STRUCTURAL-ANALYSIS OF CADMIUM-GLYCYLGLYCINE COMPLEXES STUDIED BY X-RAY-DIFFRACTION AND HIGH-RESOLUTION CD-113 AND C-13 SOLID-STATE NMR

Structural analysis of bis(glycylglycinato)cadmium(II) complexes prepa red to adjust the pH to 6 (A) and 9 (C), were studied by X-ray diffrac tion and high resolution Cd-113 and C-13 solid State NMR. In order to compare with the structure of A, structural analysis of bis(glycylglyc inato)zinc(II) complexes (B) prepared to adjust the pH to 6 was also p erformed by the same methods. The structure of A was six coordinate; a five-membered chelate ring was formed between the terminal amino-grou p and the adjacent O (peptide) and the terminal carboxygroups of the d ipeptide ligands were involved in Cd binding in the complex. The struc ture 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 t erminal aminogroups (N) of the dipeptide ligands were involved in Cd b inding in the complex. High-resolution Cd-113 and C-13 CP/MAS NMR spec tra have been measured in order to obtain detailed information about t he structures. The Cd-113 signal of A showed a low field chemical shif t compared with the peak of C. This means that the chelate ring shows decreased shielding. From the C-13 spectra of A and C, it was seen tha t 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 bonde d to a carbonyl oxygen (peptide).