Collison-Induced Dissociation of 4-imidazolone Ions; Isomers of [bn]+ Derived from Peptides After Water Loss

Collision-induced dissociation spectra of protonated peptides [(aminoacid)n + H]+ form, after the loss of water, nominal [bn]+ ions which are dependent on both peptide length and amino acid sequence. Protonated tetraglycine loses water predominantly from the first amide; loss from the second amide is a minor channel. The resultant [b4]+ ions have identical CID spectra, indicating interconversion prior to dissociation. Protonated pentaglycine loses water from the first and second amides in high abundances, and the resulting CID spectra of the [b5]+ ions are different, suggesting that interconversion is no longer the dominant pathway. The [b6]+ ions of hexaglycine, formed from the first three amides are in high abundance and their CID spectra are not identical. The substitution of an alanine or proline for a glycine of tetraglycine generally showed reduced water loss and the [b4]+ ions had different fragmentation pathways than those of the [b4]+ ions of tetraglycine.