Imidazolone formation from protonated tetrapeptides: Effects of replacing a glycine by an alanine or proline residue

Abstract

The propensity for water loss in protonated tetrapeptides that contain a hetero-residue in addition to glycines is examined. Protonated tetraglycine loses water prominently from its first amide group, with a minor contribution from the second amide group, to give a nominal [b4]+ ion. This chemistry is also apparent in protonated tetrapeptides that contain three glycine residues and one proline or alanine residue. When the alanine is at the N-terminus or in the third position of the peptide, abundances of the [b4]+ ions are low; when the alanine is in the second position, the abundance of [b4]+ ion is much higher, and is comparable to that obtained from tetraglycine. The [b4]+ ion is a protonated imidazolone formed by attack on the carbon of the first amide bond by the nitrogen of the third amide, both sites being partially obstructed by the methyl group of the alanine residue.All the [b4]+I ions (the Roman numeral refers to the residue from which the oxygen (water) was lost) dissociate by loss of an imine from the residue that is initially located at the N-terminus of the peptide. The [b4]+II ions dissociate by loss of either the imine from the second residue, via pre-dissociation rearrangement to a [b4]+I ion, or by loss of (imine plus CO) from the first residue; the latter involves a cleavage of the first amide bond. These dissociations follow the pathways observed for the protonated imidazolone ions derived from GlyGlyGlyGly. In addition to these losses, the [b4]+II ions of both GlyProGlyGly and GlyAlaGlyGly also lose HN = CH2 from the first residue, but only at higher collision energies. This requires that the [b4]+II ions rearrange to protonated imidazolones that bear an H2NCH2– group proximal to the imidazolone ring at the N-terminus, i.e., forming the [b4]+I ions of GlyProGlyGly and GlyAlaGlyGly. This rearrangement requires the transfer of an oxygen atom from the first amide group to the second α-carbon. Similarly, the [b4]+I ion of ProGlyGlyGly dissociates by loss of imines from both the first two residues; again loss of the ‘second’ imine, HN = CH2, only occurs at higher collision energies and requires rearrangement to the [b4]+I ion of GlyProGlyGly.