Showing papers by "University of Hyderabad published in 1995"

Supramolecular Synthons in Crystal Engineering—A New Organic Synthesis

Abstract: A crystal of an organic compound is the ultimate supermolecule, and its assembly, governed by chemical and geometrical factors, from individual molecules is the perfect example of solid-state molecular recognition. Implicit in the supramolecular description of a crystal structure is the fact that molecules in a crystal are held together by noncovalent interactions. The need for rational approaches towards solid-state structures of fundamental and practical importance has led to the emergence of crystal engineering, which seeks to understand intermolecular interactions and recognition phenomena in the context of crystal packing. The aim of crystal engineering is to establish reliable connections between molecular and supramolecular structure on the basis of intermolecular interactions. Ideally one would like to identify substructural units in a target supermolecule that can be assembled from logically chosen precursor molecules. Indeed, crystal engineering is a new organic synthesis, and the aim of this article is to show that rather than being only nominally relevant to organic chemistry, this subject is well within the mainstream, being surprisingly similar to traditional organic synthesis in concept. The details vary because one is dealing here with intermolecular interactions rather than with covalent bonds; so this article is divided into two parts. The first is concerned with strategy, highlighting the conceptual relationship between crystal engineering and organic synthesis and introduces the term supramolecular synthon. The second part emphasizes methodology, that is, the chemical and geometrical properties of specific intermolecular interactions.

Supramolekulare Synthone für das Kristall‐Engineering ‐ eine neue organische Synthese

Abstract: Der Kristall einer organischen Verbindung ist die hochste Form eines Supramolekuls, und sein von chemischen und geometrischen Faktoren bestimmter Zusammenbau aus Molekulen ist das perfekte Beispiel fur molekulare Erkennung im festen Zustand. Die supramolekulare Beschreibung einer Kristallstruktur beinhaltet, das die Molekule im Kristall durch nichtkovalente Wechselwirkungen zusammengehalten werden. Die Notwendigkeit fur einen rationalen Zugang zu Festkorpern von grundlegender und praktischer Bedeutung hat zum Entstehen des Kristall-Engineering gefuhrt, bei dem man versucht, die intermolekularen Wechselwirkungen und Erkennungsphanomene im Zusammenhang mit der Kristallpackung zu verstehen. Das Ziel des Kristall-Engineering ist es, auf der Grundlage der intermolekularen Wechselwirkungen zuverlassige Zusammenhange zwischen der molekularen und der supramolekularen Struktur herzustellen, und idealerweise mochte man Substrukturen in einem Zielsupramolekul identifizieren, das aus logisch gewahlten Vorlaufermolekulen zusammengesetzt werden kann. In der Tat ist Kristall-Engineering eine neue Form der organischen Synthese, und es soll in diesem Beitrag gezeigt werden, das Kristall-Engineering nicht nur von nomineller Bedeutung fur die Organische Chemie ist, sondern im Kerngebiet der Organischen Chemie liegt und konzeptionell der traditionellen organischen Synthese uberraschend ahnlich ist. Im Detail unterscheiden sich Kristall-Engineering und organischen Synthese allerdings: Beim Kristall-Engineering spielen intermolekulare Wechselwirkungen und weniger kovalente Bindungen eine Rolle. Im ersten Teil dieses Beitrags wird die Strategie behandelt: Die verwandten Konzepte des Kristall-Engineering und der organischen Synthese werden hervorgehoben und der Begriff supramolekulares Synthon eingefuhrt. Im zweiten Teil steht die Methode im Vordergrund, d. h. die chemischen und geometrischen Eigenschaften der spezifischen intermolekularen Wechselwirkungen.

