Gas-phase structures and proton affinities of N-terminal proline containing b2+ ions from protonated model peptides

- The intensity of AAAA fragment from b6 ions (PXAAAA where X = A, F, E, W and H) is higher than PA, PF, and PE fragments.
- However, it is lower than PW and PH fragments in mass spectra.
- In order to explain this experimental observation, proton affinities of fragment ions were computed considering both diketopiperazine and oxazolone structures of PX ions.
- Whether the structure of b2+ ions is oxazolone or diketo could be elucidated by comparing their proton affinities of those two isomers.