Chemical approaches to penicillin allergy—IV. Binding of carrier receptor protein with penicillin and its analogues

The availability of electrophoretically homogeneous rabbit penicillin carrier receptor protein (CRP) by affinity chromatography afforded an idealin vitro system to calculate the thermodynamic parameters of binding of penicillin and analogues with CRP as well as competitive binding of such analogues with CRP in presence of14C-penicillin G. The kinetics of association of CRP with 7-deoxy penicillin which does not bind covalently with CRP have been studied through equilibrium dialysis with14C-7-deoxybenzyl penicillin and found to be K=2·79×106M−1.−ΔG=8·106 k cal/mole as well as fluorescence quenching studies with exciter λ 280 K=3·573×106M−1,−ΔG=8·239 k cal/mole. The fluorescence quenching studies have been extended to CRP-benzyl penicillin and CRP-6-aminopenicillanic acid (6APA) systems also. The fluorescence data with benzyl penicillin indicate two conformational changes in CRP—a fast change corresponding to the non-covalent binding to CRP with 7-deoxy penicillin and a slower change due to covalent bond formation. With 6-APA the first change is not observed but the conformational change corresponding to covalent binding is only seen. Competitive binding studies indicate that the order of binding of CRP with the analogues of penicillin is as follows: methicillin > 6APA > carbenicillin >o-nitrobenzyl penicillin > cloxacillin ≈ benzyl penicillin ≈ 6-phenyl acetamido penicillanyl alcohol ≈ 7 phenyl acetamido desacetoxy cephalosporanic acid ≈p-amino benzyl penicillin ≈p-nitro benzyl penicillin > ticarcillin >o-amino benzyl penicillin > amoxycillin > 7-deoxy benzyl penicillin > ampicillin.From these data it has been possible to delineate partially the topology of the penicillin binding cleft of the CRP as well as some of the functional groups in the cleft responsible for the binding process.

Photochemical, photophysical, and theoretical studies on cyclobutanethiones..alpha.-Cleavage reactions

Photophysics and photochemistry of cyclobutanethiones 1-5 have been studied with the view to generalize the a-cleavage reactions of cyclobutanethiones. The above cyclobutanethiones possess a unit intersystem crossing efficiency from S1 to T1, a high self-quenching rate (-4 X lo9 M-' s-'), and a short triplet lifetime (<0.50 ws). Photolysis of 1-5 yields in benzene a product resulting from 1,3-transposition and in methanol two cyclic thioacetals.The origin of these products is traced to the triplet excited state. A mechanistic scheme involving a-cleavage as the primary photoprocess and diradicals and thiacarbenes as intermediates has been formulated to rationalize the formation of thioacetals and rearranged products. The proposed mechanistic scheme is supported by UHF MIND013 calculations performed on four model systems, cyclobutanethiones and 1,3-cyclobutanedithiones 18-21. These calculations indicate that formation of diradical is favored thermodynamically and kinetically for systems analogous to 19 and 21, while rearrangement to thiacarbene is likely only for those similar to 21.

Photochemistry of .alpha.,.beta.-unsaturated thiones. Cycloaddition to electron-deficient olefins from higher excited states

Electron-deficient olefins add to thioenone 1 upon m* excitation. Cycloaddition occurs to the thiocarbonyl chromophore preferentially from the less-hindered side to yield thietanes. Thietane formation is stereospecific and regioselective. This addition has been inferred to originate from the second excited singlet, S2(?rx*), state. The exciplex intermediacy has been inferred from the dependence of the fluorescence quenching rate constant on the electron-acceptor properties of the olefin. The observed site specificity and regioselectivity are rationalized on the basis of PMO theory. The observed photochemical behavior of thioenone is different from that of enones.

