LINEAR ENERGY CORRELATIONS AND FAILURES IN THE LOW-ENERGY TANDEM MASS-SPECTRA OF PROTONATED N-BENZOYLATED TRIPEPTIDES - TOOLS FOR PROBING MECHANISMS OF CAD PROCESSES

The backbone cleavages for three series of protonated N-benzoyl tripeptide ions were studied in a hybrid tandem mass spectrometer: (i) benzoyl-Gly-Gly-Xxx, where Xxx = Gly, Ala, Val, Leu, ne, Phe, Tyr, Met, Glu, Pro and Trp, (ii) benzoyl-Gly-Xxx-Gly, where Xxx = Gly, Ala, Leu, Phe, Tyr, Met and Trp, and (iii) benzoyl-Xxx-Gly-Gly, where Xxx = Gly, Ale, Val, Leu, lie, Phe, Tyr, Met, Pro and Trp. C-Terminal y-type ions and N-terminal a- and b-type ions were noted in all three cases. For benzoyl-Gly-Gly-Xxx, a linear relationship between log (y(1)/b(2)) and tbe 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. A similar relationship was noted for the benzoyl-Xxx-Gly-Gly tripeptides between log (y(2)/b(1)) and the proton affinity of the N-terminal amino acid substituent: as the proton affinity of the N-terminal residue increases, the fraction of b(1) ion formation increases. For the series benzoyl-Gly-Xxx-Gly, these relationships did not hold true. These observations point to similar reaction pathways throughout the benzoyl-Gly-Gly-Xxx series and also similar pathways throughout the benzoyl-Xxx-Gly-Gly, but pathways that are substituent dependent for benzoyl-Gly-Xxx-Gly. The increased correlation coefficients for benzoyl-Gly-Gly-Xxx and benzoyl-Xxx-Gly-Gly when compared with the free tripeptides, suggest that fewer interfering competitive reactions exist, as fewer possibilities for internal hydrogen bonding exist in the N-benzoyl derivatives versus the free compounds.