Methyl 1-methyl-1H-1,2,3-triazole-4-carboxylate

The title molecule, C5H7N3O2, has an almost planar conformation, with a maximum deviation of 0.043 (3) angstrom, except for the methyl H atoms. In the crystal structure, intermolecular C-H center dot center dot center dot O hydrogen bonds link the molecules into layers parallel to the bc plane. Intermolecular pi-pi stacking interactions [centroid-centroid distances = 3.685 (2) and 3.697 (2) angstrom] are observed between the parallel triazole rings.

Crystal And Molecular-Structure Of 8-Acetoxy-6-(2,4-Dimethoxy-5-Bromophenyl)-3-Methyl-Tricyclo[5,2,1,03,8 ]Decan-2-One

The crystal and molecular structure has been determined by the heavy-atom method and refined by the least-squares procedure to R= 8"3 % for 2033 photographically observed reflexions. The compound crystallizes in the space group P]" with two molecules in a unit cell of dimensions a = 11"68 + 0-02, b = 12"91 +0"02, c= 10"43+0"02/~, e= 114"7+ 1, fl=90-2+ 1 and 7,= 118.3+ 1 °. The unit cell also contains one molecule of the solvent, benzene. The 'cage' part of the molecule exhibits a large number of elongated bonds and strained internal valency angles. The bridgehead angle in the bicyclic heptane ring system is 89 °. The acetate group at C(16) and the methyl group at C(15) are cis to each other.

1,3,6-Trimethylpyrano[4,3-b]pyrrol-4(1H)-one

All the non-H atoms of the title compound, C10H11NO2, are almost coplanar [maximum deviation = 0.040 (3) angstrom]. The crystal structure is stabilized by C-H center dot center dot center dot O hydrogen bonds.

Crystalline ethane-1,2-diol does not have intra-molecular hydrogen bonding: Experimental and theoretical charge density studies

The X-ray structure and electron density distribution of ethane-1,2-diol (ethylene glycol), obtained at a resolution extending to 1.00 Å−1 in sin θ/λ (data completion = 100% at 100 K) by in situ cryocrystallization technique is reported. The diol is in the gauche (g′Gt) conformation with the crystal structure stabilised by a network of inter-molecular hydrogen bonds. In addition to the well-recognized O–H···O hydrogen bonds there is topological evidence for C–H···O inter-molecular interactions. There is no experimental electron density based topological evidence for the occurrence of an intra-molecular hydrogen bond. The O···H spacing is not, vert, similar0.45 Å greater than in the gas-phase with an O–H···O angle close to 90°, calling into question the general assumption that the gauche conformation of ethane-1,2-diol is stabilised by the intra-molecular oxygen–hydrogen interaction.

Helical Conformations of Hexapeptides Containing N-Terminus Diproline Segments

The role of N-terminus diproline segments in facilitating helical folding in short peptides has been investigated in a set of model hexapeptides of the type Piv-Xxx-Yyy-Aib-Leu-Aib-Phe-OMe (Piv, pivaloyl). Nine sequences have been investigated with the following N-terminus dipeptide segments: (D)Pro-Ala (4) and Pro-Psi Pro (5, Psi, pseudoproline), Ala-Ala (6), Ala-Pro (7), Pro-Ala (8), Aib-Ala (9), Ala-Aib (10). The analog sequences Piv-Pro-Pro-Ala-Leu-Aib-Phe-OMe (2) and Piv-Pro-Pro-Ala-Aib-Ala-Aib-OMe (3) have also been studied. Solid state conformations have been determined by X-ray crystallography for peptides 4, 6, and 8 and compared with the previously determined crystal structure of peptide 1 (Boc-Pro-Pro-Aib-Leu-Aib-Val-OMe); (Rai et al., JACS 2006, 128, 7916-7928). Peptides 1 and 6 adopt almost identical helical conformations with unfolding of the helix at the N-terminus Pro (1) residue. Peptide 4 reveals the anticipated (D)Pro-Ala type II' beta-turn, followed by a stretch of 3(10)-helix. Peptide 8 adopts a folded conformation stabilized by four successive 4 -> 1 intramolecular hydrogen bonds. Ala (2) adopts an alpha(L) conformation, resulting in a type II beta-turn conformation followed by a stretch of 3(10)-helix. Conformational properties in solution were probed using solvent perturbation of NH chemical shifts which permit delineation of hydrogen bonded NH groups and nuclear Overhauser effects (NOEs) between backbone protons, which are diagnostic of local residue conformations. The results suggest that continuous helical conformations are indeed significantly populated for peptides 2 and 3. Comparison of the results for peptides 1 and 2, suggest that there is a significant influence of the residue that follows diproline segments in influencing backbone folding. (C) 2010 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 94: 360-370, 2010.

