Cyclic peptides will undergo CID; however, and depending on where the
disulfide-linked cysteines are, sequence ions (i.e. those containing the
peptide N- or C-terminus and, therefore, best for providing sequence
information) are not usually abundant; instead, internal fragment ions are
often produced. This may be adequate if all you need is to confirm a known
sequence. In your case, with two disulfide bonds, you may not get much
fragmentation, but what you get may be enough to answer your questions.
You can, of course, reduce but not alkylate, thus avoiding having to desalt
your sample. Using triethylphosphine, for example, you can reduce the
disulfides and then remove the very volatile reducing agent in a speed vac
(caution, it smells really bad).
A couuple of references for cyclic peptide MS/MS:
M. Siegel, et al., Biol. Mass Spectrom. 23 (1994), 186-204.
K. Eckart, Mass Spectrom. Rev. 13 (1994), 23-55.
Ioannis Papayannopoulos
>I would like to tap the groups experience on MS/MS of peptides with
>disulfide bridges. Recently, we were brought a synthetic peptide with 2
>disulfide bridges in a 15-20-mer peptide. My first suggestion was to
>reduce/alkylate the sample, but I am wondering if anyone out there
>routinely gleans data from non-reduced forms of disulfide bridged
>peptides?
>
>On a similar theme, what approaches are taken for the interpretation of
>MS/MS of cyclic peptides?
>
>Thanks in advance.
>
>--
>Kevin Wheeler
>Sales Representative
>ThermoQuest Corporation
>PO Box 19811
>Seattle, WA 98109-6811
>Office: 206-583-0923
>wheeler@finnigan.com
>
>