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 (y1/b2) 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 y1 ion formation increases. When the C-terminal substituent was more basic than Trp, the b2 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.