Article

High-resolution neutron crystallographic studies of the hydration of the coenzyme cob(II)alamin

Department of Molecular Biology, Cell Biology and Biochemistry, Brown University, Providence, RI 02912, USA.
Acta Crystallographica Section D Biological Crystallography (Impact Factor: 7.23). 06/2011; 67(Pt 6):584-91. DOI: 10.1107/S090744491101496X
Source: PubMed

ABSTRACT The hydration of the coenzyme cob(II)alamin has been studied using high-resolution monochromatic neutron crystallographic data collected at room temperature to a resolution of 0.92 Å on the original D19 diffractometer with a prototype 4° × 64° detector at the high-flux reactor neutron source run by the Institute Laue-Langevin. The resulting structure provides hydrogen-bonding parameters for the hydration of biomacromolecules to unprecedented accuracy. These experimental parameters will be used to define more accurate force fields for biomacromolecular structure refinement. The presence of a hydrophobic bowl motif surrounded by flexible side chains with terminal functional groups may be significant for the efficient scavenging of ligands. The feasibility of extending the resolution of this structure to ultrahigh resolution was investigated by collecting time-of-flight neutron crystallographic data during commissioning of the TOPAZ diffractometer with a prototype array of 14 modular 2° × 21° detectors at the Spallation Neutron Source run by Oak Ridge National Laboratory.

0 Followers
 · 
100 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The structure of a nanospheric polyhydrido copper cluster, [Cu20(H)11{S2P(O(i)Pr)2}9], was determined by single-crystal neutron diffraction. The Cu20 cluster consists of an elongated triangular orthobicupola constructed from 18 Cu atoms that encapsulate a [Cu2H5](3-) ion with an exceptionally short Cu-Cu distance. The 11 hydrides in the cluster display three different coordination modes to the Cu atoms: six μ3-hydrides in a pyramidal geometry, two μ4-hydrides in a tetrahedral cavity, and three μ4-hydrides in an unprecedented near square-planar geometry. The neutron data set was collected for 7 days on a small crystal with dimensions of 0.20 mm × 0.50 mm × 0.65 mm using the Spallation Neutron Source TOPAZ single-crystal time-of-flight Laue diffractometer at Oak Ridge National Laboratory. The final R-factor was 8.63% for 16,014 reflections.
    Inorganic Chemistry 10/2014; 53(20). DOI:10.1021/ic501747e · 4.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Proton exchange within the MH2 moiety of (TPB)Co(H-2) (CoH2; TPB = B(o-C(6)H(4)PiPr(2))(3)) by 2-fold rotation about the MH2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP3iPr)Fe(H-2) (FeH2) (SiP3iPr = [Si(o-(C6H4PPr2)-Pr-i)(3)]). The g-values for CoH2 and FeH2 show that both have the JahnTeller (JT)-active E-2 ground state (idealized C-3 symmetry) with doubly degenerate frontier orbitals, (e)(3) = [|m(L) +/- 2>](3) = [x(2) y(2), xy](3), but with stronger linear vibronic coupling for CoH2. The observation of H-1 ENDOR signals from the CoHD complex, 2H signals from the CoD2/HD complexes, but no H-1 signals from the CoH2 complex establishes that H-2 undergoes proton exchange at 2 K through rotation around the CoH2 axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that CoH2 exhibits rotor-like behavior in solution because the underlying C3 molecular symmetry combined with H-2 exchange creates a dominant 6-fold barrier to H-2 rotation. FeH2 instead shows H-2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)-> H-2(sigma*) pi-backbonding that becomes dependent on the H-2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L2ME axis (E = Si for FeH2; E = B for CoH2). Notably, the isotropic H-1/H-2 hyperfine coupling to the diatomic of CoH2 is nearly 4-fold smaller than for FeH2.
    Journal of the American Chemical Society 09/2014; 136(42). DOI:10.1021/ja508117h · 11.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)((t)BuCOO)4((t)BuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal-metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal-oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers.
    Inorganic Chemistry 10/2014; DOI:10.1021/ic501411w · 4.79 Impact Factor

Full-text (2 Sources)

Download
34 Downloads
Available from
May 29, 2014