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Ligand Bond Lengths (ligand + bond_length)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Thermal Effects and Vibrational Corrections to Transition Metal NMR Chemical Shifts

CHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2004
Sonja Grigoleit Dr.
Abstract Both zero-point and classical thermal effects on the chemical shift of transition metals have been calculated at appropriate levels of density functional theory for a number of complexes of titanium, vanadium, manganese and iron. The zero-point effects were computed by applying a perturbational approach, whereas classical thermal effects were probed by Car,Parrinello molecular dynamics simulations. The systematic investigation shows that both procedures lead to a deshielding of the magnetic shielding constants evaluated at the GIAO-B3,LYP level, which in general also leads to a downfield shift in the relative chemical shifts, ,. The effect is small for the titanium and vanadium complexes, where it is typically on the order of a few dozen ppm, and is larger for the manganese and iron complexes, where it can amount to several hundred ppm. Zero-point corrections are usually smaller than the classical thermal effect. The pronounced downfield shift is due to the sensitivity of the shielding of the metal centre with regard to the metal,ligand bond length, which increase upon vibrational averaging. Both applied methods improve the accuracy of the chemical shifts in some cases, but not in general. [source]

Do Metal,Metal Multiply-Bonded "Ligands" Have a trans Influence?

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2008
Magnetic Comparisons of Heterometallic CrCr···Co, MoMo···Co Interactions, Structural
Abstract Reported here are two new compounds containing either a CrCr···Co [1, CrCrCo(dpa)4Cl2, dpa = 2,2,-dipyridylamide] or a MoMo···Co [2, MoMoCo(dpa)4Cl2] framework both having a multiply-bonded unit (CrCr in 1, MoMo in 2) in close proximity to the Co2+ ion and trans to a Co,Cl bond. Variable temperature magnetic susceptibility measurements reveal 1 to have a temperature-dependent spin equilibrium between a low-spin (S = 1/2) and high-spin (S = 3/2) state, whereas the Co2+ ion in 2 exists solely in its high-spin state. The crystal structures of 1 and 2 were determined. Variable temperature crystallographic data of 1 at 100 K and at room temperature reveal that the spin-transition affects not only the Co,ligand bond lengths but also the terminal Cr,ligand bond lengths. Whereas the Cr···Co distance becomes shorter by 0.13 Å in the low-spin form, the Co,Cldistance becomes longer by 0.2 Å. These observations,along with the crystal structure of 2, suggest that the multiply-bonded MM group has a trans influence on the Co2+ ion.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Atomic resolution studies of carbonic anhydrase II

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2010
Craig A. Behnke
Carbonic anhydrase has been well studied structurally and functionally owing to its importance in respiration. A large number of X-ray crystallographic structures of carbonic anhydrase and its inhibitor complexes have been determined, some at atomic resolution. Structure determination of a sulfonamide-containing inhibitor complex has been carried out and the structure was refined at 0.9,Å resolution with anisotropic atomic displacement parameters to an R value of 0.141. The structure is similar to those of other carbonic anhydrase complexes, with the inhibitor providing a fourth nonprotein ligand to the active-site zinc. Comparison of this structure with 13 other atomic resolution (higher than 1.25,Å) isomorphous carbonic anhydrase structures provides a view of the structural similarity and variability in a series of crystal structures. At the center of the protein the structures superpose very well. The metal complexes superpose (with only two exceptions) with standard deviations of 0.01,Å in some zinc,protein and zinc,ligand bond lengths. In contrast, regions of structural variability are found on the protein surface, possibly owing to flexibility and disorder in the individual structures, differences in the chemical and crystalline environments or the different approaches used by different investigators to model weak or complicated electron-density maps. These findings suggest that care must be taken in interpreting structural details on protein surfaces on the basis of individual X-ray structures, even if atomic resolution data are available. [source]

An orthorhombic form of Escherichia coli aminopeptidase P at 2.4,Å resolution

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2003
Stephen C. Graham
Aminopeptidase P (AMPP) from Escherichia coli cleaves the N-terminal residue from an oligopeptide if the second residue is proline. The active site contains a dinuclear metal centre. Following earlier structural analyses of crystals in space groups P6422 and I4122, the structure of AMPP has been solved and refined in the orthorhombic space group C2221 at 2.4,Å resolution. There are six subunits in the asymmetric unit. These are arranged in two types of tetramer. One tetramer comprises four crystallographically independent subunits, while the other comprises two pairs of subunits related by a crystallographic twofold axis. The final model of 20,994 protein atoms, 1618 water molecules and 12 metal atoms refined to residuals R = 0.195 and Rfree = 0.215. The molecular structure confirms most of the previously reported features, including the subunit,subunit interfaces in the tetramer and persistent disorder at some residues. The metal,ligand bond lengths at the active site suggest that one of the two Mn atoms is five-coordinate rather than six-coordinate. [source]