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The b5 to b9 ions support putrescine on Q267 because they carry an added mass of 71 Da. The y-ions do not have an added mass of 71, which means putrescine is not on residues VDVLVVR. The MH+3 parent ion mass is m/z 545.3059. The parent ion minus a water molecule, MH-H2O+3, has a peak at m/z 539.2938.
