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, lie, 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.