Author

# S. Ramaseshan

Other affiliations: Indian Institute of Technology Madras, Raman Research Institute, Indian Academy of Sciences

Bio: S. Ramaseshan is an academic researcher from Indian Institute of Science. The author has contributed to research in topics: Anomalous scattering & Faraday effect. The author has an hindex of 14, co-authored 88 publications receiving 854 citations. Previous affiliations of S. Ramaseshan include Indian Institute of Technology Madras & Raman Research Institute.

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##### Papers

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TL;DR: Anomalous scattering can be used for phase determination and also for determining the absolute configuration in macromolecular crystallography from the 1950s to the mid-1980s as discussed by the authors.

Abstract: When two or more crystals have the same structure except for the replacement or addition of one or a few (usually heavy) atoms, phase determination using isomorphous replacement can be resorted to. In most practical instances, the atomic form factor is a real number. When the absorption edge of an atom is close to the wavelength of the incident radiation, the form factor becomes complex and the atom becomes an anomalous scatterer. Anomalous scattering can be used for phase determination and also for determining the absolute configuration. The effects of isomorphous replacement and anomalous scattering are often complementary. Although they have been in use from the 1930s, the most useful applications of the two approaches have been in macromolecular crystallography from the 1950s. Robust methods have been developed for their use in the determination and refinement of heavy-atom positions in protein heavy-atom derivatives and in the calculation of phase angles. With the advent of tunable synchrotron radiation, methods based only on anomalous scattering have become prominent. This chapter describes the development in the area up to the mid-1980s. The subsequent developments in isomorphous replacement and anomalous scattering have been almost exclusively concerned with macromolecular crystallography. They are discussed in International Tables for Crystallography Volume F
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87 citations

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TL;DR: In this article, the authors used the Poincare sphere to evolve techniques which could be used for determining the true Faraday rotation in the presence of birefringence.

Abstract: It is pointed out that the concept of the Poincare sphere appreciably simplifies the mathematical treatment of phenomena accompanying the passage of polarized light through a medium which exhibits birefringence, optical activity or both simultaneously. This is exemplified by using the Poincare sphere to evolve techniques which could be used for determining the true Faraday rotation in the presence of birefringence. When birefringence is present, measurements made with the half-shade at the polarizer and analyzer ends are not equivalent. In either arrangement, the errors introduced as a result of birefringence are largely reduced by taking the mean of two measurements for opposite directions of the field. Formulae are also derived by which the magnitudes of the error can be calculated for the particular experimental set up, knowing the value of the birefringence. In certain cases, even this need not be known, and the true rotation can be determined purely from measurements of the apparent rotations for two different azimuths of the incident plane of polarization.

61 citations

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TL;DR: In this paper, the repulsion between ions is postulated to be due to the increase in the internal energy of the ions arising from the distortion and the compression at the points of contact with their neighbours.

51 citations

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TL;DR: In this article, the authors extended the compressible ion approach to other cubic ionic crystals and derived the lattice spacings and compressibilities of a number of perovskite-like crystals of the form A + B 2+ C 3 −.

42 citations

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TL;DR: A set of simple and physically motivated criteria for secondary structure, programmed as a pattern‐recognition process of hydrogen‐bonded and geometrical features extracted from x‐ray coordinates is developed.

Abstract: For a successful analysis of the relation between amino acid sequence and protein structure, an unambiguous and physically meaningful definition of secondary structure is essential. We have developed a set of simple and physically motivated criteria for secondary structure, programmed as a pattern-recognition process of hydrogen-bonded and geometrical features extracted from x-ray coordinates. Cooperative secondary structure is recognized as repeats of the elementary hydrogen-bonding patterns “turn” and “bridge.” Repeating turns are “helices,” repeating bridges are “ladders,” connected ladders are “sheets.” Geometric structure is defined in terms of the concepts torsion and curvature of differential geometry. Local chain “chirality” is the torsional handedness of four consecutive Cα positions and is positive for right-handed helices and negative for ideal twisted β-sheets. Curved pieces are defined as “bends.” Solvent “exposure” is given as the number of water molecules in possible contact with a residue. The end result is a compilation of the primary structure, including SS bonds, secondary structure, and solvent exposure of 62 different globular proteins. The presentation is in linear form: strip graphs for an overall view and strip tables for the details of each of 10.925 residues. The dictionary is also available in computer-readable form for protein structure prediction work.

