An Electrospray Mass Spectrometric Study of Organomercury(II) and Mercuric Interactions with Peptides Involving Cysteinyl Ligands

Positive ion electrospray mass spectrometry (MS) has been used to investigate the interaction of Hg²⁺, [MeHg]⁺ and [PhHg]⁺ cations with cysteine (Cys), glutathione (GSH) and a 27-residue polypeptide (Pp) containing one cysteinyl sulfur located at the N-terminus. MS/MS experiments showed that organomercury adduction occurs primarily at the sulfhydryl group, with some evidence for isomeric species in which the organomercury cation is bound to either an amino or a carboxylic group. Following Ellman modification of GSH and Pp, the maximum number of adducted organomercury cations was reduced by 2 and 1, respectively, indicating a 2:1 and a 1:1 interaction between [RHg]⁺ and the cysteinyl sulfur. Unlike [PhHg]⁺, [MeHg]⁺ showed an almost exclusive affinity for the cysteinyl sulfur of GSH and Pp. Both Cys and GSH reacted with Hg²⁺ to form polynuclear species. Collisional activation mass spectra of the [2Cys + Hg – H]⁺ ion indicated that the Hg(II) was most probably bridged between the two cysteinyl residues, although not necessarily via a conventional –S–Hg–S– linkage.