Structural Analysis of Cadmium–Glycylglycine Complexes Studied by X-Ray Diffraction and High Resolution 113Cd and 13C Solid State NMR

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Abstract
Structural analysis of bis(glycylglycinato)cadmium(II) complexes prepared to adjust the pH to 6 (A) and 9 (C), were studied by X-ray diffraction and high resolution 113Cd and 13C solid state NMR. In order to compare with the structure of A, structural analysis of bis(glycylglycinato)zinc(II) complexes (B) prepared to adjust the pH to 6 was also performed by the same methods. The structure of A was six coordinate; a five-membered chelate ring was formed between the terminal amino-group and the adjacent O (peptide) and the terminal carboxygroups of the dipeptide ligands were involved in Cd binding in the complex. The structure of B was similar to A, indicating that the Cd nucleus was better suited for probing the Zn nucleus-ligand structure. The structure of C was six coordinate; the four terminal carboxygroups (O) and the two terminal aminogroups (N) of the dipeptide ligands were involved in Cd binding in the complex. High-resolution 113Cd and 13C CP/MAS NMR spectra have been measured in order to obtain detailed information about the structures. The 113Cd signal of A showed a low field chemical shift compared with the peak of C. This means that the chelate ring shows decreased shielding. From the 13C spectra of A and C, it was seen that the Cd nucleus of A is bonded to a carbonyl oxygen (peptide) and the terminal carboxyl group of the dipeptide, but that for C is not bonded to a carbonyl oxygen (peptide).

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