Linear energy correlations and failures in the low‐energy tandem mass spectra of protonated N‐benzoylated tripeptides: Tools for probing mechanisms of CAD processes

Abstract

AbstractThe 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, Ile, 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, Ala, Val, Leu, Ile, 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 (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. A similar relationship was noted for the benzoyl‐Xxx‐Gly‐Gly tripeptides between log (y2/b1) and the proton affinity of the N‐terminal amino acid substituent: as the proton affinity of the N‐terminal residue increases, the fraction of b1 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.