Using interference in the frequency domain for precise determination of thickness and refractive indices of normal dispersive materials

Abstract: We present an experimental demonstration of the spectral interference phenomenon, using a spectrally broad white-light source in a Michelson interferometer configuration. We also present a theoretical analysis of the spectral interference law for the general case of amplitude-division interferometers. When a dispersive medium is introduced into one of the arms of the interferometer, the spectral interference pattern changes drastically, with a change in the frequency modulation corresponding to the dispersive nature of the medium. A zero-order fringe appears at a wavelength where the net path difference between the two arms of the interferometer is zero. We relate the thickness of the dispersive medium to the width of the zero-order fringe. From the experimental data over the entire visible region of the spectrum we obtain the refractive index n(λ) and the thickness t of the dispersive medium, calculated to an accuracy of the order of 10−5.

Anthocyanin pathway in rice (Oryza sativa L): identification of a mutant showing dominant inhibition of anthocyanins in leaf and accumulation of proanthocyanidins in pericarp.

Abstract: The present study has surveyed a collection of indica rice (Oryza sativa) lines for tissue-specific anthocyanin pigmentation pattern, which has also been used for a genetically meaningful classification. This classification helped predict probable genotypes of rice lines and, in the process, a leaf blade-specific dominant inhibitor of pigmentation (Ilb) was predicted and its presence later confirmed in two lines. We ascribe most tissue-specific accumulation of anthocyanins to the presence of a different set of Pl alleles. Cyanidin, as a major pigment, and peonidin, as a minor pigment, were detected in purple-pigmented tissues. Further, the floral organ-derived tissues always contained a higher level of anthocyanins and, correspondingly, a relatively increased proportion of peonidin. One line, N22B, with a brown pericarp was identified and shown to accumulate proanthocyanidins, but with no anthocyanins, in the pericarp. We propose that the accumulation of proanthocyanidins is due to a block in the anthocyanin biosynthetic pathway in rice at the anthocyanidin synthase-mediated conversion of leucoanthocyanidin to anthocyanidin.

Enhanced plant regeneration in grain and sweet sorghum by asparagine, proline and cefotaxime.

Abstract: Cefotaxime ( 50 and 100 mg/1 ), a cephalosporin antibiotic and the amino acids asparagine and proline (200 mg/l) enhanced the production of embryogenic callus, increased the frequency of plant regeneration, and delayed the loss of regeneration potential in immature embryo-derived callus cultures ofSorghum bicolor (L.) Moench. Although these compounds did not promote callus induction or growth of callus, they influenced plant regeneration considerably in 10 low responding genotypes of grain and high anthocyanin containing sweet sorghums.

Recognition of telugu characters using neural networks

Abstract: The aim of the present work is to recognize printed and handwritten Telugu characters using artificial neural networks (ANNs). Earlier work on recognition of Telugu characters has been done using conventional pattern recognition techniques. We make an initial attempt here of using neural networks for recognition with the aim of improving upon earlier methods which do not perform effectively in the presence of noise and distortion in the characters. The Hopfield model of neural network working as an associative memory is chosen for recognition purposes initially. Due to limitation in the capacity of the Hopfield neural network, we propose a new scheme named here as the Multiple Neural Network Associative Memory (MNNAM). The limitation in storage capacity has been overcome by combining multiple neural networks which work in parallel. It is also demonstrated that the Hopfield network is suitable for recognizing noisy printed characters as well as handwritten characters written by different "hands" in a variety of styles. Detailed experiments have been carried out using several learning strategies and results are reported. It is shown here that satisfactory recognition is possible using the proposed strategy. A detailed preprocessing scheme of the Telugu characters from digitized documents is also described.

Tb3+ fluorescence as a probe of cluster formation in lead oxyfluoride glasses

Abstract: Tb3+ fluorescence was recently shown to be a good probe of local Tb3+. The matrices in which clustering does not occur show a standard behavior. Emission spectral studies on Tb3+-doped 30Pb0-70PbF2 suggest that in favourable cases differences in the strength of interaction between Tb3+ ions can show up by systematic deviations from the standard behavior and throw light on the nature of the sites occupied.

