Tristhiolato Pseudopeptides Bind Arsenic(III) in an AsS₃ Coordination Environment Imitating Metalloid Binding Sites...

The As^III binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand L) or d-penicillamine residues (ligand L) 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^III:ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS₃-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 (logK' = 5.26 (AsL) and logK' = 3.04 (AsL)) 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 ¹H NMR data, indicate a preference for the all-endo conformers resembling the As^III environment at the semimetal binding sites in various metalloproteins.