FIGURE 2.

Representative peptide sequencing by ESI-MS/MS. This example shows the MS/MS analysis of the 3+ ion with an m/z of 624.05 that eluted from the reverse-phase column at 27.2 min and was identified as the peptidylprolyl isomerase A-derived peptide ELFADKVPKTAEN (monoisotopic mass of the unprotonated and untagged peptide = 1460.74 Da). Note that this peptide represents the D9-TMAB-labeled peptide shown in Fig. 3, C and D, and after collision-induced dissociation the a, b, and y fragments that contain TMAB tags have lost 68 Da (due to neutral loss of (C²H₃)₃N from the tag). In addition, the parent ion also shows neutral loss of 204 Da (= 3 × 68 Da) to produce the 3+ ion with m/z = 555.94 and the 2+ ion with m/z = 833.41. a^o₂ = a₂ – H₂O; y₁₁* 2+= y²⁺₁₁ – ammonia.