Dear Barb,
You should definitely be able to tell the difference between dimers
(intermolecular disulfides) and intramolecular disulfides by MS.
As others have pointed out, if the resolution on your mass spec is low,
isotopic information may not distinguish these.
Additionally, some of the dimer ions will have the exact m/z as the monomer.
However, the dimer will have some ions that the monomer will not. For
instance:
Assume peptide of MW 2000. Disulfide MW would be 1998.
(M+H)+ (M+2H)+2 (M+3H)+3
(M+4H)+4
monomer (m/z) 1999 1000 667
500.5
dimer (m/z) 3997 1999 1333
1000
If you can only scan to 2000, you would note that the dimer has an M+2H and
M+4H the same as the M+H and M+2H of the monomer. However, the +3 ion (1333
in this example) of the dimer would distinguish here as it does not appear
in the monomer series. The other monomer ions would show up in the dimer
series (at 2x the charge of the corresponding monomer ions).
If you don't see such a distinguishing ion I don't believe that any of your
peaks are the dimer. Other explanations for the three components of
apparent same mass I'll leave up to the peptide synthesis gurus.
Best of luck
Randy Wilhelm
Sr. Analytical Research Chemist
Mallinckrodt, Inc
(314)654-3965
(314)654-3355
rrwilhe@mkg.com <mailto:rrwilhe@mkg.com>
-----Original Message-----
From: Barbara Ciani [mailto:barbarac@biols.susx.ac.uk]
Sent: Friday, October 27, 2000 9:43 AM
To: Recipients of ABRF List
Subject: MS intra and inter disulfides
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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