Influence of drying on the liquid limit behaviour of a marine clay

The possible mechanisms of particle aggregation and reduction in liquid limit of the Cochin marine clay on drying are investigated. Mineralogical analysis showed the absence of halloysite in the marine specimen. Experimental results also ruled out the possibility of cementitious material being responsible for particle aggregation and reduction in clay plasticity on drying. The presence of calcium and magnesium as the predominant exchangeable ions and of a high pore salt concentration facilitates strong interparticle attraction and small particle separations; the latter leads to development of significant capillary stresses that permits an intimate contact of particles and growth of strong van der Waals' and Coulombic bonds.

Ab initio studies on proton transfer involving Schiff base and related nitrogen compounds

Proton transfer across cationic hydrogen bonds involving Schiff base, ammonia and related compounds has been studied at the 4-31G level. Proton transfer characteristics are correlated to the proton affinities of the species involved. Hydrogen bond strengths of these hydrogen bonds are correlated to the differences in the proton affinity of the donor and the acceptor. Influence of a neighbouring hydrogen bond on the proton transfer from Schiff base to ammonia and Schiff base to water is also discussed.

(2-Chloro-8-methylquinolin-3-yl)methanol

The molecule of title compound, C11H10ClNO, is close to being planar (r.m.s deviation for the non-H atoms = 0.017 angstrom). In the crystal, molecules interact by way of O-H center dot center dot center dot O hydrogen bonds, generating C(2) chains propagating in [010]. The crystal structure is consolidated by C-H center dot center dot center dot pi interactions and aromatic pi-pi stacking interactions [centroid-centroid distance = 3.661 (2) angstrom].

(2-Chloro-6-methylquinolin-3-yl)methanol

The title compound, C11H10ClNO, is close to being planar (r.m.s deviation for the non-H atoms = 0.026 angstrom). In the crystal,molecules are linked by O-H center dot center dot center dot O hydrogen bonds, generating C(2) chains, and weak C-H center dot center dot center dot pi interactions and aromatic pi-pi stacking interactions [centroid-centroid distance = 3.713 (3) angstrom] help to consolidate the sturcture.

3-[(2-Chloro-6-methylquinolin-3-yl)methyl]quinazolin-4(3H)-one

In the title molecule, C19H14ClN3O, the quinoline and quinazoline ring systems form a dihedral angle of 80.75 (4)degrees. In the crystal, the molecules are linked by pairs of C-H center dot center dot center dot N hydrogen bonds into centrosymmetric dimers, generating R-2(2)(6) ring motifs. The structure is further stabilized by C-H center dot center dot center dot pi interactions and pi-pi stacking interactions [centroid-centroid distances = 3.7869 (8) and 3.8490 (8) angstrom].

X-Ray structure of a ternary complex, copper(II)–lnosine 5-monophosphate–benzimidazole. The first example for the absence of direct metal–nucleotide interaction

The unprecedented absence of direct metal–nucleotide interaction has been observed in the X-ray structure of the ternary metal nucleotide system [Cu(bzim)(H2O)5]2+[IMP]2–·3H2O [IMP = inosine 5-monophosphate(2–), bzim = benzimidazole). The complex crystallizes in the space group P21 with a= 7.013(2), b= 13.179(9), c= 14.565(9)Å, = 94.82(4)°, and Z= 2. The structure was solved by the heavy-atom method and refined by full-matrix least squares on the basis of 1 761 observed (I? 3i) reflections to final R and R values of 0.034 and 0.036 respectively. The CuII has a distorted octahedral co-ordination with a nitrogen of the bzim ligand [Cu–N 1.947(5)Å] and three oxygens of water molecules in the basal plane [mean Cu–O 2.017(3)Å] and two more water oxygens at axial positions [Cu–O 2.194(6) and 2.732(5)Å]. The nucleotide base stacks with the bzim ligand at an average distance of 3.5 Å and an angle of 22°. In the lattice, N(7) of the base is linked to a lattice water through a hydrogen bond, while all the phosphate oxygens are involved in hydrogen bonds with co-ordinated as well as lattice water molecules. The co-ordination behaviour of IMP to CuII is compared in structures containing different -aromatic amines in order to assess the influence of the ternary ligand in complex formation. The present results indicate that, apart from the commonly observed phosphate binding, other modes of co-ordination are possible, these being influenced mainly by the -accepting properties of the ternary ligand.

