Positive and negative ion electrospray mass spectrometric studies of some amino acids and glutathione, and their interactions with alkali metal ions and methylmercury(II)

Positive and negative ion electrospray mass spectrometry (ESMS) has been used to investigate solutions of some simple amino acids (glycine, histidine, methionine and cysteine) and the small peptide glutathione. Negative ion ESMS detected the simple anions [AA-H-] in each case (AA = amino acid) and often the dimeric species [(AA)2-H]- were also observed. In the positive ion mode ESMS allowed the observation of the protonated cluster ions [H(AA)x]+ (x = 1-4). Some interactions between alkali metal ions and the amino acids were observed, but in general the peaks in the ES mass spectra were weak and the alkali metal complexation could not compete with protonation. In contrast, methylmercury(II) reacted strongly with the amino acids and glutathione. In addition to species of 1:1 stoichiometry, all the amino acids gave peaks in their ES mass spectra corresponding to the ions [(MeHg)2(AA-H)]+, while glutathione gave a clear indication of the formation of a 3:l species.