FTIR, XAS, and XRD study of cadmium complexes with l-cysteine

Cysteine (cys) plays a vital role in the detoxification of cadmium (Cd), but the coordination of this metal to cys is not well understood, although extensively studied. In this investigation, our results showed that the ratio Cd:cys 1:2 precipitate as a amorphous white powder, the Cd:cys 1:4 provided for a viscous liquid without precipitating, while the Cd:cys 1:6 resulted in long, clear, needle-like crystals. FTIR and XAS analysis indicated that cys was coordinated to Cd through S ligands on the cys. EXAFS of the Cd:cys products showed small differences in the coordination environment. The complex from Cd:cys 1:2 reaction displayed two different Cd–S interactions at 2.69 and 2.54 Å, each consisting of two neighboring atoms. The complex from the Cd:cys 1:4 reaction showed four equally distant Cd–S interactions at 2.47 Å. The comples from the Cd:cys 1:6 reaction exhibited a Cd–S interaction of four S ligands at approximately 2.54 Å. All the Cd:cys complexes had a tetrahedral arrangement of S ligands around the central Cd atom. The XRD demonstrated that the Cd:cys 1:2 and 1:4 reaction products were amorphous and no diffraction peaks were observed. However, the Cd:cys 1:6 reaction product showed strong diffraction peaks, crystallizing into a monoclinic unit cell (C2/C).

Graphical abstractCadmium cysteine complexes are important in biological systems. Through the reaction of cadmium with different mole ratios of cysteine only Cd–S interactions were observed. These interactions were found to range from 2.47 to 2.69 Å, under different the different mole ratios of cadmium to cysteine from 1:1 to 1:6 (Cd:Cys).Figure optionsDownload full-size imageDownload as PowerPoint slide