First your peptide itself: the predicted MW you mention (978.1) is the
average MW; the monoisotopic MW calculated for your sequence is 977.5, in
good agreement with your ESI data, if the latter is for the most abundant
ion in the peptide ion cluster, which, at this mass, would be the 12-C
isotope.
The N-terminal Gln will form some pyroGlu (15% is certainly a reasonable
amount), especially in 1% Tfa, over a period of two days, even at 4oC. The
mass of the pyroGlu-containing peptide will be 17 u less than the original
(loss of ammonia), so 960.5. Are you sure that the ion at 961.5 is a
doubly charged ion? It seems to me that you may be looking at the singly
charged pyroGlu peptide. If this is the case (which, depending on your
instrument's mass resolution, should be obvious from the spacing of the
isotope peaks), then the ion at m/z 1923.1 is the mass spectrometric
"dimer" at m/z (2xMW+1), which should be 1922.0, but your data system may
be giving you the m/z value for the largest peak in the isotope cluster,
which, at this mass, would be the one 13-C containing isotope, at m/z 1923.
There was a discussion in this forum, about 5-6 months ago if I remember
correctly, on the nature of such mass spectrometric "dimers", which are not
true covalent dimers. The spectrum of the Gln-containing peptide should
have some "dimer" as well, at m/z (2x977.5)+1, although, for reasons I
could not explain, it may be that the pyroGlu-containing species yields a
more abundant such "dimer" (unlikely in my opinion).
Regards,
Ioannis Papayannopoulos
CytoMed, Inc.
Cambridge, MA
At 12:16 PM 4/28/98 -0400, VERNON SHOUP wrote:
>In one of my peptide maps, I have the following peptide:
>
> Predicted MW Observed MW (ESI)
> QYFYETK 978.1 977.4 (M+1 = 978.4)
>
>My tryptic digest is adjusted to 1% TFA to quench the digestion. Over two
days at 4C, about 15% of the peptide seems to convert to a peptide with a
mass of 1922.1. m/e
> Minor M+1 1923.1
> Major M+2 961.5
>
>This mass of 1922.1 is about 32-33 mass units less than twice the MW of
the original peptide. Does anyone have any ideas about what's going on?
I've considered dimerization with the loss of a couple of waters, but
mechanisms or possible structures elude me!
>
>Vernon
>
>Vernon A. Shoup
>QC, Regeneron Pharmaceuticals
>Rensselaer, NY 12144
>
>(518)488-6012
>vernon.shoup@regpha.com
>
>
>