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![[Figure 6]](rv5012fig6mag.jpg)
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Figure 6
Sub-millisecond irradiation of a megaDalton protein assembly. (a) Extracted ion chromatogram showing the increase of the doubly protonated +16 Da modified product of peptide 496–511, TQAIQSAAESTEMLLR (eluted at 4.0 min, 882.95 m/z) with the increase in irradiation time on the sample solution containing 10 µM mmcpn in 10 mM phosphate buffer, pH 7, containing 1 mM ATP, 1 mM TCEP, ∼5% glycerol and 150 mM NaCl. For easy visualization, the abundances of doubly protonated native peaks (eluted at 4.3 min, 874.95 m/z) are made equal for all the irradiation time points and the corresponding modified peak abundances are adjusted accordingly by multiplication factors. The peak areas are calculated from the extracted ion chromatograms of raw data. The fraction of unmodified peptide at a given exposure is calculated as the unmodified peak area divided by the sum of unmodified and modified peak areas. The ∼3.5% background modification for peptide 496–511 is normalized in the dose-response plot (inset). The solid line represents a single exponential fit with rate constants k = 253.7 s−1, with individual points representing the mean of three independent measurement with standard error. (b) The sites of modification are identified by the MS/MS of the double protonated modified precursor ion of 882.95 m/z. The signature +16 m/z shift on y and b fragment ions shown in red indicate modification at M508. (c) Pictorial representation of modified Met residues on two adjacent subunits out of the 16 stacked subunits in the protein assembly (3kfb ) (Pereira et al., 2010  ). |
![[Figure 6]](rv5012fig6.jpg)
 | JOURNAL OF SYNCHROTRON RADIATION |
ISSN: 1600-5775
