Tristhiolato Pseudopeptides Bind Arsenic(III) in an AsS Coordination Environment Imitating Metalloid Binding Sites in Proteins.

The As binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand ) or d-penicillamine residues (ligand ) as potential coordinating groups for soft semimetals or metal ions, was studied by experimental (UV, CD, NMR, and ESI-MS) and theoretical (DFT) methods. All of the experimental data, obtained with the variation of the As:ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS-type coordination mode. The UV-monitored titration of the ligands with arsenous acid at pH = 7.0 provided an absorbance data set that allowed for the determination of apparent stability constants of the forming species. The obtained stabilities (log' = 5.26 (As) and log' = 3.04 (As)) reflect high affinities, especially for the sterically less restricted cysteine derivative. DFT calculated structures correlate well with the spectroscopic results and, in line with the H NMR data, indicate a preference for the all- conformers resembling the As environment at the semimetal binding sites in various metalloproteins.