The influence glutamic acid in protonated b3 → b2 formation from VGEIG and related analogs.

A direct pathway for the fragmentation of peptide b3 fragment ions to b2 ions has, until now, not been identified. Experimental evidence for the formation of a b3 anhydride structure and isomerization to an extended macrocycle is demonstrated here by comparison of the completely different fragmentation patterns of the b3 ions generated from protonated VGEIG and its methyl ester. In particular, the absence of a b2 ion in the fragmentation spectrum of the methyl ester b3 indicates that facile fragmentation of an anhydride-type b3 is responsible for virtually all b2 ions formed. The stability of this b3 structure and the ease with which it fragments to the b2 may be responsible for the relatively high abundance of the b3 and b2 ions. IRMPD action spectroscopy measurements indicate the presence of a ring protonated oxazolone in the b2 population. VGEIG and three related analogs, VALEIG, VADEIG, and V(Aib)EIG were studied by QCID-HDX-SORI experiments in an FT-ICR instrument, and provide significant evidence for extensive alpha proton scrambling in an ion-molecule complex formed between the b2 and neutral loss fragment following formation of the b2. MS3 and HDX of VG(2,2-d2)EIG indicate that the scrambled b2 ions have the same structure as the unscrambled b2. Based on these data and with the support of molecular modeling, we propose a new mechanism for this scrambling, in which the alpha protons are transferred in a multistep pathway during an ion-molecule complex formed between the b2 and amino-terminated anhydride ring neutral loss component.