Electrospray mass spectrometry has been used to investigate the interaction of the organomercury(II) cations MeHg⁺ and PhHg⁺, and the metal ions Ag⁺, Zn²⁺, Cd²⁺ and Pb²⁺, with a polypeptide and two proteins, bovine ubiquitin and horse heart myoglobin. The addition of these metal ions resulted in the formation of metal replacement ions as well as the normally observed multiply protonated species.

The high affinity of the organomercury(II) ions for the peptide was clearly shown by the very low concentrations (10µM) required to produce organomercury-containing adducts in the absence of solely protonated ions. However, the binding abilities of the organomercury ions PhHg⁺ and MeHg⁺ are very different. Up to six PhHg⁺ ions, compared to only two MeHg⁺ ions, were observed to bind to the peptide, which suggests that they have different affinities for different sites on the polypeptide chain.

A most probable site for organomercury adduction is the sulfhydryl group of cysteine. The polypeptide contains only one cysteine residue. When this was modified by blocking the sulfhydryl group, no organomercury adducts were observed at low salt concentrations. For higher concentrations, one less organomercury ion was found to bind to the peptide, confirming the strong interaction between organomercury ions and cysteine.

Silver(I) ions are also known to have strong affinities for thiols, and up to six Ag⁺ ions were observed to bind to the peptide. At 20 µM salt concentrations very strong signals corresponding to the adduction of two Ag⁺ ions were observed. For the modified peptide, one less Ag⁺ ion was found to adduct, with very intense signals corresponding to the adduction of one Ag⁺ ion. This again confirms metal adduction at the cysteinyl sulfur.

The addition of the Zn²⁺ and Cd²⁺ produced only very weak interactions with both the modified and the un-modified peptide. In both cases the adduction of two metal ions was observed, demonstrating the absence of any interaction with cysteine.

The order of affinity of the organomercury(II) and metal salts towards horse heart myoglobin and bovine ubiquitin, neither of which contains a cysteine residue, was also investigated. The organomercury salts PhHg⁺ and MeHg⁺ showed very high affinities for the proteins compared to the other metal ions, with the Zn²⁺ ion having the lowest affinity. The protein affinities of the metal ions Cd²⁺, Pb²⁺, and Ag⁺ were very similar. From experiments involving protein solutions electrosprayed in the presence of equimolar concentrations of two metal salts, the relative order of affinity of the metal ions studied was determined as PhHg⁺ > MeHg⁺ > Ag⁺ > Pb²⁺ > Cd²⁺ > Zn²⁺.