A simple convenient method for the resolution of racemic 2,2′-dihydroxy-1,1′-binaphthyl using (S)-proline

Abstract: The racemic mixture of 2,2′-dihydroxy-1,1′-binaphthyl has been resolved to obtain the R(+) and S(−) enantiomers in essentially pure forms by refluxing with (S)-proline (1eq.) in benzene in three successive operations.

Anisotropic three-dimensional XY model and vortex-loop scaling.

Abstract: Scaling equations are obtained for thermally excited vortex loops in the anisotropic three-dimensional (3D) XY model with interplane/intraplane coupling ratio ${\mathit{K}}_{\mathrm{\ensuremath{\perp}}}$/${\mathit{K}}_{\mathrm{\ensuremath{\parallel}}}$=${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}2}$. For high-${\mathit{T}}_{\mathit{c}}$ superconductors, ${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}2}$, related to the Ginzburg-Landau masses, is in a strong anisotropy regime, ${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}1}$0.5, and a length scale ${\mathit{r}}_{0}$=${\ensuremath{\gamma}}_{0}$${\mathit{a}}_{0}$ naturally arises (${\mathit{a}}_{0}$= lattice constant). (i) For loop major axes ag${\mathit{r}}_{0}$, the dominant excitations are 3D elliptical vortex loops cutting multiple planes, with a\ensuremath{\sim}${\ensuremath{\xi}}_{\mathrm{\ensuremath{-}}}$(T)=${\mathit{a}}_{0}$\ensuremath{\Vert}\ensuremath{\varepsilon}${\mathrm{\ensuremath{\Vert}}}^{\mathrm{\ensuremath{-}}\ensuremath{ u}}$ where \ensuremath{\Vert}\ensuremath{\varepsilon}\ensuremath{\Vert}\ensuremath{\equiv}\ensuremath{\Vert}(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$\ensuremath{\Vert}, and \ensuremath{ u}=0.67. The vorticity segment components (\ensuremath{\mu}=x^,y^,z^) are ${\mathit{J}}_{\mathrm{\ensuremath{\mu}}}$(r)=0,\ifmmode\pm\else\textpm\fi{}1, and interact via a Biot-Savart-like law. The renormalized anisotropy ${\ensuremath{\gamma}}_{\mathit{l}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\rightarrow}1, asymptotically isotropic, as l=ln(a/${\mathit{a}}_{0}$)\ensuremath{\rightarrow}\ensuremath{\infty}: anisotropy is irrelevant. (ii) For loop scales ${\mathit{r}}_{0}$gag2${\mathit{a}}_{0}$ the dominant excitations are quasi-2D rectangular loops with short sides ${\mathit{J}}_{\mathit{z}}$(r)=\ifmmode\pm\else\textpm\fi{}1 cutting single planes, that are thus effectively decoupled at finite scales ${\mathit{r}}_{0}$. The ${\mathit{J}}_{\mathit{z}}$=\ifmmode\pm\else\textpm\fi{}1 vortex components, of in-plane separation \ensuremath{\Vert}R${\ensuremath{\rightarrow}}_{\mathrm{W}}$mN\ensuremath{\Vert}${\mathit{r}}_{0}$, interact via a logarithmic [ln(\ensuremath{\Vert}R${\ensuremath{\rightarrow}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\Vert}/${\mathit{r}}_{0}$)] plus linear [${\mathit{Q}}_{0}$(\ensuremath{\Vert}R${\ensuremath{\rightarrow}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\Vert}/${\mathit{a}}_{0}$-1)] potential. As ${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\rightarrow}0, the coefficient ${\mathit{Q}}_{0}$\ensuremath{\rightarrow}0, and the Kosterlitz-Thouless limit is recovered. (iii)The 3D transition temperature ${\mathit{T}}_{\mathit{c}}$ versus ${\mathit{K}}_{\mathrm{\ensuremath{\perp}}}$/${\mathit{K}}_{\mathrm{\ensuremath{\parallel}}}$, calculated for strong anisotropies from ``2D'' (3D) scaling for scales ${\mathit{r}}_{0}$ (g${\mathit{r}}_{0}$), matches existing Monte Carlo data well. Critical regions \ensuremath{\Vert}${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$\ensuremath{\Vert}\ensuremath{\le}${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}1/\ensuremath{ u}}$ are estimated. Contact is made with ideas of an intrinsic critical current arising from linear \ensuremath{\sim}${\ensuremath{\gamma}}_{0}^{\mathrm{\ensuremath{-}}1}$${\mathit{R}}_{\mathrm{\ensuremath{\parallel}}}$ vortex segment effective potentials.

