DeltaMass lysine epsilon amino to imine + 12 amu 12.0 12.010000228881835938 140.0949554443359375 140.19000244140625 C 1 H 0 N 0 O 0 C 7 H 12 N 2 O 1 From DeltaMass: Average Mass: 12 Average Mass Change: 12 References: Mathews, W. Rodney; Runge, Thomas A.; Haroldsen, Peter E.; Gaskell, Simon J. (1989) Characterization of impurities in a synthetic renin substrate peptide by fast-atom bombardment mass spectrometry and hybrid tandem mass spectrometry. Rapid Commun. Mass Spectrom. 3(9), 314-19 Notes: Fast-atom bombardment mass spectrometry of a synthetic renin substrate decapeptide (Pro-His-Pro-Phe-His-Leu-Val-Ile-His-D-Lys) indicated the presence of several side products, including a component 12 Da higher in mass. Low-energy collisionally activated ***decompn*** analyses were performed using a hybrid tandem instrument and demonstrated that the heavier side product had two components, in which the structural modification was either at the N- or the C-terminus. Addnl. analyses of the N-acetyl deriv. indicated that for each component the structural modification blocked a site of N-acetylation. It is suggested that the formation of these side products is attributable to the generation of formaldehyde, during removal of the histidine protecting group (benzyloxymethyl), which reacts with the N-terminus of the peptide to give an imidazolidinone structure or with the D- ***lysine***.epsilon.-amine group to yield an ***imine*** . While the precise genesis of the side-products remains speculative, it is clear that the combined strategy of derivatization and tandem mass spectrometry has allowed structural conclusions concerning individual components of an isobaric mixt. K MOD:00964 N6-(methylidene)-lysine PSI-MOD artifact modification from DeltaMass