A linear free‐energy correlation in the low‐energy tandem mass spectra of protonated tripeptides Gly–Gly–Xxx

The backbone cleavages of protonated tripeptide ions of the series Gly—Gly—Xxx, where Xxx  Gly, Ala, Val, d ‐Leu, l ‐Leu, Ile, Phe, Tyr, Trp, Pro, Met and Glu, 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 ( y 1 / b 2 ) and the proton affinity of the C ‐terminal amino acid substituents was found: as the proton affinity of the C ‐terminal residue increases, the fraction of y 1 ion formation increases. When the C ‐terminal substituent was more basic than Trp, the b 2 ion was not observed. It is likely that the site of protonation changes from peptide bond to side‐chain for just these residues, Lys, His and Arg.