You have isolated the proteins from two other adjacent spots

You have isolated the proteins from two other adjacent spots after 2D electrophoresis and digested them with trypsin. Following MALDI-TOF mass spectrometry, the 2 two proteins were found to be identical except for one peak. For this peak, the mass-to-charge ratio (m/z) values differed by 80 g/mol (or daltons), a value that does not correspond to any discrete difference in amino acid sequence. (For example, glutamic acid i instead of valine at one position would give an m/2 difference of around 30.) Can you suggest a possible difference between the two peptides that might account for the observed m/z difference? m/z (mass to chare ratio)

Solution

Answer:

The Glycine residue has molecular weight = 57.0513 and the Histidine residue has molecular weight = 137.1393

So the difference in molecular weight = 137.1393 - 57.0513 = 80.088

So the possible difference of mass -to-charge ratio (m/z) in a the above one peak is might be the histidine instead of glycine in one position for which the difference in m/z is around 80.

*The residue means after removal of water molecule from aminoacid.