Cadmium(II) Complex Formation with Cysteine and Penicillamine

Abstract

The complex formation between cadmium(II) and the ligands cysteine (H2Cys) or penicillamine (H2Pen = 3, 3′-dimethylcysteine) in aqueous solutions, containing CCd(II) ∼ 0.1 mol dm-3 and CH2L = 0.2 – 2 mol dm-3, was studied at pH = 7.5 and 11.0 by means of 113Cd-NMR and Cd K- and L3-edge X-ray absorption spectroscopy. For all cadmium(II)-cysteine mole ratios the mean Cd-S and Cd-(N/O) bond distances were found in the ranges 2.52 – 2.54 Å and 2.27 – 2.35 Å, respectively. The corresponding cadmium(II)-penicillamine complexes showed slightly shorter Cd-S bonds, 2.50 – 2.53 Å, but with the Cd-(N/O) bond distances in a similar wide range, 2.28 – 2.33 Å. For the mole ratio CH2L / CCd(II) = 2, the 113Cd chemical shifts, in the range 509 – 527 ppm at both pH values, indicated complexes with distorted tetrahedral CdS2N(N/O) coordination geometry. With a large excess of cysteine (mole ratios CH2Cys / CCd(II) ≥ 10) complexes with CdS4 coordination geometry dominate, consistent with the 113Cd NMR chemical shifts, δ ∼ 680 ppm at pH 7.5 and 636 - 658 ppm at pH 11.0, and their mean Cd-S distances of 2.53 ± 0.02 Å. 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, and at pH 11.0 a minor amount of the [Cd(Cys)3]4- complex with CdS3N coordination is formed. For the corresponding penicillamine solutions with mole ratios CH2Pen / CCd(II) ≥ 10, the 113Cd-NMR chemical shifts, δ ∼ 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 Å and Cd-O 2.30 – 2.33 Å, indicate that [Cd(penicillaminate)3]n- complexes with chelating CdS3(N/O) coordination dominate already at pH 7.5, and become mixed with CdS2N(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.

Physiologically relevant plasma d , l -homocysteine concentrations mobilize Cd from human serum albumin

“…This result is remarkable because, in contrast to Cys and hCys, only a single SMW Cd complex eluted from the column. In order to rationalize this finding, previous studies revealed that in an aqueous solution at pH 7.0, the interaction of Cd with Cys results in the formation of the monomeric Cd species (possibly Cys 4 Cd) and the multimeric Cd species (possibly Cd 3 Cys 9 ) [52]. Since in the latter Cd complex Cd is coordinated to a thiol and a carboxylate group, blocking the carboxylate group with the ester moiety (Fig.…”

Probing the coordination behavior of Hg2+, CH3Hg+, and Cd2+ towards mixtures of two biological thiols by HPLC-ICP-AES

“…Based on these EXAFS data, the Cd species that eluted from the HPLC column is identified as the approximately tetrahedral (GS) 4 Cd. Even though the XAS data were acquired at 10 K, the formation of (GS) 4 Cd at room temperature has been recently reported [73] and it is therefore very likely that this species was also formed on the column.…”

Penicillamine-modified sensor for the voltammetric determination of Cd(II) and Pb(II) ions in natural samples

“…Particularly, D-penicillamine (Pen -Cuprimine, Depen), that has long been used as the drug of choice in the treatment of Wilson's disease or hepatolenticular degeneration, is a potential and effective chelator for some transition metals [12][13][14].…”