Crystal and molecular structure of synthetic cholesterol compounds vitamin D3-analogues (I) 24(S)-hydroxy coprastan-3-one & (II) 24(R)-hydroxy coprastan-3-one

The title compound I (24-(S)-Hydroxy Coprastan-3-one) crystallises in orthorhombic space group P2(1)2(1)2(1) with Z = 4. The unit cell dimensions are a = 6.701(2)Angstrom, b = 11.506(8)Angstrom, c = 32.183(4)Angstrom, V = 2481(2)Angstrom (3), D-cal = 1.077 Mg/m(3). The tide compound II (24-(R)-Hydroxy Coprastan-3-one) crystallises in orthorhombic space group P212121 with two molecules per assymetric unit and with Z = 8. The Unit cell dimensions are a = 10.954(2)Angstrom, b = 21.757(6)Angstrom, c = 21.130(7)Angstrom, V = 5035.0(2)Angstrom (3), D-cal = 1.062 Mg/m(3). In compound I and in both the molecules of compound II, the rings A, B & C are in chair conformation and the five membered ring D is in envelope conformation. The priority sequence attached to the chiral carbon C24 has "S" designation in compound I and "R" designation in compound II. The structures are stabilized by C-H . . .O and O-H---O hydrogen bonds.

Crystal structure of Rv2118c: an AdoMet-dependent methyltransferase from Mycobacterium tuberculosis H37Rv

Rv2118c belongs to the class of conserved hypothetical proteins from Mycobacterium tuberculosis H37Rv. The crystal structure of Rv2118c in complex with S-adenosyl-Image -methionine (AdoMet) has been determined at 1.98 Å resolution. The crystallographic asymmetric unit consists of a monomer, but symmetry-related subunits interact extensively, leading to a tetrameric structure. The structure of the monomer can be divided functionally into two domains: the larger catalytic C-terminal domain that binds the cofactor AdoMet and is involved in the transfer of methyl group from AdoMet to the substrate and a smaller N-terminal domain. The structure of the catalytic domain is very similar to that of other AdoMet-dependent methyltransferases. The N-terminal domain is primarily a β-structure with a fold not found in other methyltransferases of known structure. Database searches reveal a conserved family of Rv2118c-like proteins from various organisms. Multiple sequence alignments show several regions of high sequence similarity (motifs) in this family of proteins. Structure analysis and homology to yeast Gcd14p suggest that Rv2118c could be an RNA methyltransferase, but further studies are required to establish its functional role conclusively.

Electronic structure of ${\rm Ca}_{1-x}{\rm Sr}_x{\rm VO}_3$: A tale of two energy scales

We investigate the electronic structure of Ca1-xSrxVO3 using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to a distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy scales, namely the low-energy scale of thermal excitations ( $\sim\!k_{\rm B}T$) and the high-energy scale related to Coulomb and other electronic interactions.

Vertical thermal structure of the atmosphere during active and weak phases of convection over the north Bay of Bengal: Observation and model results

Owing to the lack of atmospheric vertical profile data with sufficient accuracy and vertical resolution, the response of the deep atmosphere to passage of monsoon systems over the Bay of Bengal. had not been satisfactorily elucidated. Under the Indian Climate Research Programme, a special observational programme called 'Bay of Bengal Monsoon Experiment' (BOBMEX), was conducted during July-August 1999. The present study is based on the high-resolution radiosondes launched during BOBMEX in the north Bay. Clear changes in the vertical thermal structure of the atmosphere between active and weak phases of convection have been observed. The atmosphere cooled below 6 km height and became warmer between 6 and 13 km height. The warmest layer was located between 8 and 10 km height, and the coldest layer was found just below 5 km height. The largest fluctuations in the humidity field occurred in the mid-troposphere. The observed changes between active and weak phases of convection are compared with the results from an atmospheric general circulation model, which is similar to that used at the National Centre for Medium Range Weather Forecasting, New Delhi. The model is not able to capture realistically some important features of the temperature and humidity profiles in the lower troposphere and in the boundary layer during the active and weak spells.

Novel double insertion of carbon disulfide into two Ru-H bonds of [(dppm)(2)Ru(H)(2)] (dppm = Ph2PCH2PPh2): Synthesis and crystal structure of a methanedithiolate complex

Addition of excess carbon disulfide to cis/trans-[(dPPM)(2)Ru(H)(2)] results in the methanedithiolate complex [(dppm)(2)Ru(eta(2)-S2CH2)] 4 via the intermediacy of cis-[(dppm)(2)Ru(H)(SC(S)H)] 2. The X-ray crystal structure of this species has been determined.

Stereochemistry of the marine sesquiterpene 2-thiocyanatoneopupukeanane: Crystal structure of neopupukean-2-yl 4-nitrobenzoate

Single crystal X-ray diffraction analysis of neopupukean-2-yl p-nitrobenzoate unambiguously established the stereochemistry of the thiocyanate group as endo in the marine sesquiterpene 2-thiocyanatoneopupukeanane.

