Charge remote fragmentation of peptides following attachment of a fixed positive charge: A matrix-assisted laser desorption/ionization postsource decay study

A fixed positive charge can be placed at the N-terminus of a peptide by addition of a tris[(2,4,6-trimethoxyphenyl)phosphonium]acetyl group. The usefulness of these charged derivatives has been demonstrated in fast atom bombardment mass spectrometry and in matrix-assisted laser desorption/ionization mass spectrometry. After ion formation and acceleration, these derivatized peptide ions dissociate and their fragment ions can be analyzed in a postsource decay experiment by using a time-of-flight mass spectrometer. The matrix-assisted laser desorption/ionization-postsource decay spectra are very different from what may be expected based on fragment ions observed from protonated peptide molecules. Cleavage of CHR-C(O) bonds dominates to form a series of a-type ions. Mechanistic possibilities are evaluated. When aspartic acid residues are encountered, the chemistry radically changes. This appears to be due to the formation of geometrically accessible intermediates that can dissociate via low energy processes. This charge-remote chemistry parallels that for much simpler systems, resulting in spectra that are very easy to interpret. (C) 1997 American Society for Mass Spectrometry.