Hello everyone,
I have been working on my peptide sequence RAHAGDIQMK(Dnp)-OH and performed last decoupling with (95%TFA, 2.5 H2O, 2.5% Triisopropylsilyl) and at the end washed with ether. Expected mass was 1291.57 and I got 1378.56.
I am trying to figure out what is this 86.25 in my results? this mass matches Serine but I am pretty sure i never used this aminoacid.
I thought maybe one of my protecting group didnt cleaved properly and is shown in the graph. I looked for masses of Fmoc, t-Bu, Trt, Pbf but still no one matches with this mass.
I dont know what else i could look up to or could be possible reason for this big mass difference?
Hi,
Even though I do not have an explanation for the mass difference you observe on your peptide readily available, a few things come to mind when looking at your data:
What peak from the UV trace did you select to generate the mass spectrum provided? What masses do you detect in the other peaks observed?
A serine doesn't seem obvious, as there are no other serines in your sequence, so I assume no serine was available during synthesis?
How accurate is your measurement? It could also be a combination of things, e.g. an additional alanine and oxidation of your methionine (although that would result in a slighlty larger mass difference..)
Were data acquired on a Waters TOF or Q-TOF? In case of the latter, I would definitely advise to run MS/MS of your peptide, preferably of the doubly charged precursor (at m/z 690.26).
Hope this helps, good luck with the puzzle!
First, knowing which mass spectrometer was used here would help in finding an explanation. I assume the ionization source was electrospray, which gives leads to an expected m/z of 1292.6 for [M+H]+. Your peak is 1378.65, which means your molecule weighs 1377.6: uneven mass, so uneven amount of nitrogen atoms -> hence it cannot be one amino acid residue that was added (unless it would contain 2 nitrogens, but this does not make sense experimentally or based on these data).
I am not sure if the 690 peak is your double charges species, because this would give you a mass of 689.86.
How was the peptide synthesis performed? Manually? Automated? What coupling reagents? Capping with Ac2O after each coupling step?
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