FIGURE 9.

Mass spectrometric identification of carnosine-propanal-modified paired basic amino acid-cleaving system protein. Representative HCD fragmentation spectrum for +3-charged ion with a monoisotopic m/z 545.63898 Da was observed at +4.99 millimass unit (mmu)/9.14 ppm mass error using a 1D-LC-LTQ-Oribitrap-ELITE mass spectrometer. Analysis of the collected data by Sequest and Mascot (v1.20) identified a peptide (XCorr, 1.70; probability, 0.00) with the sequence WLQQQEVkRR, K₈-C₁₂H₁₆N₄O₃ (264.1224 Da), and a MH⁺ of 1634.90238 Da. The fragmentation of the peptide yielded four c-ion and three z-ion series with sub 2.5 ppm mass accuracy. The inset shows the putative structures of the adducts that could be due to the formation of lysine-propanal-β-alanyl-histidine (i) or β-alanyl-histidine-propanal-lysine (ii) adducts.