28 Da Shifts Observed in MALDI-MS Spectrum

Michael S Curtis (Michael_S_Curtis@bms.com)
Sun, 28 Mar 1999 15:16:59 -0500

I'm working with a tryptic peptide isolated from a spot on a 2D TLC
phosphopeptide map. In the MALDI-MS mass spectrum of this peptide I
observe two sets of major peaks all differing by 28 Da; 600, 628, 656
m/z and 820, 848, 876, 904 m/z. In addition I see a series of minor
peaks running from 686 to 792 m/z in 28 Da increments. I can't for the
life of me figure out where this 28 Da business is coming from. The
peptide was extracted from the cellulose TLC plate using water
/butanol/pyridine/acetic acid, concentrated and reconed in 50/50/0.1
ACN/H2O/TFA. Mass spectrum was acquired using alpha-CN as matrix.

I've checked delta mass; it lists four possibilities, none which seem
likely (ethyl, formylation, N,N-dimethylation of Arg or Lys, and
2,4-BisTrp-6,7-dione from Tryptophan). Any help in figuring this out
would be greatly appreciated.

Thanks.

Mike Curtis
SUNY HSC / Bristol-Myers Squibb Co.
Syracuse, NY