X-Ray crystallographic and spectroscopic studies of the binding of methylmercury by the sulphur amino-acid DL-penicillamine: models for methylmercury poisoning of proteins

Abstract

Methyl-DL-penicillaminatomercury(II) monohydrate, [HgMe(pen)]·H₂O (pen =DL-penicillaminate), and µ-DL-penicillaminato-bis[methylmercury(II)], [Hg₂Me₂(pen)], were prepared from appropriate amounts of methylmercury chloride (or hydroxide) and DL-penicillamine in 50% aqueous ethanol solutions. Crystals of [HgMe(pen)]·H₂O are colourless monoclinic prisms with a= 23.154, b= 9.819, c= 19.321 Å, and β= 106.4°; space group C2/c, Z= 16. Crystals of [Hg₂Me₂(pen)] are colourless orthorhombic needles with a= 19.695, b= 19.695, b= 10.478, and c= 11.971; space group, Pbcn, Z= 8. Structures of both compounds were solved from single-crystal diffractometer data with 1 047 and 1 254 independent observed reflections for [HgMe(pen)]·H₂O and [Hg₂Me₂(pen)] respectively. Final R factors were 0.079 and 0.075 respectively. In [HgMe(pen)]·H₂O, the amino-acid, present in the zwitterionic form -SC(CH₃)₂CH([graphic omitted]H₃)COO^–, is co-ordinated to methylmercury via a deprotonated sulphydryl group and in [Hg₂Me₂(pen)] the methylmercury groups are bonded to the deprotonated sulphydryl and amino-groups. The important average bond distances are: Hg–CH₃, 2.07; Hg–S, 2.36; and Hg–N, 2.13 Å. The two bonds about the mercury atoms deviate slightly but significantly from linearity because of weak interactions from neighbouring sulphur or oxygen atoms. The vibrational and n.m.r. spectra are discussed in the light of the crystal structures. Diagnostic criteria of bonding modes are assessed.