Silver(I) Complex Formation with Cysteine, Penicillamine, and Glutathione

The complex formation between silver(I) and cysteine (H(2)Cys), penicillamine (H(2)Pen), and glutathione (H(3)Glu) in alkaline aqueous solution was examined using extended Xray absorption fine structure (EXAFS) and Ag-109 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 angstrom consistently emerged, while for penicillamine (pH 9) the average Ag-S bond distance gradually increased from 2.40 to 2.44 +/- 0.02 angstrom. EXAFS and Ag-109 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 A and delta(Ag-109) 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 angstrom and delta(Ag-109) 922 ppm indicate that mononuclear AgS2 coordinated complexes dominate. For Ag(I)-glutathione solutions (C-Ag(I) = 0.01 mol dm(-3), pH similar to 11), mononuclear AgS2 coordinated species with a mean Ag-S bond distance of 2.36 +/- 0.02 angstrom dominate for L/Ag mole ratios from 2.0 to 10.0. The crystal structure of the silver(I)-cysteine compound (NH4)Ag-2(HCys)(Cys)center dot G(2)O (1) precipitating at pH similar to 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)center dot 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 Ag-109 NMR chemical shifts, to assist assignments of the Ag(I) coordination in solution.