Mass difference of -18Da

Dr. Artem Evdokimov eudokima at
Fri Apr 27 07:24:11 EST 2001


Did you run the mass spectrum on a freshly dissolved peptide, maybe
injected under neutral conditions ? Does it show the same two peaks ?

Could you elaborate a bit more on your solvent system, type of acid and
the pH/temperature of storage. Also was the sample exposed to sunlight
for extended periods of time ?

+18 could be plus one fluoride, minus one hydrogen (is there TFA present
it also could be a tightly bound water molecule. Methionine oxidation
would also give you extra peaks - i.e. when the not-so-stable selenoxide
undergoes further oxidation and decomposition. Do you have any other
peaks in your MS ?

-18 does sound like loss of water - and it can happen via several
mechanisms, not just cyclisation of the ends. 

Heather Peto wrote:
> Hi
> 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 :)

|Dr. Artem Evdokimov   Protein Engineering |
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