CD-113 SHIELDING TENSORS OF CADMIUM COMPOUNDS .7. X-RAY STRUCTURE AND CD-113 NMR-STUDIES OF POLY(BIS(ACETATO)BIS(IMIDAZOLE)CADMIUM(II)) - A MODEL-COMPOUND FOR CD-SUBSTITUTED CARBOXYPEPTIDASE-A AND THERMOLYSIN

The structure of poly(bis(acetato)bis(imidazole)cadmium(II)) [Cd(OAc)2(Im)2] was determined by X-ray diffraction methods. Cd(OAc)2(Im)2 crystallizes in the space group P21/c, and the monoclinic unit cell constants are a = 7.6563 Å, b = 10.2048 Å, c = 17.5709 Å, β = 95.764°, Z = 4. The coordination about the metal atom is best described as a distorted octahedron. The two imidazole ligands and an asymmetrical chelating acetate ligand define an equatorial plane. The axial positions are occupied by two monodentate acetate ligands that form bridges to the neighboring metal atoms producing infinite chains. 113Cd solid-state NMR experiments were performed on Cd(OAc)2(Im)2 to determine the magnitude and orientation of the 113Cd shielding tensor. The principle elements of the shielding tensor determined from both powder and oriented single-crystal experiments in the molecular frame of Cd(OAc)2(Im)2 are σ11 =-40.62 ppm, σ22 = 101.37 ppm, and σ33 = 248.75 ppm. As anticipated from the crystal symmetry, two symmetry-related tensors were observed in the single-crystal experiment. Some simple empirical rules provided a vehicle for the assignment of the symmetry-related tensors to lattice sites. The most deshielded element σ33 is 75° off the plane containing the two imidazole nitrogens and cadmium metal ion, while the most shielded element σ11 is 63° from the best least-squares line containing the axial Cd-O bonds. The results presented in this paper can be utilized in predicting the orientation of the 113Cd shielding tensor in Cd-substituted carboxypeptidase-A. Since the enzyme thermolysin has a similar metal environment as carboxypeptidase-A, Cd(OAc)2(Im)2 may also be used as a model compound of Cd-substituted thermolysin. This work represents part of our ongoing investigation on 113Cd shielding tensors via single-crystal NMR experiments on model compounds resembling the metal environment of metalloproteins. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.