Glycine zinc sulfate pentahydrate: redetermination at 10 K from time-of-flight neutron Laue diffraction

Single crystals of glycine zinc sulfate pentahydrate [systematic name: hexaaquazinc tetraaquadiglycinezinc bis(sulfate)], [Zn(H2O)6][Zn(C2H5NO2)2(H2O)4](SO4)2, have been grown by isothermal evaporation from aqueous solution at room temperature and characterized by single-crystal neutron diffraction. The unit cell contains two unique ZnO6octahedra on sites of symmetry -1 and two SO4tetrahedra with site symmetry 1; the octahedra comprise one [tetraaqua-diglycine zinc]2+ion (centred on one Zn atom) and one [hexaaquazinc]2+ion (centred on the other Zn atom); the glycine zwitterion, NH3+CH2COO−, adopts a monodentate coordination to the first Zn atom. All other atoms sit on general positions of site symmetry 1. Glycine forms centrosymmetric closed cyclic dimers due to N—H...O hydrogen bonds between the amine and carboxylate groups of adjacent zwitterions and exhibits torsion angles varying from ideal planarity by no more than 1.2°, the smallest values for any known glycine zwitterion not otherwise constrained by a mirror plane. This work confirms the H-atom locations estimated in three earlier single-crystal X-ray diffraction studies with the addition of independently refined fractional coordinates andUijparameters, which provide accurate internuclearX—H (X= N, O) bond lengths and consequently a more accurate and precise depiction of the hydrogen-bond framework.

X-ray and neutron powder diffraction analyses of Gly·MgSO4·5H2O and Gly·MgSO4·3H2O, and their deuterated counterparts

We have identified a new compound in the glycine–MgSO4–water ternary system, namely glycine magnesium sulfate trihydrate (or Gly·MgSO4·3H2O) {systematic name:catena-poly[[tetraaquamagnesium(II)]-μ-glycine-κ2O:O′-[diaquabis(sulfato-κO)magnesium(II)]-μ-glycine-κ2O:O′]; [Mg(SO4)(C2D5NO2)(D2O)3]n}, which can be grown from a supersaturated solution at ∼350 K and which may also be formed by heating the previously known glycine magnesium sulfate pentahydrate (or Gly·MgSO4·5H2O) {systematic name: hexaaquamagnesium(II) tetraaquadiglycinemagnesium(II) disulfate; [Mg(D2O)6][Mg(C2D5NO2)2(D2O)4](SO4)2} above ∼330 K in air. X-ray powder diffraction analysis reveals that the trihydrate phase is monoclinic (space groupP21/n), with a unit-cell metric very similar to that of recently identified Gly·CoSO4·3H2O [Tepavitcharovaet al.(2012).J. Mol. Struct.1018, 113–121]. In order to obtain an accurate determination of all structural parameters, including the locations of H atoms, and to better understand the relationship between the pentahydrate and the trihydrate, neutron powder diffraction measurements of both (fully deuterated) phases were carried out at 10 K at the ISIS neutron spallation source, these being complemented with X-ray powder diffraction measurements and Raman spectroscopy. At 10 K, glycine magnesium sulfate pentahydrate, structurally described by the `double' formula [Gly(d5)·MgSO4·5D2O]2, is triclinic (space groupP\overline{1},Z= 1), and glycine magnesium sulfate trihydrate, which may be described by the formula Gly(d5)·MgSO4·3D2O, is monoclinic (space groupP21/n,Z= 4). In the pentahydrate, there are two symmetry-inequivalent MgO6octahedra on sites of \overline 1 symmetry and two SO4tetrahedra with site symmetry 1. The octahedra comprise one [tetraaquadiglcyinemagnesium]2+ion (centred on Mg1) and one [hexaaquamagnesium]2+ion (centred on Mg2), and the glycine zwitterion, NH3+CH2COO−, adopts a monodentate coordination to Mg2. In the trihydrate, there are two pairs of symmetry-inequivalent MgO6octahedra on sites of \overline 1 symmetry and two pairs of SO4tetrahedra with site symmetry 1; the glycine zwitterion adopts a binuclear–bidentate bridging function between Mg1 and Mg2, whilst the Mg2 octahedra form a corner-sharing arrangement with the sulfate tetrahedra. These bridged polyhedra thus constitute infinite polymeric chains extending along thebaxis of the crystal. A range of O—H...O, N—H...O and C—H...O hydrogen bonds, including some three-centred interactions, complete the three-dimensional framework of each crystal.

Tuneable graphene nanopores for single biomolecule detection

The architecture of a tuneable graphene nanopore device (left) and the highly sensitive detection of the carboxyl group in a glycine zwitterion as it translocates through the pore (right).