Fluorescence studies on a supramolecular porphyrin bearing anthracene donor moieties

Abstract: A tetraphenylporhyrin bearing four anthracene donor moieties (5,10,15,20-tetra(3-(9-methyloxyanthracenyl)phenyl)porphyrin, I) was synthesized and fully characterized. Spectroscopic and electrochemical data and results of metallation and intermolecular fluorescence quenching experiments suggest that there is no appreciable interaction between the porphyrin and anthracene moieties in I. Although intermolecular quenching of anthracene fluorescence by 5,10,15,20-(tetraphenyl)porphyrin is not obvious, the fluorescence of the anthracene donors in I is quenched by more than 90%. On the basis of the fluorescence excitation pectral data, the quenching observed is attributed to intramolecular singlet-singlet energy transfer from anthracene to the porphyrin. However, neither Forster's dipole-dipole formalism nor Dexter's electron exchange mechanism can explain adequately this intramolecular energy transfer. Thermodynamic considerations and solvent-dependent fluorescence data indicate that photoinduced electron transfer from singlet anthracene to the porphyrin can compete with energy transfer in this supramolecular system.

Thermodynamic analysis of biotin binding to avidin. a high sensitivity titration calorimetric study

Abstract: Titration calorimetric studies on the binding of biotin to avidin were performed in phosphate buffered saline, pH 7.4. From the temperature dependence of the binding enthalpy (delta H), the delta Cp value was determined. While the delta H value of -23.4 kcal/mol at 25 degrees C is in close agreement with the previously determined value of -22.5 kcal/mol (Suurkusk, J. & Wadso, I. (1972) Eur. J. Biochem. 28, 438-441), the delta Cp value of -461 cal/mol biotin/K is significantly at variance with the value of -237 cal/mol biotin/K obtained in the previous study. A comparison of the thermodynamic data obtained for the avidin-biotin system with that of the streptavidin-biotin system revealed that the higher binding affinity of avidin for biotin is due to a smaller (negative) entropy of binding.

Microstructural and magnetisation study in melt-grown YBaCuO samples

Abstract: The effect of Y 2 BaCuO 5 (211) and silver on the microstructure of melt-grown YBa 2 Cu 3 O 7− x (123) has been systematically investigated. An optimal amount (28 mol%) of 211 in the starting composition of 123 with 5 wt.% of silver gave critical current densities >10 4 A/cm 2 . Magnetisation measurements using a SQUID magnetometer have been utilised to characterise the superconducting properties of the material at various stages of optimisation. Bean's and Kim's critical-state models are used to analyse the magnetisation of the optimised composition (123 + 28 mol% + 5 wt.% Ag). An anticlockwise tilt observed in the magnetisation curve above 70 K in the optimised sample has been explained as being due to the presence of an additional paramagnetic contribution arising from 211 particles present in the 123 matrix.

Coherent population trapping at low light levels.

Abstract: A three-level lambda ($\ensuremath{\Lambda}$) system driven by two coherent fields at the Raman resonance condition evolves to a nonabsorbing eigenstate of the Hamiltonian called the coherent population trapping (CPT) state. We study the various factors dictating the dynamical evolution of the coherences and the populations to the CPT state. We demonstrate the formation of the CPT state even at low intensities, though it takes much longer to form such a state. For the case of unequal decay rates we demonstrate an interesting sharp dip in the steady-state response of the medium at the Raman resonance condition.