Computer modelling approach to study the modes of binding of alpha- and beta-anomers of D-galactose, D-fucose and D-glucose to L-arabinose-binding protein

The modes of binding of alpha- and beta-anomers of D-galactose, D-fucose and D-glucose to L-arabinose-binding protein (ABP) have been studied by energy minimization using the low resolution (2.4 A) X-ray data of the protein. These studies suggest that these sugars preferentially bind in the alpha-form to ABP, unlike L-arabinose where both alpha- and beta-anomers bind almost equally. The best modes of binding of alpha- and beta-anomers of D-galactose and D-fucose differ slightly in the nature of the possible hydrogen bonds with the protein. The residues Arg 151 and Asn 232 of ABP from bidentate hydrogen bonds with both L-arabinose and D-galactose, but not with D-fucose or D-glucose. However in the case of L-arabinose, Arg 151 forms hydrogen bonds with the hydroxyl group at the C-4 atom and the ring oxygen, whereas in case of D-galactose it forms bonds with the hydroxyl groups at the C-4 and C-6 atoms of the pyranose ring. The calculated conformational energies also predict that D-galactose is a better inhibitor than D-fucose and D-glucose, in agreement with kinetic studies. The weak inhibitor D-glucose binds preferentially to one domain of ABP leading to the formation of a weaker complex. Thus these studies provide information about the most probable binding modes of these sugars and also provide a theoretical explanation for the observed differences in their binding affinities.

Crystal structure of L-lysyl-L-glutamic acid dihydrate

L-Lysyl-L-glutamic acid dihydrate, C11N3O5H21·2H2O, crystallizes in the monoclinic space group P21 with a = 12.474(2), b = 5.020(1), c = 13.157(2) Å, β= 114.69(1)° and Z = 2. The crystal structure was solved by direct methods and refined to an R value of 0.037 using full matrix least-squares method. The molecule exists as a double zwitterion with both the amino and carboxyl groups ionised. The peptide has a folded conformation with its Lys residue trans and Glu residue gauche−gauche+. The side chains of the Lys and Glu residues correspond to all trans and folded (g−g−g−) conformations respectively. The terminal carboxyl group forms hydrogen bonds with the ξ-amino group of the lysine side chain. The head-to-tail interaction often seen in peptide crystals is absent in the present structure. In the extended crystal structure water molecules form channels along the b direction and are enclosed within helically arranged hydrogen bonds formed by the lysine side chain and the peptide backbone.

Asynchronous behaviour in some three-atom linear concerted radical-exchange reactions

A few simple three-atom thermoneutral radical exchange reactions (i.e. A + BC --> AB + C) are examined by ab initio SCF methods. Emphasis is laid on the detailed analysis of density matrices rather than on energetics. Results reveal that the sum of the bond orders of the breaking and forming bonds is not conserved to unity, due to development of free valence on the migrating atom 'B' in the transition state. Bond orders, free valence and spin densities on the atoms are calculated. The present analysis shows that the bond-cleavage process is always more advanced than the bond-formation process in the transition state. Further analysis shows a development of the negative spin density on the migrating atom 'B' in the transition state. The depletion of the alpha-spin density on the radical site "A" in the reactant during the reaction lags behind the growth of the alpha-spin density on the terminal atom "C" of the reactant bond, 'B-C' in the transition state. But all these processes are completed simultaneously at the end of the reaction. Hence, the reactions are asynchronous but kinetically concerted in most cases.

Photoacoustic investigation of the optical absorption and thermal diffusivity in SixTe100−x glassesstar

The photoacoustic technique is used to determine the optical energy gap E0 of bulk SixTe100−x glasses in the glass-forming region 10 ≤ x ≤ 28. The thermal diffusivity α of these samples has also been measured. The variation of E0 and α with x is reported. It is found that E0 increases with x nearly linearly with a sharp decrease in the rate of increase beyond x = 20. The thermal diffusivity also increases with x up to x = 20 but decreases for compositions with higher values of x. The observed behaviour is explained on the basis of a chemical bond approach. It is accounted for in terms of the increase in the number of Te---Te bonds and formation of SiTe4 tetrahedra with an increase in the chalcogen content.