what is your opinion, if my peptide mass is 1137.474 and in MALDI mass obtained is 1183.663? The difference is 46 does it mean the addition of 2Na? As Na is 23
Hi,
If your peptide ion indeed contains two sodium ions, you should be observing a doubly charged ion [M+2Na]2+; is that the case? If not, then something else is going on..
Are you expecting a lot of sodium in your sample? Usually, sodium adducts can be observed but then in the presence of the protonated ion.
What's the peptide sequence and what were the conditions you used to acquire the MALDI spectrum?
Hope this helps!
Hi,
What is the molecular formula of your peptide? Is 1137.474 a monoisotopic mass or average molecular mass? Exactly, if there have been two Na+ added, the ion formed would be doubly-charged and appeared at m/z (1137.474+2x22.9898)/2 = 591.7268 and not at 1183.663. By the way, is a [M+Na]+ ion present in your spectrum? (that one should be singly-charged and appear more less at m/z = M+23).
Dear Grazyna Bartkowiak madam, the molecular formula of peptide is given below
C51H71N13O13S2
Exact Mass: 1137.474
Mol. Wt.: 1138.319
is it M+2Na-H as the difference is 45.351.
No their is no [M+Na]+ ion present
Dear Dr. Ashok,
The peptide's exact mass is calculated precisely - but after substraction of one H and addition of two Na it should be C51H70N13O13S2Na2, which gives a signal at m/z 1182.445, so it means there is a certain discrepancy. Maybe, something else is going on (citing Eef Dirksen).
If you only see the monoisotopic peak at m/z 1183.663, but no other peaks which could be attributed to (M+H)+, (M+Na)+, etc, then most likely it doesn't correspond to (M-H+2Na)+. When salts are present in the sample, you will generally see peaks attributable protonated, mono, di, tri sodiated with varying intensities generally decreasing in the same order.
If your MALDI is well calibrated 2 sodiums should be +44Da not 46 Da. You can determine if your mass is the monoisotopic or not , opening the window of your spectra peak and observe the lower value. Also, check which parameters did you use for your measures (linear mode or reflector mode), some Maldi methods only give you the average of the mass and not the monoisotopic. This link can help you,
https://www.abrf.org/index.cfm/dm.home?AvgMass=all
I already obtained those 2 sodiums peaks with several matrixes (alpha cyano for example) but indeed there were also all the other peaks (M+H; M+Na,...). If possible, you could check in negative ionisation and with other matrixes, it should help you to be sure the ion is really your peptide.
What about adding potassium ion? (KCl, KF..)
If your peptide really has 2 sodiums, potassium will replace sodium. Then it may gives you different peak.
Is your peptide a synthetic salted compound in origin or completely desalted?
Dear Javier Dotor sir
The peptide is purified by HPLC. Hope it should be desalted
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