Predominant localization of mitochondria enriched with glycine-decarboxylating enzymes in bundle sheath cells of Alternanthera tenella, a C3–C4 intermediate species

Abstract: Mesophyll protoplasts and bundle sheath cells were prepared by enzymatic digestion of leaves of Alternanthera tenella, a C 3 -C 4 intermediate species. The intercellular distribution of selected photosynthetic, photorespiratory and respiratory (mitochondrial) enzymes in these meso-phyll and bundle sheath cells was studied. The activity levels of photosynthetic enzymes such as PEP carboxylase (EC 4.1.1.31) or NAD-malic enzyme (EC 1.1.1.39) and photorespiratory enzymes such as glycolate oxidase (EC 1.1.3.1) or NADH-hydroxypyruvate reductase (EC 1.1.1.29) were similar in the two cell types. The activity levels of mitochondrial TCA cycle enzymes such as citrate synthase (EC 4.1.3.7) or fumarase (EC 4.2.1.2) were 2- to 3-fold higher in bundle sheath cells. On the other hand, the activity levels of mitochondrial photorespiratory enzymes, namely glycine decarboxylase (EC 2.1.2.10) and serine hydroxymethyltransferase (EC 2.1.2.1), were 6-9-fold higher in bundle sheath cells than in mesophyll protoplasts. Such preferential localization of mitochondria enriched with the glycine-decarboxylating system in the inner bundle sheath cells would result in efficient refixa-tion of CO 2 from not only photorespiration but also dark respiration before its exit from the leaf. We propose that predominant localization of mitochondria specialized in glycine decarboxylation in bundle sheath cells may form the basis of reduced photorespiration in this C 3 -C 4 intermediate species.

Bessel-beam representation for partially coherent fields

Abstract: The Bessel-beam representation for partially coherent fields is introduced as an alternative to the familiar angular spectrum representation. This alternative representation is applied to far fields generated by partially coherent secondary sources, nondiffracting partially coherent fields, and statistically homogeneous fields. A scheme for generating nondiffracting partially coherent fields is also discussed.

Electromagnetic-field-induced transparency in high-density exciton systems.

Abstract: In a system of high-density excitons modeled as a system of anharmonic oscillators we demonstrate the (a) existence of the field-induced transparency, (b) changes in the dispersion characteristics, (c) the possibility of probe gain due to the presence of the pump laser, and (d) the possibility of gain enhancement in regions close to the bistability critical points.

Purification and partial characterization of a lectin from Euphorbia neriifolia latex.

Abstract: A lectin was purified from Euphorbia neriifolia latex to homogeneity by affinity chromatography on Sepharose 4B. The protein appears to be a dimer with approximate M(r) of 60,000 on gel filtration and showing a single band at M(r) 32,000 in SDS-PAGE, and contains 12.3% carbohydrate. It agglutinated trypsinized human and rabbit erythrocytes, but not sheep erythrocytes. However, sialidase-treated sheep erythrocytes were agglutinated. The galactose and galactose containing sugars inhibited the heamagglutination with increased beta-anomeric specificity. The Euphorbia lectin possesses mitogenic activity with murine spleen lymphocytes but it does not inhibit protein synthesis in rabbit reticulocyte lysate.

Excess noise acquired by a laser beam after propagating through an atomic-potassium vapor.

Abstract: We have found that an intense shot-noise-limited laser beam tuned near the 4 {sup 2}{ital S}{sub 1/2}{leftrightarrow}4 {sup 2}{ital P}{sub 3/2} potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell The noise is maximum for laser detunings of approximately {plus_minus}1 GHz and falls to nearly the shot-noise limit for detunings greater than {plus_minus}3 GHz We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam The predictions of this theory are in good agreement with the experimental data