9780521516358book.pdf

9.5.4 Carbon isotopes and finding the charge state of a peptide

Since the mass detector operates on the basis of mass-to-charge ratio (m/z), mass

assignment is normally made assuming a single charge per ion (i.e.m/z¼mþ1in

positive ion mode). However, since there is around 1.1%^13 C natural abundance, with

increasing size, peptides will have a greater chance of containing at least one^13 C and

two^13 C, etc. A peptide of 20 residues has approximately equal peak heights of the ‘all

(^12) C peptide’ and of the peptide with one (^13) C.
A singly charged peptide will show adjacent peaks differing in one mass unit; a
doubly charged peptide will show adjacent peaks differing in half a mass unit and so
on (Fig. 9.23 and Table 9.3). In the example illustrated, the peptide has a mass
Table 9.3Mass differences due to isotopes in multiply
charged peptides
Charge on peptide Apparent mass
Mass difference between
isotope peaks
Single charge [(MþH)/1] 1 Da
Double charge [(Mþ2H)/2] 0.5 Da
Triple charge [(Mþ3H)/3] 0.33 Da
ncharges [(MþnH)/n]1/nDa
m/z
324.95 +4
325.17
325.42
325.68
(324.93)
m/z
324.5 325.0 325.5 326.0 326.5
0
20
40
60
80
432.93 +3 100
433.26
433.58
(432.90)
0
20
40
60
80
100
% Intensity
m/z
433 434
+1
(1296.69)
1296.65
1297.64
1298.64
1296 1298 1300
0
20
40
60
80
100
% Intensity
648.82 +2
649.32
649.81
650.31
(648.85)
648 649 650 651
m/z
0
20
40
60
80
100
Fig. 9.23Spectra of a multiply charged peptide. Finding the charge state of a peptide involveszooming in
on a particular part of the mass spectrum to obtain a detailed image of the mass differences between different
peaks that arise from the same biomolecule, due to isotopic abundance. This is mainly due to^12 C and its
(^13) C isotope, as described in the text.
385 9.5 Structural information by tandem mass spectrometry