Barbara--
I can think of one instance in which you may not see correct spacing between
isotopic peaks of a multiply charged species. Our MS detector is an LCQ
classic which we generally run in centroid mode. If you do only a full MS
in centroid mode (i.e. no zoom scan), the spacing of the centroid peaks are
usually about 1 mass unit apart, even for multiply charged species.
However, your masses should be slightly different for monomers, dimers and
trimers, even if you can't tell which is which by the isotope spacing. Say
your peptide mass is 1000, the M+H peak for the oxidized cyclic peptide
should be 1000-2H+1=999. The M+2H for the dimer should be
(999+999+2)/2=1000. As for the appearance of three different peaks if they
are truly the same mass, is there any chance one (or more) of the chiral
centers has racemized? Or could the peptide have cyclized by some other
mechanism, e.g. glutamate to pyroglutamate?
Kara Pearson
NIH/NHLBI
Bethesda, MD 20892
>From: Barbara Ciani <barbarac@biols.susx.ac.uk>
>To: Recipients of ABRF List <abrf@aecom.yu.edu>
>Subject: MS intra and inter disulfides
>Date: Fri, 27 Oct 2000 15:43:04 +0100
>
>Dear all,
>
>I am having big troubles during the oxidation of a peptide with one Cys at
>each of the termini.
>The HPLC trace tells me that I have 3 peaks and the Mass-spec says that
>all 3 are the same thing: cyclic monomer disulfide linked (also the
>distance between the C13 and C12 isotopes is +/- 1 Da as it should be).
>My problem is how reliable is the isotope difference rule?
>In relation to the mass-spec:If I have dimers should I see a peak
>of the correspondent single charged
>ion or should I see just the double charged dimer?(that would have the
>same ion mass of the monomer?).
>In relation to the HPLC: why 3 different peaks for the same thing?
>
>Hope someone can help.
>
>Barbara
>
>
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