The physical properties of SnS films grown on lattice-matched and amorphous substrates

The development of high-quality tin monosulphide (SnS) layers is one of the crucial tasks in the fabrication of efficient SnS-based optoelectronic devices. Reduction of strain between film and the substrate by using an appropriate lattice-matched (LM) substrate is a new attempt for the growth of high-quality layers. In this view, the SnS films were deposited on LM Al substrate using the thermal evaporation technique with a low rate of evaporation. The as-grown SnS films were characterized using appropriate techniques and the obtained results are discussed by comparing them with the properties of SnS films grown on amorphous substrate under the same conditions. From structural analysis of the films, it is noticed that the SnS films deposited on amorphous substrate have crystallites that were oriented along different directions. However, the SnS crystallites grown on Al substrate exhibited epitaxial growth along the 101] direction. Photoluminescence (PL) and Raman studies reveal that the films grown on Al substrate have better optical properties than those of the films grown on amorphous substrates. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Droplet Epitaxy of InN Quantum Dots on Si(111) by RF Plasma-Assisted Molecular Beam Epitaxy

InN quantum dots (QDs) were fabricated on Si(111) substrate by droplet epitaxy using an RF plasma-assisted MBE system. Variation of the growth parameters, such as growth temperature and deposition time, allowed us to control the characteristic size and density of the QDs. As the growth temperature was increased from 100 C to 300 degrees C, an enlargement of QD size and a drop in dot density were observed, which was led by the limitation of surface diffusion of adatoms with the limited thermal energy. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the QDs size and density. The chemical bonding configurations of InN QDs were examined by X-ray photo-electron spectroscopy (XPS). Fourier transform infrared (FTIR) spectrum of the deposited InN QDs shows the presence of In-N bond. Temperature-dependent photoluminescence (PL) measurements showed that the emission peak energies of the InN QDs are sensitive to temperature and show a strong peak emission at 0.79 eV.

Doping Transition Metal (Mn or Cu) Ions in Semiconductor Nanocrystals

Following growth doping strategy and using dopant oxides nanocrystals as dopant sources, we report here two different transition-metal ions doped in a variety of group II-VI semiconductor nanocrystals. Using manganese oxide and copper oxide nanocrystals as corresponding dopant sources, intense photoluminescence emission over a wide range of wavelength has been observed for different host nanocrystals. Interestingly, this single doping strategy is successful in providing such highly emissive nanocrystals considered here, in contrast with the literature reports that would suggest synthesis strategies to be highly specific to the particular dopant, host, or both. We investigate and discuss the possible mechanism of the doping process, supporting the migration of dopant ions from dopant oxide nanocrystals to host nanocrystals as the most likely scenario.

Third Polymorph of Phenylacetylene

A third polymorph of phenylacetylene (Z' = 6), which is obtained by quenching the liquid, shows some similarities to the two earlier known forms, and the adoption of a high Z' value in this crystal form is discussed.

Self-assembled flower-like nanostructures of InN and GaN grown by plasma-assisted molecular beam epitaxy

Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be similar to 28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.

Two-stage photoquenching in semi-insulating GaAs:EL2

The photoquenching of EL2 in semi‐insulating gallium arsenide is seen to be a complex process, where at low temperatures the initial slow quenching is followed by a switch to fast quenching. A possible explanation involving lattice strain mediated cooperative structural relaxation arising out of transition to the metastable state is proposed.

Formation of lead pyrophosphate glass and the role of anion disproportionation

Thermal, spectroscopic and electrical properties of lead pyrophosphate glass prepared by melt quenching have been examined. A model based on the structural disproportionation of the P2O 7 4− ions has been proposed and is shown to consistently explain all the observations. The equilibrium of various anionic species has been discussed on the basis of their electronegativities which are in turn related to their basicities.

