNA double helix by proteins.

Study of the Random Vibration of Nonlinear Systems by the Gaussian Closure Technique

Abstract: A technique is developed to study random vibration of nonlinear systems. The method is based on the assumption that the joint probability density function of the response variables and input variables is Gaussian. It is shown that this method is more general than the statistical linearization technique in that it can handle non-Gaussian excitations and amplitude-limited responses. As an example a bilinear hysteretic system under white noise excitation is analyzed. The prediction of various response statistics by this technique is in good agreement with other available results.

The structure of turbulent boundary layers along mildly curved surfaces

Abstract: This paper describes a detailed study of the structure of turbulence in boundary layers along mildly curved convex and concave surfaces. The surface curvature studied corresponds to δ/Rw = ± 0·01, δ being the boundary-layer thickness and Rw the radius of curvature of the wall, taken as positive for convex and negative for concave curvature. Measurements of turbulent energy balance, autocorrelations, auto- and cross-power spectra, amplitude probability distributions and conditional correlations are reported. It is observed that even mild curvature has very strong effects on the various aspects of the turbulent structure. For example, convex curvature suppresses the diffusion of turbulent energy away from the wall, reduces drastically the integral time scales and shifts the spectral distributions of turbulent energy and Reynolds shear stress towards high wavenumbers. Exactly opposite effects, though generally of a smaller magnitude, are produced by concave wall curvature. It is also found that curvature of either sign affects the v fluctuations more strongly than the u fluctuations and that curvature effects are more significant in the outer region of the boundary layer than in the region close to the wall. The data on the conditional correlations are used to study, in detail, the mechanism of turbulent transport in curved boundary layers. (Published Online April 12 2006)

Production of cast aluminium-graphite particle composites using a pellet method

Abstract: Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm–2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm–2 and the optimum size and percentage of aluminium powder were 400 to 1000mgrm and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.

Rapid distortion of axisymmetric turbulence

Abstract: A generalization of the isotropic theory of Batchelor & Proudman (1954) is developed to estimate the effect of sudden but arbitrary three-dimensional distortion on homogeneous, initially axisymmetric turbulence. The energy changes due to distortion are expressed in terms of the Fourier coefficients of an expansion in zonal harmonics of the two independent scalar functions that describe the axisymmetric spectral tensor. However, for two special but non-trivial forms of this tensor, which represent possibly the simplest kinds of non-isotropic turbulence and specify the angular distribution but not the wavenumber dependence, the energy ratios have been determined in closed form. The deviation of the ratio from its isotropic value is the product of a factor containing R, the initial value of the ratio of the longitudinal to the transverse energy component, and another factor depending only on the geometry of the distortion. It is found that, in axisymmetric and large two-dimensional contractions, the isotropic theory gives nearly the correct longitudinal energy, but (when R > 1) over-estimates the increase in the transverse energy; the product of the two intensities varies little unless the distortion is very large, thus accounting for the stress-freezing observed in rapidly accelerated shear flows.Comparisons with available experimental data for the spectra and for the energy ratios show reasonable agreement. The different ansatzes predict results in broad qualitative agreement with a simple strategem suggested by Reynolds & Tucker (1975), but the quantitative differences are not always negligible.

Structure of DNA predicted from stereochemistry of nucleoside derivatives.

Abstract: CRYSTALLOGRAPHIC data on the constituents of nucleic acids can be very useful in predicting secondary structures. Until recently only data from the crystal structures of mononucleosides and mononucleotides have been used in building models of secondary structures for nucleic acids and polynucleotides. However, such data only provide information about the stereochemistry of nucleotides and not the relative orientation of the nucleotides around the 3′ and 5′ phosphodiester bonds. This relative orientation is the key factor which determines the secondary structure of nucleic acids and details of this orientation can be obtained from single-crystal study of dinucleoside monophosphates and other higher oligomers. So far, crystal structure data on the dinucleoside monophosphates GpC (sodium salt)1, GpC (calcium salt)2, ApU3, UpA4, a dinucleotide pTpT5, a trinucleoside diphosphate ApApA6, and a tetranucleotide pApTpApT (M.A. Viswamitra, personal communication) are available. We have analysed the conformations of these molecules and present here stereochemical criteria for the backbone of the poly-nucleotide chains and the nucleic acids. Application of these criteria to nucleic acids indicates type II structure for DNA, as proposed previously by us7 as an alternative to the double helix.

Equation for stress-strain curve of concrete confined in circular steel spiral

Abstract: The paper presents some results of tests carried out on concrete confined in circular steel spirals subjected to monotonic compression. An equation is proposed for the stress-strain curves of such concrete. The parameters of the equation are determined and the curve given by the equation found to represent satisfactorily the test results of this investigation as well as those of an earlier study.

Turbulence Measurements in Boundary Layers Along Mildly Curved Surfaces

Abstract: This paper presents results of turbulence measurements in boundary layers over surfaces of mild longitudinal curvature. The study indicates that convex wall curvature decreases both the length and velocity scales of turbulent motions, whereas concave curvature has the opposite effect. White being qualitatively similar to those brought about by stronger wall curvature, mild curvature effects are found to be much larger than what one expects from a linear interpolation between the effects of zero and strong curvature. It is also observed that curvature has a relatively larger effect on the Reynolds shear stress than on the turbulent kinetic energy. The present study, however, indicates that it is still possible to use some of the phenomenological models of turbulence (e.g., the mixing length model, the Prandtl-Kolmogorov model), provided an appropriate curvature model is available for describing the effect of curvature on the relevant length scale in the boundary layer. The present data are used to test the validity and limitations of such a curvature model (based on an analogy between streamline curvature and buoyancy) currently in use.

The superfluid as a source of all interactions

Abstract: The superfluid state of fermion-antifermion fields developed in our previous papers is generalized to include higher orbital and spin states. In addition to single-particle excitations, the system is capable of having real and virtual bound or quasibound composite excitations which are akin to bosons of spinJP equal to0−, 1−, 2+, etc. These pseudoscalar, vector, and tensor bosons can be massive or massless and provide the vehicles for strong, electromagnetic, weak, and gravitational interactions. The concept that the basic (unmanifest) fermion-antifermion interaction can lead to a multiplicity of manifest interactions seems to provide a basis for a unified field theory.

Immunological activity of parthenin and its diasteriomer in persons sensitized by Parthenium hysterophorus L.

Abstract: Persons suffering from allergic dermatitis caused by direct contact with Parthenium hysterophorus (Compositae) showed delayed immunologic reaction to the sesquiterpene lactone parthenin but not to its diasteriomer, hymenin.

Limit theorems for semi-Markov processes

Abstract: A new construction of regeneration times is exploited to prove ergodic and renewal theorems for semi-Markov processes on general state spaces. This work extends results of the authors in Ann. Probability (6 (1978), 788-797).

A generalised cole-hopf transformation for nonlinear parabolic and hyperbolic equations

Abstract: The Cole-Hopf transformation has been generalized to generate a large class of nonlinear parabolic and hyperbolic equations which are exactly linearizable. These include model equations of exchange processes and turbulence. The methods to solve the corresponding linear equations have also been indicated.