Top-Down Mass Analysis of Protein Tyrosine Nitration: Comparison of Electron Capture Dissociation with “Slow-Heating” Tandem Mass Spectrometry Methods

Figure 3. ECD (a) and IRMPD (b, c) of mono- (b) and bis-nitrated (a, c) cytochrome c. N-acetylated glycine, nitrated tyrosines, and cysteines
bound to the heme group are circled.

limited cleavage of the disulfide bonds by ECD. Cleavage of all
four S-S bonds in lysozyme would require the capture of at least
four electrons. We did record up to four consecutive electron
capture events taking place for the 10+ charge state of nitrated
lysozyme, which was indicated by the presence of (M + 10H)⁶⁺
charge-reduced ions in the ECD mass spectra. There is a
possibility that both the disulfide bond and backbone cleavages
occurred, but the resulting ECD fragments were held together
by noncovalent interactions in the protein. However post-ECD
IR activation, which we had previously used to release ECD
fragments from the charge-reduced ions,⁴⁰ did not produce new
fragments. Therefore we conclude that multiple disulfide bond

7290 Analytical Chemistry, Vol. 82, No. 17, September 1, 2010

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