A comprehensive study on sedimentation rate and sediment age of Satopanth Tal Garhwal Himalaya,using 210Pb and 137Cs techniques

Journal of Radioanalytical and Nuclear Chemistry - Radionuclides dating have been applied in the chronological series of sediment core recovered from Satopanth Tal to estimate the sedimentation...     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

NMR structure determination and calcium binding effects of lipopeptide antibiotic daptomycin

Daptomycin is an acidic lipopeptide antibiotic, whose three-dimensional structure and mechanism of action is currently unknown. Recently daptomycin, trade name Cubicin, was approved as a drug for the treatment of skin-related infections (M. Larkin Lancet, 2003, 3, 677) and became the first antibiotic of its class to be used in the clinic (A. Raja et al., Nature Rev. Drug Discov., 2003, 2, 943-944). We have carried out a systematic high field NMR study of daptomycin and its binding to calcium ions which is essential for antibiotic activity. In this first report, we demonstrate the sequence-specific resonance assignment of daptomycin under resolved NMR measurement conditions. In addition to this, we have determined the 3D structure of apo-daptomycin and demonstrated a 1 : 1 stoichiometry on the binding to calcium ions. We have also demonstrated that the binding of calcium ions does not result in major conformational changes, but does induce aggregation. This may be an important factor in the mode of action of daptomycin.     

Activity of carboxypeptidase a bound to a modified cellulosic matrix

Carboxypeptidase A immobilized on acid chloride of oxidized cellulose showed the following features: (a) as indicated by the linearity of reaction kinetics, the immobilized enzyme action is not diffusion controlled; (b) greater flow rates are achievable with less clogging during continual usage since the enzyme is attached to a porous screen; (c) ease of handling; and (d) no apparent electrostatic interaction with the support material that is uncharged. The immobilized enzyme retained 60% of the original activity. The half-life of free enzyme was only 20 min, whereas for immobilized enzyme it was enhanced up to 2 h 48 min. It could be recovered and repeatedly used.     

Standardization protocols and optimized precursor sets for the efficient application of automated parallel synthesis to lead optimization: a Mitsunobu example

A strategy has been developed for the efficient application of automated parallel synthesis to specific aspects of the lead optimization processes employed in drug discovery. The method involves the synthesis of collections of compounds using sets of precursors designed to encompass established medicinal chemistry principles and that have been concurrently optimized with respect to a specific chemical transformation. The strategy is illustrated using an automated Mitsunobu protocol employing sets of aliphatic alcohols and phenols as precursors. The former has been formatted to perform simple alkyl homologation exercises, with the latter being designed for use in diversity-based studies.     

Macroreticular Phenolic Cationites

Macroreticular cation exchangers can be prepared by polymerizing sulfonated phenols with formaldehyde under acidic conditions. The resins are highly porous with large surface areas. Their properties compare well with those based on styrene and divinylbenzene copolymers.     

Hydrocarbon chain conformation in an intercalated surfactant monolayer and bilayer

Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS₃ at two different grafting densities. Low grafting densities are obtained on direct intercalation of CTA ions into CdPS₃ to give Cd_0.93PS₃(CTA)_0.14. Intercalation occurs with a lattice expansion of 4.8 ? with the interlamellar surfactant ion lying flat forming a monolayer. Intercalation at higher grafting densities was effected by a two-step ion-exchange process to give Cd_0.83PS₃(CTA)_0.34, with a lattice expansion of 26.5 ?. At higher grafting densities the interlamellar surfactant ions adopt a tilted bilayer structure.¹³C NMR and orientation-dependent IR vibrational spectroscopy on single crystals have been used to probe the conformation and orientation of the methylene ‘tail’ of the intercalated surfactant in the two phases. In the monolayer phase, the confined methylene chain adopts an essentially all-trans conformation with most of the trans chain aligned parallel to the gallery walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and aligned to the confining surface. In the bilayer phase, most bonds in the methylene chain are in trans conformation. It is possible to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene. These high energy conformers disappear on cooling leaving all fifteen methylene units of the intercalated cetyl trimethyl ammonium ion in trans conformational registry at 40 K.