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Letter: Formation and Fragmentation Reactions of Platinum-Terpyridine Complexes of Glycine and Cysteine and Their Methyl Derivatives via Electrospray Ionization Mass Spectrometry

Under electrospray ionization (ESI) conditions, several ions are observed from a 1.5 × 10−6 M solution of [Pt(terpy)Cl]Cl in water including: [Pt(terpy)Cl]+ (base peak), [Pt(terpy)HO]+ and [Pt(terpy)H2O]2+. When a 30- to 40-fold excess of glycine or its N- or O-methyl derivatives was added, the ESI mass spectrum was still dominated by the [Pt(terpy)Cl]+ ion, although a number of new ions were observed including [Pt(terpy)M]2+, [Pt(terpy)M – H]+, [M + H]+ and [2M + H]+ ions (where M = the amino acid). In contrast, when a 10-fold excess of cysteine was used, the [Pt(terpy)Cl]+ ion virtually disappeared and the dominant ion became the [Pt(terpy)M – H]+ ion. These results are consistent with the known solution chemistry of [Pt(terpy)Cl]Cl, which shows a preference for binding via the thiolate of cysteine residues. In addition, collision-induced dissociation was performed on both the [Pt(terpy)M]2+ and [Pt(terpy)M – H]+ ions of glycine and cysteine. Isotopic labeling and methyl derivatives were used to elucidate the mechanisms of their fragmentation reactions.