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Mass Spectrometry of <strong>Protein</strong> Phosphorylation 613<br />

Fig. 1. (A) Ion chromatograms of the tryptic peptide T 98 (SVTAVDGNEQLYHQQ-<br />

GDPQQSNEK, comprising residues 595–618) derived from the C-terminal tail domain of<br />

Drosophila melanogaster interphase lamin. The LC/MS data were scanned for ions with m/z<br />

predicted for the triply charged unphosphorylated (top) and the triply charged phosphorylated<br />

peptide (the calculated mass of the singly charged peptide is 2672.2 Da). The bottom panel<br />

shows the reconstructed ion current of the sum of all ions present during the LC/MS run. (B)<br />

Spectrum of the triply and doubly charged T 98 and its phosphorylated form (labeled with an<br />

asterisk). The spectrum was generated by summing all scans acquired during the elution of T 98<br />

and phospho-T 98. (Modified from ref. 17).<br />

peptide with a signal approx 27 Da higher for the triply charged (80 Da divided by 3),<br />

and 40 Da (80 Da divided by 2) higher for the doubly charged ion. Note that in the<br />

corresponding ion chromatograms, the phosphopeptide elutes slightly earlier than the non<br />

phosphorylated peptide, an effect that is frequently observed due to the higher polarity of<br />

the phosphate group.<br />

3.6. Stoichiometry of Phosphorylation<br />

Stoichiometries of phosphorylation can be quantified by dividing the sum of the<br />

peak areas of all intensities of a given phosphopeptide (all charge states must be

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