14,077 citations

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TL;DR: Automatic pattern recognition (model building) combined with refinement, allows a structural model to be obtained reliably within a few CPU hours and is demonstrated with examples of a few recently solved structures.

Abstract: In protein crystallography, much time and effort are often required to trace an initial model from an interpretable electron density map and to refine it until it best agrees with the crystallographic data. Here, we present a method to build and refine a protein model automatically and without user intervention, starting from diffraction data extending to resolution higher than 2.3 A and reasonable estimates of crystallographic phases. The method is based on an iterative procedure that describes the electron density map as a set of unconnected atoms and then searches for protein-like patterns. Automatic pattern recognition (model building) combined with refinement, allows a structural model to be obtained reliably within a few CPU hours. We demonstrate the power of the method with examples of a few recently solved structures.

2,463 citations

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24 Sep 2002Abstract: CRYSTALLINE MATERIALS Introduction Physical Properties Optical Properties Mechanical Properties Thermal Properties Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties GLASSES Introduction Commercial Optical Glasses Specialty Optical Glasses Fused Silica Fluoride Glasses Chalcogenide Glasses Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties Special Glasses POLYMERIC MATERIALS Optical Plastics Index of Refraction Nonlinear Optical Properties Thermal Properties Engineering Data METALS Physical Properties of Selected Metals Optical Properties Mechanical Properties Thermal Properties Mirror Substrate Materials LIQUIDS Introduction Water Physical Properties of Selected Liquids Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Commercial Optical Liquids GASES Introduction Physical Properties of Selected Gases Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Atomic Resonance Filters APPENDICES Safe Handling of Optical Materials Abbreviations, Acronyms, and Mineralogical or Common Names for Optical Materials Abbreviations for Methods of Preparing Optical Materials and Thin Films Fundamental Physical Constants Units and Conversion Factors

1,262 citations

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01 Nov 1956

TL;DR: In this paper, the superposition of two coherent beams in different states of elliptic polarisation is discussed in a general manner, and the extent of mutual interference varies from a maximum for identically polarised beams (c = 0) to zero for oppositely polarised ones (C = π ).

Abstract: The superposition of two coherent beams in different states of elliptic polarisation is discussed in a general manner. If A and B represent the states of polarisation of the given beams on the Poincare sphere, and C that of the resultant beam, the result is simply expressed in terms of the sides,a, b, c of the spherical triangle ABC. The intensity I of the resultant beam is given by: I=I1 + I2 +2√I1+I2cos½Ccosδ; the extent of mutual interference thus varies from a maximum for identically polarised beams (c = 0), to zero for oppositely polarised beams (c = π ). The state of polarisation C of the resultant beam is located by sin2 ½a = (I1/I) sin2 ½c and sin2 ½b = (I2/I) sin2 ½c. The 'phase difference' δ is equal to the supplement of half the area of the triangle C'BA (where C' is the point diametrically opposite to C). These results also apply to the converse problem of the decomposition of a polarised beam into two others. The interference of two coherent beams after resolution into the same state of elliptic polarisation by an elliptic analyser or compensator is discussed; as also the interference (direct,and after resolution by an analyser) of n coherent pencils in different states of polarisation.

1,142 citations

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TL;DR: The problem of determining the three-dimensional structure of thousands of atoms is reduced to that of initially solving for a few anomalous scattering centers that can be used as a reference for developing the entire structure.

Abstract: Resonance between beams of x-ray waves and electronic transitions from bound atomic orbitals leads to a phenomenon known as anomalous scattering. This effect can be exploited in x-ray crystallographic studies on biological macromolecules by making diffraction measurements at selected wavelengths associated with a particular resonant transition. In this manner the problem of determining the three-dimensional structure of thousands of atoms is reduced to that of initially solving for a few anomalous scattering centers that can then be used as a reference for developing the entire structure. This method of multiwavelength anomalous diffraction has now been applied in a number of structure determinations. Optimal experiments require appropriate synchrotron instrumentation, careful experimental design, and sophisticated analytical procedures. There are rich opportunities for future applications.

1,113 citations