Peptide charge state

MH+, protonated molecular ion z, peptide charge state p, probability of a random match MW, protein molecular weight Sp, peptide preliminary score... 

G. Neubauer and R.J. Anderegg, Identifying charge states of peptides in liquid chromatography/electrospray ionization mass spectrometry, Anal. Chem., 66 (1994) 1056-1061. 

Separation by differential electrophoretic mobilities, charge state, and Stokes radii of peptides chromatographic interaction of peptides with the stationary phase both chromatographic interactions and electrophoretic mobility of the peptides... 

The question often arises, especially concerning the analysis of peptides and proteins, how is the charge state of the observed ion determined The answer is straightforward. The spectrum in Figure 10 below shows two peaks, one at m/z 1978 and another at m/z 990. 

Fortunately, as larger peptides and proteins are analyzed a distribution of ions is obtained. Even though we cannot look at the individual charge states from the isotopic pattern (because the resolving power of the instruments are too low) we can deduce the charge state by looking at two consecutive peaks in the mass spectrum [m/z and m/(z+1)]. All commercial instruments now allow for these calculations to be carried out very easily. 

Early mass spectrometers were simply equipped with a SQ mass analyser merely suitable for full scan MS mode and selected ion monitoring. Even though SQ spectrometers are highly superior to UV-detectors with respect to selectivity, they still bear the risk of deterioration by matrix compounds of similar m/z values. Especially, peptides and small proteins may cause a series of diverse m/z values due to their multiple charge states after ES ionization potentially interfering with the analytes or IS [103, 105],... 

The molecular weight of the peptide is deduced from its electrospray spectrum (Figure 9.2A). If we suppose that the observed peaks in this spectrum correspond to different charge state of the peptide, than we can calculate the number of charges z for the peak detected at m. z = j(mi - l)/( i2 -m i) where j corresponds to the number of peaks +1 separating m and m2 z has to be rounded to the nearest integer. The molecular weight is then M = zi(m - 1). 

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