Studies on n=2 Aurivillius phases: structure of the series Bi3-xLaxTiNbO9 (0 <= x <= 1)

The crystal structures of the solid solutions of Bi3-xLaxTiNbO9 (0 less than or equal to x less than or equal to 1) have been analyzed by powder X-ray diffraction with supporting evidence from selected area electron diffraction (SAD). The structure of the starting member (x = 0) is verified to be in the orthorhombic space group A2(1) am while the end member (x = 1) is determined to crystallize in the centrosymmetric orthorhombic space group Pmcb. The structure of x = 1 phase is solved by ab initio powder diffraction. The intermediate compositions belong to the space group A2(1) am as confirmed by Rietveld refinements. Rietveld refinements on all the compositions reveal that the La3+ ion is disordered only in the A site and not in the [Bi2O2](2+) layer. The tilt in the Ti/NbO6 octahedra decreases with increasing x. (C) 2003 Elsevier B.V. All rights reserved.

Tuning the structure of surfactant complexes with DNA and other polyelectrolytes

We have carried out small-angle X-ray diffraction studies on complexes formed by the anionic polyelectrolytes, namely, sodium salts of double and single stranded (ds and ss) DNA, poly( glutamic acid) ( PGA), poly( acrylic acid) (PAA), and poly( styrene sulfonate) (PSS) with a cationic surfactant system consisting of cetyltrimethylammonium bromide ( CTAB) and sodium 3-hydroxy-2-naphthoate (SHN). All complexes have a two-dimensional (2D) hexagonal structure at low SHN concentrations. DNA-CTAB-SHN complexes exhibit a hexagonal to lamellar transition near the SHN concentration at which CTAB-SHN micelles show a cylinder to bilayer transformation. On the other hand, PGA and PAA complexes form a 2D centered rectangular phase at higher SHN concentrations, and PSS complexes show a primitive rectangular structure. These results provide a striking example of polyion specificity in polyelectrolytesurfactant interactions.

Beta-structure of polypeptides in non-aqueous solutions. I. Spectral characteristics of the polypeptide backbone

The optical rotatory features of the beta-structure of the polypeptides in non-aqueous solutions and films cast from these solutions have been investigated. The beta-structure of poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-O-carbo-bands of their films. The optical rotatory dispersion (ORD) and circular dichroism (CD) spectra of these polypeptides are found to be very similar in both film and solution. In solvents promoting the beta-structure, the polypeptides are characterized by CD troughs in the n-pi* transition region of the peptide chromophore. The ORD spectra are found to be positive in sign throughout the visible and accessible ultraviolet regions and are interpreted in terms of the possible existence of a relatively much larger positive pi-pi* CD bands as compared with the negative n-pi* band. The rotatory data obtained in the non-aqueous solution are compared with those obtained for other poly peptides in aqueous solutions, with respect to the type and extent of beta-structure present.

Beta-structure of polypeptides in non-aqueous solutions. II. Side-chain orientation

The specific side-chain orientations of the phenyl group in the polypeptides poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-O-carbobenzoxy-L-serine in the beta-structure have been studied by spectral measurements in solutions. All the three polypeptides exhibit aromatic CD bands, indicating the asymmetric placement of the side-chain phenyl rings when the polypeptide backbone takes up the antiparallel beta-structure. Supporting evidence for this is derived from n.m.r. spectra of the polypeptides, which show upfield shift of the phenyl protons due to the stacking of the aromatic rings. Molecular model building studies reveal the stacking of alternate phenyl groups along the polypeptide chain.

Conformational studies of the phosphazenyl side chain in cyclophosphazenes. Part 3. Crystal and molecular structure of N3P3Cl4(NEt2)(NPPh3)

The crystal structure of N3P3Cl4(NEt2)(NPPh3) has been determined. The crystals are orthorhombic, space group Pbca, with a= 8.208(1), b= 21.890(1), c= 31.722(2)Å, Z= 8, and m.p. = 146.5 °C. The structure was solved by direct methods and refined to a final R value of 0.045 for 2 025 independent reflections. The analysis reveals significant variations in the ring P–N bond lengths. The two nitrogenous substituents, NPPh3 and NEt2, reside on the same phosphorus atom. The latter, NEt2, has an almost exact type II conformation (the plane NC2 almost perpendicular to the local NPN plane)(the first observed for a dialkylamino-group in cyclophosphazenes), the former, NPPh3, deviates from type II towards type III (in type III the plane Pring–N–Pexo makes an angle of ca. 45° with the local N–P–N ring plane). The present structure is compared with others of triphenylphosphazenyl-cyclophosphazenes and the conformation of the NPPh3 substituent and its electron supply in the ground and perturbed states are discussed.