The loss of NH3 on glutamine amino acid residues is also readily abundant, with this modification detected at Q100 (74.64% -Figure ), Q119 (73.38% -Figure ), and Q606 (5.44% -Figure ). Gln can naturally slowly undergo deamidation and it is suggested that mechanistically the deamidation process is similar to that of Asn, including the possible formation of a glutarimide-like intermediate . However, the abundance of such an intermediate compared to either Gln or its deamidated product glutamic acid (E, Glu) is generally minimal. Alternatively, it has been shown that peptides containing N-terminal glutamine can undergo partial deamidation in solution and thus it is possible some of the glutamine NH3 loss is occurring during the peptide digestion. Additionally, it has been shown that during collision induced dissociation (CID) MS analysis, NH3 loss readily occurs on peptides containing Gln to form a variety of ringed structures . Furthermore, deamidation is the predominant neutral mass loss on N-terminal glutamine when a peptide has no "mobile protons", meaning the charge state of the peptide is less than or equal to the number of basic amino acids (Arg, Lys, His) contained within it . Upon examination of the peptides containing glutamine NH3 loss, all peptides contain N-terminal Gln. The two with high abundance of this modification (>70%)