The b n ions observed from the fragmentation of the protonated peptides are believed to be oxazolones formed by nucelophilic attack by a carbonyl oxygen on the adjacent peptide bond. Here, we report possible mechanisms for water loss from protonated tetraglycine to generate b 4 ions with different structures. Our density functional calculations show that the reaction starts with nucleophilic attack on the N-terminal amide group by the nitrogen of the second amide producing N 1 -protonated 3,5-4H-imidazol-4-one, has the lowest critical barrier. This is the product observed in infrared multiple-photon dissociation spectroscopy. Formation of an oxazolone by dehydration of the carboxylic group has an almost identical barrier and is probably the minor product as deduced from the collision-induced dissociation spectrum of protonated tetraglycines whose amide oxygens have been individually labeled with 18 O. Losses of water from the other amides have slightly higher barriers. Formation of an oxazole initiated by nucleophilic attack by an amide oxygen on an adjacent protonated amide has a lower energy barrier to form an early tetrahedral intermediate but overall is not competitive as the subsequent proton transfer step has a barrier that is higher than those on the other pathways.