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.