Silver(I) complex formation with cysteine, penicillamine, and glutathione

Abstract

The complex formation between silver(I) and cysteine (H2Cys), penicillamine (H2Pen), and glutathione (H3Glu) in alkaline aqueous solution was examined using extended X-ray absorption fine structure (EXAFS) and (109)Ag NMR spectroscopic techniques. The complexes formed in 0.1 mol dm(-3) Ag(I) solutions with cysteine and penicillamine were investigated for ligand/Ag(I) (L/Ag) mole ratios increasing from 2.0 to 10.0. For the series of cysteine solutions (pH 10-11) a mean Ag-S bond distance of 2.45 ± 0.02 Å consistently emerged, while for penicillamine (pH 9) the average Ag-S bond distance gradually increased from 2.40 to 2.44 ± 0.02 Å. EXAFS and (109)Ag NMR spectra of a concentrated Ag(I)-cysteine solution (C(Ag(I)) = 0.8 mol dm(-3), L/Ag = 2.2) showed a mean Ag-S bond distance of 2.47 ± 0.02 Å and δ((109)Ag) 1103 ppm, consistent with prevailing, partially oligomeric AgS3 coordinated species, while for penicillamine (C(Ag(I)) = 0.5 mol dm(-3), L/Ag = 2.0) the mean Ag-S bond distance of 2.40 ± 0.02 Å and δ((109)Ag) 922 ppm indicate that mononuclear AgS2 coordinated complexes dominate. For Ag(I)-glutathione solutions (C(Ag(I)) = 0.01 mol dm(-3), pH ∼11), mononuclear AgS2 coordinated species with a mean Ag-S bond distance of 2.36 ± 0.02 Å dominate for L/Ag mole ratios from 2.0 to 10.0. The crystal structure of the silver(I)-cysteine compound (NH4)Ag2(HCys)(Cys)·H2O (1) precipitating at pH ∼10 was solved and showed a layer structure with both AgS3 and AgS3N coordination to the cysteinate ligands. A redetermination of the crystal structure of Ag(HPen)·H2O (2) confirmed the proposed digonal AgS2 coordination environment to bridging thiolate sulfur atoms in polymeric intertwining chains forming a double helix. A survey of Ag-S bond distances for crystalline Ag(I) complexes with S-donor ligands in different AgS2, AgS2(O/N), and AgS3 coordination environments was used, together with a survey of (109)Ag NMR chemical shifts, to assist assignments of the Ag(I) coordination in solution.

Figure 1

(Left) In the Ag(I)-cysteinate compound (NH₄)Ag₂(HCys)(Cys)·H₂O (1) layers of Ag-thiolates with AgS₃N (Ag1) and AgS₃ (Ag2) coordination in the a-b plane of the unit cells are connected via hydrogen bonds that are shown as dashes (right) from carboxylate oxygen to water molecules (O5) and ammonium ions (N3). Thermal ellipsoids (right) are shown with 70% probability, and bond distances in Å.

Figure 2

In the crystal structure of Ag(HPen)·H₂O (2), the Ag(HPen) monomers connect with nearly linear AgS₂ coordination forming two intertwined polymeric strands in a double helix along the a-axis. For clarity only the thiolate sulfur and its nearest carbon atom are shown (except for S3 and S7). Dashed lines indicate the closest Ag⋯Ag and Ag⋯O interactions (see text). Thermal ellipsoids are shown with 70% probability, and bond distances in Å.

Figure 3

Ag K-edge EXAFS spectra and corresponding Fourier-transforms for Ag(I) alkaline aqueous solutions (pH 9 – 11), containing cysteine (top), penicillamine (middle) and glutathione (below) with L/Ag ratios 2 – 10 (see Table 2). The curve-fitting results (Exp.; Fit ) are presented in Table 5 (labelled with *).

Figure 4

¹⁰⁹Ag NMR spectra of concentrated Ag(I) aqueous solutions with L/Ag ratio ~ 2 for cysteine (P) and penicillamine (Q); see Table 2.