Enrico:
Tetramethylguanidium by-product formation can hardly explain your
observations because they are always derived from truncated sequences
and not from the full-length peptide.
Mark Carter's suggestion with minor reservations is closer to the point.
+98 is sulfuric acid exactly, so it should be sulfate (not sulfite) salt
of your peptide.
In the presence of water, primary Pbf-derived cleavage product,
arylsulfonic acid, should undergo partial proto-desulfation and give
rise to free sulfuric acid. The latter is non-volatile and much stronger
than TFA, and, in my chemical opinion, peptide mono and disulfates may
be stable enough to be seen in MS.
To avoid such artifacts, it seems reasonable to "revive" a
post-cleavage batchwise treatment of peptide solution with acetate forms
of Amberlite CG-4B, IRA-410, Dowex 1, etc.
Regards,
Igor Rodionov
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