Fragmentation reactions of b(5) and a (5) ions containing proline--the structures of a(5) ions.

A detailed study has been made of the b(5) and a(5) ions derived from the amides H-Ala-Ala-Ala-Ala-Pro-NH(2), H-Ala-Ala-Ala-Pro-Ala-NH(2), and H-Ala-Ala-Pro-Ala-Ala-NH(2). From quasi-MS(3) experiments it is shown that the product ion mass spectra of the three b(5) ions are essentially identical, indicating macrocyclization/reopening to produce a common mixture of intermediates prior to fragmentation. This is in agreement with numerous recent studies of sequence scrambling in b ions. By contrast, the product ion mass spectra for the a(5) ions show substantial differences, indicating significant differences in the mixture of structures undergoing fragmentation for these three species. The results are interpreted in terms of a mixture of classical substituted iminium ions as well as protonated C-terminal amides formed by cyclization/rearrangement as reported recently for a(4) ions (Bythell, Maître , Paizs, J . Am. Chem. Soc. 2010, 132, 14761-14779). Novel fragment ions observed upon fragmentation of the a(5) ions are protonated H-Pro-NH(2) and H-Pro-Ala-NH(2) which arise by fragmentation of the amides. The observation of these products provides strong experimental evidence for the cyclization/rearrangement reaction to form amides and shows that it also applies to a(5) ions.