Mass Spectrometry of Putrescine, Spermidine, and Spermine Covalently Attached to Francisella tularensis Universal Stress Protein and Bovine Albumin

Figure 8

False positive putrescine adduct on cysteine is actually acrylamide adduct on cysteine. The MS/MS spectrum for VEHEVLVGNPAAEIVEYAEESNC(71.04)DVIVLGSHATH from rUsp/His₆ shows Cys 100 covalently modified by acrylamide. The modification occurred spontaneously during polyacrylamide gel electrophoresis. The parent ion mass in charge state +4 is 997.23 m/z. (a) A y-ion sequence y1–y16. The interval between y11 and y12 is equal to the mass of cysteine (103 Da) plus the mass of propionamide (71.04 Da). The masses of peptides y12–y16 all are consistent with the presence of an added 71.04 Da. (b) The b-ion sequence b2–b14. None of these peptides includes the 71.04 Da added mass. There is also a doubly charged y-ion sequence y16–y20. The masses of these doubly charged peptides are consistent with the added mass of 71.04 Da. The y-ion, b-ion, and doubly charged y-ion sequences account for all of the residues in the peptide. Unassigned fragments correspond to internal fragments or to loss of amine, water, or CO from assigned fragments. A scheme showing addition of acrylamide to cysteine is in Figure 9.

(a)

(b)