Cadmium(II) Complex Formation with Cysteine and Penicillamine

The complex formation between cadmium(II) and the ligands cysteine (H(2)Cys) and penicillamine (H(2)Pen = 3, 3'-dimethylcysteine) in aqueous solutions, having C(Cd(II)) similar to 0-1 mol dm(-3) and C(H2L) = 0.2-2 mol dm(-3), was studied at pH = 7.5 and 11.0 by means of (113)Cd NMR and Cd K- and L(3)-edge X-ray absorption spectroscopy. For all cadmium (II)-cysteine molar ratios, the mean Cd-S and Cd-(N/O) bond distances were found in the ranges 2.52-2.54 and 2.27-2.35 angstrom, respectively. The corresponding cadmium(II)-penicillamine complexes showed slightly shorter Cd-S bonds, 2.50-2.53 angstrom, but with the Cd-(N/O) bond distances in a similar wide range, 2.28-2.33 angstrom. For the molar ratio C(H2L)/C(Cd(II)) = 2, the (113)Cd chemical shifts, in the range 509-527 ppm at both pH values, indicated complexes with distorted tetrahedral CdS(2)N(N/O) coordination geometry. With a large excess of cysteine (molar ratios C(H2Cys)/C(Cd(II)) >= 10), complexes with CdS(4) coordination geometry dominate, consistent with the (113)Cd NMR chemical shifts, delta similar to 680 ppm at pH 7.5 and 636-658 ppm at pH 11.0, and their mean Cd-S distances were 2.53 +/- 0.02 angstrom. At pH 7.5, the complexes are almost exclusively sulfur-coordinated as [Cd(S-cysteinate)(4)](n-) while at higher pH, the deprotonation of the amine groups promotes chelate formation. At pH 11.0, a minor amount of the [Cd(CYS)(3)](4-) complex with CdS(3)N coordination is formed. For the corresponding penicillamine solutions with molar ratios C(H2Pen)/C(Cd(II)) >= 10, the (113)Cd NMR chemical shifts, delta similar to 600 ppm at pH 7.5 and 578 ppm at pH 11.0, together with the average bond distances, Cd-S 2.53 +/- 0.02 angstrom and Cd-(N/O) 2.30-2.33 angstrom, indicate that [Cd(penicillaminate)(3)](n-) complexes with chelating CdS(3)(N/O) coordination dominate already at pH 7.5 and become mixed with CdS(2)N(N/O) complexes at pH 11.0. The present study reveals differences between cysteine and penicillamine as ligands to the cadmium(II) ion that can explain why cysteine-rich metallothionines are capable of capturing cadmium(II) ions, while penicillamine, clinically useful for treating the toxic effects of mercury(II) and lead(II) exposure, is not efficient against cadmium(II) poisoning.