x-ray studies on crystalline complexes involving amino-acids and peptides .10. head-to-tail sequences in the crystal-structure of l-lysine acetate

l-Lysine acetate crystallises in the monoclinic space group P21 with a = 5.411 (1), b = 7.562(1), c= l2.635(2) Å and β = 91.7(1). The crystal structure was solved by direct methods and refined to an R value of 0.049 using the full matrix least squares method. The conformation and the aggregation of lysine molecules in the structure are similar to those found in the crystal structure of l-lysine l-aspartate. A conspicuous similarity between the crystal structures of l-arginine acetate and l-lysine acetate is that in both cases the strongly basic side chain, although having the largest pK value, interacts with the weakly acidic acetate group leaving the α-amino and the α-carboxylate groups to take part in head-to-tail sequences. These structures thus indicate that electrostatic effects are strongly modulated by other factors so as to give rise to head-to-tail sequences which have earlier been shown to be an almost universal feature of amino acid aggregation in the solid state.

The rα structure, chemical shift anisotropy, and the abnormal orientation of methyldichlorophosphine from the NMR spectrum

NMR studies of methyldichlorophosphine have been undertaken in the nematic phase of mixed liquid crystals of opposite diamagnetic anisotropies. The rα structure is derived. The proton chemical-shift anisotropy has been determined from the studies without the use of a reference compound and without a change of experimental conditions. It is shown that the molecule orients in the liquid crystal with positive diamagnetic anisotropy in such a way that the C3 symmetry axis of the CH3P moiety is preferentially aligned perpendicular to the direction of the magnetic field, unlike other similar systems. This is interpreted in terms of the formation of a weak solvent-solute molecular complex. The heteronuclear indirect spin-spin coupling constants are determined. The sign of the two-bond JPH is found to be positive.

cis-trans Enantiomerism in the Diels-Alder cycloadducts of 6-arylfulvenes with maleic anhydride: resolution of the exo adducts via the N-((1S)-1-(naphth-1-yl)ethyl)imide derivatives: assignment of the absolute configurations based on the crystal structure of an imide diastereomer

A new case of the uncommon cis-trans enantiomerism is presented. The titled anhydride adducts were prepared in good yields by the known reaction of three 6-arylfulvenes with maleic anhydride (aryl = phenyl, p-tolyl and p-anisyl). The exo adducts were converted to the corresponding imides by reaction with (1S)-1-(naphth-1-yl)ethylamine in similar to 80% yields, and the resulting diastereomeric imides separated by silica gel column chromatography. They were hydrolysed and recyclised to the chiral anhydrides, in `one-pot' with 10% NaOH-EtOH, followed by treatment with 2 M HCl, in similar to 40% yields. The titled anhydrides were thus obtained in homochiral form, in enantiomeric purities (generally) of similar to 90% as indicated by chiral HPLC. The chiral anhydrides were also converted to the corresponding imides (presumably stereospecifically), by treatment with ammonia solution in excellent yields. The crystal structure of one of the above diastereomeric imides (derived from 6-phenylfulvene) was determined, and based on the known (S)-configuration of the naphthylethylamine moiety, the `configurations' of the original anhydride adducts were assigned. (c) 2005 Elsevier Ltd. All rights reserved.

Novel coordination behaviour of gem-bis (pyrazolyl) cyclotriphosphazenes as tridentate NNN-donor ligands: The crystal structure of [Mo(CO)3{N3P3Ph4(3,5-Me2C3HN2)2}]

Reactions of group 6 metal carbonyls with bis(pyrazolyl) phosphazenes yield metal tricarbonyl complexes, [M(CO)3.L] [L = N3P3Ph4 (3, 5-Me2C3HN2)2 (1) or N3P3(MeNCH2CH2O)2 (3,5-Me2C3HN2)2(4)]. The structure of the complex [Mo(CO)3.1], determined by single-crystal X-ray analysis, shows that the (pyrazolyl) phosphazene acts as a tridentate ligand; the two pyridinic pyrazolyl nitrogen atoms and a phosphazene ring nitrogen atom are coordinated to the metal. A similar structure is proposed for the complexes [M(CO)3.4] (M = Mo or W] on the basis of their spectroscopic data.

The crystal structure of the dipotassium salt of uridine 5'-diphosphate

C9H12N2Ot2P22-. 2K + .3H20 is orthorhombic, P2~2~2p with a = 18.977 (5), b - 22.597 (6), c = 8.995 (2) A, Z = 8. The structure was refined to R = 0.059 for 2587 observed reflexions. The two molecules of the asymmetric unit have very similar conformations with a 2'- endo sugar pucker and a folded pyrophosphate chain. They form a dimer, coordinated by the K + ions but without direct bridging between the base and the pyrophosphate within each individual molecule. One uracil base has the keto-enol and the other the diketo form. The extended structure shows alternating hydrophobic and hydrophilic regions.

