1. Error in synthesis, amino acid misincorporation, or mutation (you don't
say whether your peptide is synthetic or natural/recombinant). I assume
you have checked these possibilities.
2. Deamidation of two Asn residues to form aspartic acid imides (loss of
two ammonia molecules). Does your sequence have Asn-Gly (or Asn-Ala or
Asn-Ser) sequences?
3. Loss of H2S from one cysteine before disulfide bond formation to form
dehydroalanine. This would result in your peptide having one free
cysteine, which should be easy to check for, plus it would likely form an
intermolecular disulfide, but this might be difficult to differentiate from
the mass spectrometric non-covalent dimer. However, this species (the
monomer, that is) should have a mass of 3459 [3491 + 2 (two reduced Cys) -
34 (loss of H2S)].
Ioannis Papayannopoulos
CytoMed, Inc.
Cambridge, MA
At 03:43 PM 7/6/98 +0100, Bryan Dunbar wrote:
>Hi guys,
> I have been sequencing a 31-residue polypeptide which contains 6
>cysteines. The calculated mass from the sequence is 3491, assuming all
>the Cysteines are involved in disulphides, which by homology, is pretty
>definite. However, when we look at this thing in the Voyager, we always
>get a mass of 3457 - anybody have any bright ideas about this "loss" of
>34??
>Best regards,
>Bryan.
>--
>Bryan Dunbar,
>Protein Facility,
>Room WT27,
>Department of Molecular & Cell Biology,
>University of Aberdeen,
>Polwarth Building,
>Foresterhill,
>Aberdeen,
>SCOTLAND.
>AB25 2ZD.
>TEL: 01224-273103
>FAX: 01224-273104
>e-mail; b.dunbar@abdn.ac.uk
>
>