Search

Neutron diffraction data have been collected at 12, 50, 150 and 295 K for the dipeptide glycyl-L-alanine, C5H10N2O3, in order to obtain accurate positional and anisotropic displacement parameters for the H atoms. The values of these parameters serve as a benchmark for assessing the equivalent parameters obtained from a so-called Hirshfeld-atom refinement of X-ray diffraction data described elsewhere [Capelli et al. (2014). IUCrJ, 1, 361-379]. The flexibility of the glycyl-L-alanine molecule in the solid and the hydrogen-bonding interactions as a function of temperature are also considered. [ABSTRACT FROM AUTHOR]

The monoclinic C2 crystal structure of an α-cyclodextrin/1-undecanol host-guest inclusion complex was solved using single-crystal neutron diffraction. Large high-quality crystals were specially produced by optimizing temperature-controlled growth conditions. The hydrate crystallizes in a channel-type structure formed by head-to-head dimer units of α-cyclodextrin molecules stacked like coins in a roll. The alkyl chain of the guest lipid is entirely embedded inside the tubular cavity delimited by the α-cyclodextrin dimer and adopts an all- trans planar zigzag conformation, while the alcohol polar head group is outside close to the α-cyclodextrin primary hydroxyl groups. The cyclodextrin dimer forms columns, which adopt a quasi-square arrangement much less compact than the quasi-hexagonal close packing already observed in the less hydrated α-cyclodextrin channel-type structures usually found with similar linear guests. The lack of compactness of this crystal form is related to the high number of interstitial water molecules. The replacement of 1-undecanol by 1-decanol does not modify the overall crystal structure of the hydrate as shown by additional X-ray diffraction investigations comparing the two host-guest assemblies. This is the first study that analyses the entire hydrogen-bonding network involved in the formation of a cyclodextrin dimer surrounded by its shell of water molecules. [ABSTRACT FROM AUTHOR]

The crystal structures of ammonium tetraphenylborate, NH4+.BH4-, are determined by neutron diffraction at 20 and 293 K. At both temperatures, all four N - H vectors of the ammonium ion are time-average-oriented at the midpoints of the phenyl rings of neighboring anions. The N-H … Ph distances. H … M 2.067 and N … M 3.023 Å, are exceptionally short (M = aromatic midpoint). Even at 20 K the ammonium ion performs large amplitude motions which allow the N- H vectors to sample the entire face of the aromatic system. [ABSTRACT FROM AUTHOR]

Two isostructural complexes Ln(DOTAM)(H 2 O)(triflate) 3 ·3H 2 O (Ln = Eu, Nd) have been characterized by neutron diffraction analysis at different temperatures by means of two techniques, white-beam Laue at 65 K and monochromatic at 20 K, 65 K and 220 K. The high accuracy in the hydrogen-atom localization revealed the H-bonds network formed by the metal-coordinated water, the water molecules, the anions, and the macrocycle, as well as the non-symmetric orientation of the coordinated water with respect to the Ln-O w (w = water) axis. The accurate determination of Nd-H and Eu-H distances allows derivation of an averaged Gd-H value (2.974 Å at 220 K) that is in the range of the smaller values commonly used in relaxivity calculations for Gd-based contrast agents. [ABSTRACT FROM AUTHOR]

The biological addition of oligosaccharide moieties to asparagine residues of N-glycoproteins influences the properties and bioactivities of these macromolecules. The low-temperature neutron crystal structures of three N-glycoprotein linkage models and analogues provide accurate characterization of the three-dimensional structure of the conserved GlcNAc-Asn linkage. These first crystal structures of N-acetylated carbohydrates obtained by neutron diffraction provide high-resolution geometrical parameters that can be used for force-field parametrization and subsequent molecular dynamics simulation of N-glycoproteins. The correct localization of hydrogen atoms demonstrates the occurrence of trifurcated hydrogen bonds and hydrophobic contacts. [ABSTRACT FROM AUTHOR]

Four isomorphous complexes of formula [M(L)4(H2O)2]SO4·2H2O (M Co, 1a; Ni, 1b; Cu, 1c; Zn, 1d) have been isolated and characterized by single-crystal X-ray diffraction and neutron diffraction using the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin as well as by thermogravimetric analysis. The structures contain a discrete, strongly hydrogen-bonded water tetramer which causes a significant distortion of the metal coordination sphere in each case. Partial atomic charges and hardness analysis (PACHA) calculations reveal that the shortest hydrogen bonds are not the strongest in this constrained, cyclic solid-state structure and show that the distortion at the metal center is caused by the drive to maintain the integrity of the water tetramer. The system undergoes a disorder-order transition on slow cooling that provides insight into the nature of communication between water squares. [ABSTRACT FROM AUTHOR]

The article reports that dihydrogen complexes were a curiosity 20 years ago when the first paper by G.J. Kubas demonstrated that dihydrogen could coordinate to a metal center without H-H bond breaking. The dihydrogen bonding in W(η2-H2(CO)3(PiPr3)2 was established clearly by single-crystal neutron diffraction, showing a H-H bond distance of 0.82 Å. A number of polyhydride species were then reformulated, and the number of dihydrogen complexes grew swiftly. Most complexes contain only one dihydrogen ligand in the octahedral coordination sphere of a d6.

Reports on the synthesis and structural analysis by single-crystal neutron diffraction of a phenol...nitroxide hydrogen bonded complex. Hydrogen atom abstraction by peroxy radicals from vitamin E and related phenolic antioxidants; Interaction of radicals with phenolic antioxidants under steric and electronic environments.