Interactions of tris buffer with nucleotides: the crystal structure of tris(hydroxymethyl)methylammonium adenosine 5'-diphosphate dihydrate

The crystal and molecular structures of the Tris salt of adenosine 5'-diphosphate were determined from X-ray diffraction data. The crystals are monoclinic, space P21, and Z = 2 with a=9.198 (2) A, b=6.894 (1) A, c=18.440 (4) A, and beta = 92.55 (2) degrees. Intensity data were collected on an automated diffractometer. The structure was solved by the heavy-atom technique and refined by least squares to R = 0.047. The ADP molecule adopts a folded conformation. The conformation about the glycosidic bond is anti. The conformation of the ribose ring is close to a perfect C(2')-endo-C-(3')-exo puckering. The conformation about C(4')-C(5') is gauche-gauche, similar to other nucleotide structures. The pyrophosphate chain displays a nearly eclipsed geometry when viewed down the P-P vector, unlike the staggered conformation observed in crystal structures of other pyrophosphates. The less favorable eclipsed conformation probably results from the observed association of Tris molecules with the polar diphosphate chain through electrostatic interactions and hydrogen bonds. Such interactions may play an important role in Tris-buffered aqueous solutions of nucleotides and metal ions.

The primary structure of sheep liver cytosolic serine hydroxymethyltransferase and an analysis of the evolutionary relationships among serine hydroxymethyltransferases

The complete amino-acid sequence of sheep liver cytosolic serine hydroxymethyltransferase was determined from an analysis of tryptic, chymotryptic, CNBr and hydroxylamine peptides. Each subunit of sheep liver serine hydroxymethyltransferase consisted of 483 amino-acid residues. A comparison of this sequence with 8 other serine hydroxymethyltransferases revealed that a possible gene duplication event could have occurred after the divergence of animals and fungi. This analysis also showed independent duplication of SHMT genes in Neurospora crassa. At the secondary structural level, all the serine hydroxymethyltransferases belong to the alpha/beta category of proteins. The predicted secondary structure of sheep liver serine hydroxymethyltransferase was similar to that of the observed structure of tryptophan synthase, another pyridoxal 5'-phosphate containing enzyme, suggesting that sheep liver serine hydroxymethyltransferase might have a similar pyridoxal 5'-phosphate binding domain. In addition, a conserved glycine rich region, G L Q G G P, was identified in all the serine hydroxymethyltransferases and could be important in pyridoxal 5'-phosphate binding. A comparison of the cytosolic serine hydroxymethyltransferases from rabbit and sheep liver with other proteins sequenced from both these sources showed that serine hydroxymethyltransferase was a highly conserved protein. It was slightly less conserved than cytochrome c but better conserved than myoglobin, both of which are well known evolutionary markers. C67 and C203 were specifically protected by pyridoxal 5'-phosphate against modification with [C-14]iodoacetic acid, while C247 and C261 were buried in the native serine hydroxymethyltransferase. However, the cysteines are not conserved among the various serine hydroxymethyltransferases. The exact role of the cysteines in the reaction catalyzed by serine hydroxymethyltransferase remains to be elucidated.

Impact of chemical treatment on the surface, structure, optical and electrical properties of SnS thin films

The impact of chemical treatment on the surface morphology and other physical properties of tin monosulphide (SnS) thin films have been investigated. The SnS films treated with selected organic solvents exhibited strong improvement in their crystalline-quality and considerable decrease in electrical resistivity. Particularly, the films treated with chloroform showed very low electrical resistivity of similar to 5 Omega cm and a low optical band gap of 1.81 eV as compared to untreated and treated SnS films with other chemicals. From these studies we realized that the chemical treatment of SnS films has strong impact on their surface morphology and also on other physical properties. (C) 2012 Elsevier B.V. All rights reserved.

An encaged sigma* orbital in 3-methoxy-2,4,10-trioxaadamantane. Preferred exo-anomeric effect as revealed by the crystal structure

In the titled tricyclic orthocarbonate derivative, the three endocyclic C-O bonds are longer than the exo-cyclic C-O bond (similar to 4.40 angstrom vs. similar to 1.37 angstrom). This indicates an anomeric-type interaction between the two electron lone pairs on the exocyclic oxygen atom and the antibonding orbitals of the two antiperiplanar endocyclic C-O bonds. The remaining endocyclic C-O bond - marginally shorter than the other two apparently adds to this effect. Intriguingly, the antibonding orbital of the exocyclic C-O bond extends into the interior of the adamantyl cage, and is stereoelectronically prevented from overlapping with any of the six adjacent lone pairs. The results also seem to indicate a preference for interaction between a single donor oxygen atom and multiple acceptor antibonding orbitals rather than vice versa. The results add insightfully to the substantial body of evidence favouring the antiperiplanar lone pair hypothesis (ALPH). (C) 2014 Elsevier B.V. All rights reserved.

