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

Figure 4

MS/MS spectrum for peptide AE(70.06)FVEVTLKLVTDLTK with putrescine covalently bound to Glu 250 of bovine serum albumin through the action of EDC. The parent ion mass in charge state +3 is 588.35 m/z. (a) A y-ion sequence, y1–y11, which identifies the peptide. This portion of the peptide does not include the labeled glutamate. (b) A b-ion sequence, b2–b8. All fragments in this sequence contain the added mass of 70.06 Da for the presence of putrescine. The mass of the b2 fragment (271.18) is consistent with alanine-glutamate-putrescine. The y-ion and b-ion sequences account for all residues in the peptide. The residue preceding the N-terminal side in the albumin sequence is lysine (dehydro mass 128.09), which cannot be mistaken for the added mass of 70 Da. Two nonsequence fragments at 155.12 and 200.14 Da are consistent with signature ions that are unique for the glutamate-putrescine construct. Most of the remaining, unassigned fragments correspond to internal fragments or to loss of amine, water, or CO from assigned fragments. Schemes that illustrate the formation of the signature ions are shown in Section 3.7.

(a)

(b)