Evaluating the Role of the Ligand in the Fragmentation Reactions of Platinum(II) Complexes of Aliphatic Dipeptides. Electrospray Ionization Tandem Mass Spectrometry of [Pt(L 3 )M] 2+ and [Pt(L 3 )M — H] + Ions [L 3 = (H 3 N) 3,Diethylenetriamine and Terpyridine;M = Gly-Gly,Ala-Gly and Gly-Ala] †

Abstract

Incubation of simple aliphatic dipeptides (M) with [Pt(L₃)Cl]⁺ complexes (where L₃ = terpyridine, diethylenetriamine or triamine) results in the formation of platinated dipeptides as detected by electrospray ionization mass spectrometry. Although the [Pt(L₃)M – H]⁺ and [Pt(L₃)M]²⁺ ions represent less than 1% of the total-ion current, they can readily be mass selected and subjected to collision-induced dissociation in an ion trap mass spectrometer. The nature of the coordinating ligands and the charge state have profound effects on the types of fragmentation reactions observed. Thus, the tridentate ligands terpyridine (terpy) and diethylenetriamine (dien) induce dissociation of the peptide bond and the C-terminal OH bond in the [Pt(L₃)M]²⁺ ions, whereas the complex containing the three unidentate ammonia ligands fragments via neutral and charged ammonia loss. [Pt(terpy)M – H]⁺ ions fragment via peptide bond cleavage and also through a C-terminal rearrangement reaction for gly-ala and glycyl-N-methylglycine, but not for ala-gly. In contrast, [Pt(dien)M – H]⁺ ions fragment via loss of the neutral dipeptide while [Pt(NH₃)₃M – H]⁺ ions fragment via loss of NH₃.