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

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 109Ag 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 109Ag NMR spectra of a concentrated Ag­(I)–cysteine solution (CAg(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(109Ag) 1103 ppm, consistent with prevailing,
partially oligomeric AgS3 coordinated species, while for
penicillamine (CAg(I) = 0.5 mol dm–3, L/Ag = 2.0) the mean Ag–S bond distance of
2.40 ± 0.02 Å and δ­(109Ag) 922 ppm indicate
that mononuclear AgS2 coordinated complexes dominate. For
Ag­(I)–glutathione solutions (CAg(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 109Ag NMR chemical
shifts, to assist assignments of the Ag­(I) coordination in solution.