A Study of b1+H2O and b1-Ions in the Product Ion Spectra of Dipeptides Containing N-Terminal Basic Amino Acid Residues

The product ion spectra of approximately 200 dipeptides were acquired under low-energy conditions using a triple quadrupole mass spectrometer. The spectra of dipeptides containing an N-terminal arginine (R), histidine (H), or lysine (K) were observed to yield a b1 + H2O ion corresponding to the protonated basic amino acid. This was equivalent to the y1-ion in the corresponding C-terminal isomer. The formation of a b1 + H2O ion was not a significant fragmentation channel in any dipeptides analyzed including those containing a C-terminal basic amino acid unless they also contained an N-terminal basic amino acid. Occurring simultaneously and under equal energy conditions an apparent b1-ion was formed, which has its corresponding C-terminal equivalent in the y1-H2O ion. Energy resolved mass spectrometry (ERMS), deuterium labeling, and accurate mass experiments as well as data reported were used to show the relationships between the b1+H2O and b1-ions in the dipeptides containing an N-terminal basic amino acid and the y1 and y1-H2O ions in the corresponding C-terminal isomers.