Linear energy correlation in the low‐energy tandem mass spectra of protonated tripeptides Xxx–Gly–Gly but failure for Gly–Xxx–Gly

The backbone cleavages of protonated tripeptide ions of the series Xxx–Gly–Gly, where Xxx–Gly, Ala, Val, Leu, Ile, Phe, Tyr, Met, Pro and Trp, were studied in a hybrid tandem mass spectrometer. C ‐Terminal y ‐type ions and N ‐terminal a ‐ and b ‐type ions were noted. A linear relationship between log (intensity of y 2 divided by the sum of all other product ion intensities) and the proton affinity of the N ‐terminal amino acid substituent was found for the series Xxx–Gly–Gly: as the proton affinity of the N ‐terminal residue increases, the fraction of y 2 ion formation decreases. The series of protonated tripeptides Gly–Xxx–Gly, where Xxx = Gly, Ala, Leu, Phe, Tyr, Met and Trp, were also studied.