If anyone is interested, we found a reference (see below) for the strange
modification of 12 mass units that I described in my original note. In our
case it looks like imine formation on a Lys residue.
Mark Hail
Bristol-Myers Squibb
Princeton, NJ
> >L7 ANSWER 1 OF 1 CA COPYRIGHT 1998 ACS
> >AN 112:62735 CA
> >TI Characterization of impurities in a synthetic renin substrate
> > peptide by fast-atom bombardment mass spectrometry and hybrid tandem
> > mass spectrometry
> >AU Mathews, W. Rodney; Runge, Thomas A.; Haroldsen, Peter E.; Gaskell,
> > Simon J.
> >CS Upjohn Co., Kalamazoo, MI, 49001, USA
> >SO Rapid Commun. Mass Spectrom. (1989), 3(9), 314-19
> > CODEN: RCMSEF; ISSN: 0951-4198
> >DT Journal
> >LA English
> >AB 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.
> >
> >
> >> >All,
> >> >
> >> >I have a 3kD peptide with the sequence NH2-Pro-Lys-Lys-yadayadayada. In
> >> >a forced degradation study, peptide solid was subjected to 40 C/75%
> >> >humidity for 8 weeks (yikes). The peptide is an acetate salt. MS
> >> >indicates that the peptide undergoes a mass modification of 12 u. MS/MS
> >> >fragmentation appears to indicate that "the modification of 12" is on
> >> >the first lysine. About the only thing that I can come up with that
> >> >adds 12 is the formation of an immine from the lysine amino group (i.e.,
> >> >(CH2)4-NH2 --> (CH2)4-N=CH2). Has anyone seen anything like this before
> >> >or can you suggest alternative explanations? I can't seem to find any
> >> >reference of this in the usual places. Thanks for your help!