Electron Transfer Dissociation Mass Spectrometry of Acidic Phosphorylated Peptides Cationized with Trivalent Praseodymium.

The lanthanide ion praseodymium, Pr(III), was employed to study metallated ion formation and electron transfer dissociation (ETD) of 27 biological and model highly acidic phosphopeptides. All phosphopeptides investigated form metallated ions by electrospray ionization (ESI) that can be studied by ETD to yield abundant sequence information. The ions formed are [M+Pr-H](2+), [M+Pr](3+), and [M+Pr+H](4+). All biological phosphopeptides with a chain length of seven or more residues generate [M+Pr](3+). For biological phosphopeptides, [M+Pr](3+) undergoes more backbone cleavage by ETD than [M+Pr - H](2+) and, in some cases, full sequence coverage occurs. Acidic model phosphorylated hexa-peptides and octa-peptides, composed of alanine residues and one phosphorylated residue, form exclusively [M+Pr - H](2+) by ESI. Limited sequence information is obtained by ETD of [M+Pr - H](2+) with only metallated product ions being generated. For two biological phosphopeptides, [M+Pr+H](4+) is observed and may be due to the presence of at least one residue with a highly basic side chain that facilitates the addition of an extra proton. For the model phosphopeptides, more sequence coverage occurs when the phosphorylated residue is in the middle of the sequence than at either the N- or C-terminus. ETD of the metallated precursor ions formed by ESI generates exclusively metallated and nonmetallated c- and z-ions for the biological phosphopeptides, while metallated c-ions, z-ions, and a few y-ions form for the model phosphopeptides. Most of the product ions contain the phosphorylated residue indicating that the metal ion binds predominantly at the deprotonated phosphate group. The results of this study indicate that ETD is a promising tool for sequencing highly acidic phosphorylated peptides by metal adduction with Pr (III) and, by extension, all nonradioactive lanthanide metal ions.