X-Ray Studies On The Bilayer Structure Of Trypsin-Treated Rat-Brain Myelin

Trypsin-treated rat brain myelin was subjected to biochemical and X-ray studies. Untreated myelin gave rise to a pattern of three rings with a fundamental repeat period of 155 Angstrom consisting of two bilayers per repeat period, whereas myelin treated with trypsin showed a fundamental repeat period of 75 Angstrom with one bilayer per repeat period. The integrated raw intensity of the h=4 reflection with respect to the h=2 reflection is 0.38 for untreated myelin. The corresponding value reduced to 0.23, 0.18, 0.17 for myelin treated with 5, 10, 40 units of trypsin per mg of myelin, respectively, for 30 min at 30 degrees C. The decrease in relative raw intensity of the higher-order reflection relative to the lower-order reflection is suggestive of a disordering of the phosphate groups upon trypsin treatment or an increased mosaicity of the membrane or a combination of both these effects, However, trypsin treatment does not lead to a complete breakdown of the membrane, The integrated intensity of the h=1 reflection, though weak, is above the measurable threshold for untreated myelin, whereas the corresponding intensity is below the measurable threshold for trypsin-treated myelin, indicating a possible asymmetric to symmetric transition of the myelin bilayer structure about its centre after trypsin treatment.

Hydrogen bonding in peptide helices - Analysis of two independent helices in the crystal structure of a peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe

The crystal structure determination of the heptapeptide Boc-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe reveals two peptide helices in the asymmetric unit, Crystal parameters are: space group P2(1), a = 10.356(2) Angstrom, b = 19.488(5) Angstrom, c = 23.756(6) Angstrom, beta = 102.25(2)degrees), V = 4685.4 Angstrom(3), Z = 4 and R = 5.7% for 7615 reflections [I>3 sigma(I)]. Both molecules adopt largely alpha-helical conformations with variations at the C-terminus, Helix type Is determined by analysing both 4-->1 and 5-->1 hydrogen-bond interactions and comparison with the results of analysis of protein structures. The presence of two 4-->1 hydrogen-bond interactions, besides four 5-->1 interact ions in both the conformations provides an opportunity to characterize bifurcated hydrogen bonds at high resolution, Comparison of the two helical conformations with related peptide structures suggests that distortions at the C-terminus are more facile than at the N-terminus.

The crystal structure of eulytite Pb3BiV3O12

The crystal structure of Pb3BiV3O12 was solved using single-crystal X-ray diffraction technique. The compound crystallizes in the cubic system View the MathML source (No. 220) with eulytite structure with a = 10.7490(7) Å, V = 1241.95(14) Å3 and Z = 4. The final R1 value of 0.0198 (wR2=0.0384) was achieved for 359 independent reflections during the structure refinement. The Pb2+ and Bi3+ cations occupy the special position (16c) while the oxygen anions occupy the general position (48e) in the crystal structure. Unlike many other eulytite compounds, all the crystallographic positions are fully occupied. The structure consists of edge-shared Pb/Bi octahedra linked at the corners to independent [VO4]3− tetrahedra units, generating a eulytite-type network in the crystal lattice.

The crystal structure of the amino-terminal pentapeptide of suzukacillin. Occurrence of a four-fold peptide helix

The monohydrate of the protected amino-terminal pentapeptide of suzukacillin, t-butoxycarbonyl--aminoisobutyryl-L-prolyl-L-valyl--aminoisobutyryl-L-valine methyl ester, C29H51N5O8, crystallizes in the orthorhombic space group P212121 with a= 10.192, b= 10.440, c= 32.959 Å, and Z= 4. The structure has been solved by direct methods and refined to an R value of 0.101 for 1 827 observed reflections. The molecule exists as a four-fold helix with a pitch of 5.58 Å. The helix is stabilised by N–H O hydrogen bonds, two of the 51 type (corresponding to the -helix) and the third of the 41 type (310 helix). The carbonyl oxygen of the amino-protecting group accepts two hydrogen bonds, one each from the amide NH groups of the third (41) and fourth (51) residues. The remaining 51 hydrogen bond is between the two terminal residues. The lone water molecule in the structure is hydrogen bonded to carbonyl oxygens of the prolyl residue in one molecule and the non-terminal valyl residue in a symmetry-related molecule.

Sequence-dependent conformation of an A-DNA double helix: The crystal structure of the octamer d(G-G-T-A-T-A-C-C)

The crystal structures of the synthetic self-complementary octamer d(G-G-T-A-T-A-C-C) and its 5-bromouracil-containing analogue have been refined to R values of 20% and 14% at resolutions of 1·8 and 2·25 Å, respectively. The molecules adopt an A-DNA type double-helical conformation, which is minimally affected by crystal forces. A detailed analysis of the structure shows a considerable influence of the nucleotide sequence on the base-pair stacking patterns. In particular, the electrostatic stacking interactions between adjacent guanine and thymine bases produce symmetric bending of the double helix and a major-groove widening. The sugar-phosphate backbone appears to be only slightly affected by the base sequence. The local variations in the base-pair orientation are brought about by correlated adjustments in the backbone torsion angles and the glycosidic orientation. Sequence-dependent conformational variations of the type observed here may contribute to the specificity of certain protein-DNA interactions.