Interpretation of matrix-assisted laser desorption/ionization postsource decay spectra of charge-derivatized peptides: Some examples of tris[(2,4,6-trimethoxyphenyl) phosphonium]-tagged proteolytic digestion products of phosphoenolpyruvate carboxykinase

The fragmentation of peptides, to which a positive charge is attached at the N-terminus, was studied by matrix-assisted laser desorption/ionization postsource decay mass spectrometry. In these experiments, the tris[(2,4,6-trimethoxyphenyl)phosphonium] acetyl group is covalently attached. The main advantage of this modification is that the resulting spectra are simplified and the fragment ions observed consist predominantly of a(n)-type ions. We report the results for charge-derivatized peptides formed following enzymatic digestion of phosphoenolpyruvate carboxykinase. Specific fragmentation of bonds within aspargine and threonine residues are observed and are discussed. The understanding of the mechanistic aspects of the fragmentation process is essential to formulate a simple and straightforward mass spectrometric strategy for peptide sequencing using these charged derivatives.