Evidence of Diketopiperazine and Oxazolone Structures for HA b2 + Ion

Peptide fragmentation can lead to an oxazolone or diketopiperazine b2 + ion structure. IRMPD spectroscopy combined with computational modeling and gas-phase H/D exchange was used to study the structure of the b2 + ion from protonated HAAAA. The experimental spectrum of the b2 + ion matches both the experimental spectrum for the protonated cyclic dipeptide HA (a commercial diketopiperazine) and the theoretical spectrum for a diketopiperazine protonated at the imidazole pi nitrogen. A characteristic band at 1875 cm−1 and increased abundance of the peaks at 1619 and 1683 cm−1 indicate a second population corresponding to an oxazolone species. H/D exchange also shows a mixture of two populations consistent with a mixture of b2 + ion diketopiperazine and oxazolone structures.