Mass difference of -18Da

Heather Peto hp217 at
Fri Apr 27 05:45:47 EST 2001


I have a peptide of 28 AA's. After leaving it in acidic conditions for a
week I run the mass spect and a proportion
gains a mass of 18Da (I assume that to be either Methionine oxidation
(+16Da, (but where are the other two Daltons?) or the addition of water
chemically, but where and how?),

But interestingly I also get a significant peek with -18Da!
What could this -18 be? One guess of my own is that it is a cyclisation
of the N-and C-termini with the by product of water. (How could I test
this?, does the low pH (pH3) facilitate this?)
Any other guesses? The +18 and -18 peaks are roughly the same size.

Heather :)

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