I have recently started working on top-down proteomics and while working obtains many chromatograms of envelope shapes. Even the software has the option to deconvolute and identifies the molecular weight and charge state of the protein. But I am interested in understanding the basic concept of the calculation. Any help for this concept will be appreciable.
Hi,
Your question and the attached spectrum don't exactly match as these seem to deal with two different concepts: the one in your question is about the MS data of a charged protein that shows several charge states.
The number of charges can be determined by taking two adjacent charge states and performing the following calculation: n/((n+1)-n), where n is the m/z of the first charge state and (n+1) that of the adjacent charge state. If you then know the charge state of n, you can calculate its mass by multiplying that charge state with the corresponding m/z and subtract the number of protons that make up the charge state (i.e. 20 in the case of a 20+ ion).
Your attached example however, shows the isotope distribution of a single charge state. The charge of that one can be determined by subtracting two adjacent m/z values and take the invert of that (1/((n+1)-n)), which in this case'll tell you that that specific ion is probably 16+. Its mass can be determined by taking the monoisotopic mass (which is the very first m/z in the envelope), multiply that by 16 and substract 16: (946.00384*16)-16 = 15120.06 Da.
Hope this helps!
Hi,
It is not possible to identify the protein based on the spectrum you attached. Even though these seem to be accurate data that allow for the calculation of an accurate mass, it is not known whether this particular protein has some post-translational modifications that contribute to this mass (and its pI).
The theoretical pI can only be determined if you know the amino acid sequence (or could be estimated from a 2D gel image).
So, I think you will need some additional data, for example sequence information from a top-down fragmentation experiment, in order to be able to confidently identify this protein.
Good luck!
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