Methylmercury(II) complexes with tryptophan and its N-acetyl derivative

Abstract

CH3HgOH reacts with tryptophan in aqueous ethanol to form a 2:1 neutral complex in which CH3Hg groups are bound to the terminal amino group and to the deprotonated indole nitrogen. When this complex is dissolved in water, 1H and 13C nuclear magnetic resonance show that the indole CH3Hg group hydrolyzes, leading to an amino-bound 1:1 species. Under the same conditions, N-acetyltryptophan forms a neutral 1:1 compound where the metal is bound to a carboxylate oxygen, and a neutral 2:1 complex in which an extra CH3Hg group is fixed to the deprotonated indole ring. It is shown by 1H and 13C nuclear magnetic resonance that the 1:1 species is stable in DMSO-d6, but that the 2:1 compound is readily hydrolyzed to the 1:1 compound by traces of water. The solid-state infrared and Raman spectra are discussed. In the crystal structure of the 2:1 tryptophan complex, the ligand adopts a conformation favoring a short intramolecular π interaction between Hg and the indole ring. No such π interaction is found in the crystal structure of the 1:1 carboxylate-bonded complex with acetyltryptophan.