Dear Colin,
how did you check your mass? ESI or MALDI? I've observed such a mass shift
of a peptide which looked perfectly o.k. in MALDI when I measured it by
ESI-MS. It was simply an acetate salt which we could prove by applying a
low collision energy to the peptide which resulted in a dramatic decrease
of the adduct signal and yielded the free peptide base. What makes me
wondering a little bit is the new HPLC peaks you mentioned. If its just a
salt, this should probably not be observed. Have you checked the peaks
seperately by MS? What's their content? You also mention "extra masses". Do
you observe multiple addition of AcOH? If it is a real modification of the
peptide, you should observe some kind of gaussian distribution of the
intensity of different numbers of incorporated modifications due to
statistical effects. Furthermore, have you already made a MS/MS experiment
of the modified peptides? If its a covalent attachment, you should be able
to identify the modification sites by this way.
We routinely work up our peptides with acetic acid containing solutions and
I have so far never observed a covalent modification of a peptide under
these conditions.
Hope that helps a bit. Yours sincerely,
Marcus Macht
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Dr. Marcus Macht
University of Cologne
Centre for molecular medicine - Service laboratory
Joseph-Stelzmann-Str. 52
50931 Cologne, Germany
Tel.: +49 221 478-6995
Fax: +49 221 478-6977
e-mail: Marcus.Macht@uni-koeln.de
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