Studies on nucleotidases in plants: Fluorescence and kinetic properties of nucleotide pyrophosphatase from mung bean (Phaseolus aureus) seedlings

Nucleotide pyrophosphatase of mung bean seedlings has earlier been isolated in our laboratory in a dimeric form (Mr 65,000) and has been shown to be converted to a tetramer by AMP and to a monomer by p-hydroxymercuribenzoate. All the molecular forms were enzymatically active with different kinetic properties. By a modified procedure using blue-Sepharose affinity chromatography, we have now obtained a dimeric form of the enzyme which is desensitized to AMP interaction. The molecular weight of the desensitized form of the enzyme was found to be the same as that of the native dimeric enzyme. However, the desensitized enzyme functioned with a linear time course, contrary to the biphasic time course exhibited by the native enzyme. In addition, it was not converted to a tetramer on the addition of AMP, had only one binding site for adenine nucleotides, and p-hydroxy-mercuribenzoate had no effect on the time course of the reaction or on the molecular weight of the enzyme. The temperature optimum of the desensitized enzyme was found to be 67 °C in contrast to the optimum of 49 °C for the native dimer. Fifty percent of the tryptophan residues of the desensitized enzyme were not accessible for quenching by iodide. Fluorescence studies gave Kd values of 0.34, 2.2, and 0.8 mImage for AMP, ADP, and ATP, which were close to the Ki values of 0.12, 2.2, and 0.9 mImage , respectively, for these nucleotides. The binding and inhibition studies with AMP and its analogs showed that the 6-amino group and the 5′-phosphate group were essential for the inhibition of the enzyme activity.

Silver- and Gold-Related Deep Levels in Gallium Arsenide

Characterization of silver- and gold-related defects in gallium arsenide is carried out. These impurities were introduced during the thermal diffusion process and the related defects are characterized by deep-level transient spectroscopy and photoluminescence. The silver-related center in GaAs shows a 0.238 eV photoluminescence line corresponding to no-phonon transition, whereas its thermal ionization energy is found to be 0.426 eV. The thermal activation energy of the gold-related center in GaAs is 0.395 eV, but there is no corresponding luminescence signal.

Computational Methods for Studies of Quantum Dots and Boson Condensates

This thesis presents ab initio studies of two kinds of physical systems, quantum dots and bosons, using two program packages of which the bosonic one has mainly been developed by the author. The implemented models, \emph{i.e.}, configuration interaction (CI) and coupled cluster (CC) take the correlated motion of the particles into account, and provide a hierarchy of computational schemes, on top of which the exact solution, within the limit of the single-particle basis set, is obtained. The theory underlying the models is presented in some detail, in order to provide insight into the approximations made and the circumstances under which they hold. Some of the computational methods are also highlighted. In the final sections the results are summarized. The CI and CC calculations on multiexciton complexes in self-assembled semiconductor quantum dots are presented and compared, along with radiative and non-radiative transition rates. Full CI calculations on quantum rings and double quantum rings are also presented. In the latter case, experimental and theoretical results from the literature are re-examined and an alternative explanation for the reported photoluminescence spectra is found. The boson program is first applied on a fictitious model system consisting of bosonic electrons in a central Coulomb field for which CI at the singles and doubles level is found to account for almost all of the correlation energy. Finally, the boson program is employed to study Bose-Einstein condensates confined in different anisotropic trap potentials. The effects of the anisotropy on the relative correlation energy is examined, as well as the effect of varying the interaction potential.}

Improved growth of GaN layers on ultra thin silicon nitride/Si (111) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth. (C) 2010 Elsevier Ltd. All rights reserved.

Dielectric and Impedance Studies on 0.5Li(2)O-0.5K(2)O-2B(2)O(3) Glasses

Transparent glasses in the system 0.5Li(2)O-0.5K(2)O-2B(2)O(3) (LKBO) were fabricated via the conventional melt quenching technique. Amorphous and glassy nature of the samples was confirmed by X-ray diffraction and differential scanning calorimetry (DSC) respectively. Complex dielectric and impedance studies were conducted on the samples at different temperatures in the 100 Hz-10 MHz frequency range. ac conductivity was calculated from the dielectric data and the conductivity relaxation was found to obey the Jonscher's law. The Nyquist's plots (Z `'(omega) vs. Z'(omega)) showed single suppressed semicircles at all the temperatures under study indicating the non ideal Debye type relaxation process to be active. Activation energies for conduction and relaxation process were calculated using the Arrhenius relation. The UV-visible optical transmission spectra was shown a wide transmission window and calculated optical band gap was found to be 5.67 eV.