Mercury(II) Complexation of Cysteine, Methyl Cysteineate, and S-Methylcysteine in Acidic Media

Bis(cysteineato)mercury(II) hydrochloride hemihydrate, [Formula: see text] was obtained by reaction of ethanolic solutions of L-cysteine hydrochloride with either HgCl2 in 95% EtOH, Hg(OAc)2 in MeOH, Hg(NO3)2 in HOAc, or by reaction of L-cysteine (free base) with HgCl2. Reaction of L-cysteine (free base) with Hg(OAc)2 gave a mercury(II) cysteineate with composition (C3H6O2NS)5Hg2. Bis(S-methyl-L-cysteineate)mercury (II), (C4H8O2NS)2Hg, was obtained by reaction of S-methyl-L-cysteine with Hg(OAc)2. 1H- and 13C-n.m.r. spectral data are reported and compared for acidic solutions of cysteine, methyl cysteineate, S-methylcysteine, and their mercury(II) complexes. The proton spectra for S-methyl-L-cysteine and L-cysteine hydrochloride have been analyzed as ABC systems and relative rotamer populations have been determined. In acidic solution, mercury bonding is localized to sulfur and the complexes appear to have an extended molecular configuration resulting from a trans orientation of the COOH group and the Hg site.