Substituent effects on the gas-phase fragmentation reactions of protonated peptides containing benzylamine-derivatized lysyl residues.

Motivated by the need for chemical strategies designed to tune peptide fragmentation to selective cleavage reactions, benzyl ring substituent influence on the relative formation of carbocation elimination (CCE) products from peptides with benzylamine-derivatized lysyl residues has been examined using collision-induced dissociation (CID) tandem mass spectrometry. Unsubstituted benzylamine-derivatized peptides yield a mixture of products derived from amide backbone cleavage and CCE. The latter involves side-chain cleavage of the derivatized lysyl residue to form a benzylic carbocation [C(7)H(7)](+) and an intact peptide product ion [(MH(n))(n+) - (C(7)H(7))(+)]((n-1)+). The CCE pathway is contingent upon protonation of the secondary ε-amino group (N(ε)) of the derivatized lysyl residue. Using the Hammett methodology to evaluate the electronic contributions of benzyl ring substituents on chemical reactivity, a direct correlation was observed between changes in the CCE product ion intensity ratios (relative to backbone fragmentation) and the Hammett substituent constants, σ, of the corresponding substituents. There was no correlation between the substituent-influenced gas-phase proton affinity of N(ε) and the relative ratios of CCE product ions. However, a strong correlation was observed between the π orbital interaction energies (ΔE(int)) of the eliminated benzylic carbocation and the logarithm of the relative ratios, indicating the predominant factor in the CCE pathway is the substituent effect on the level of hyperconjugation and resonance stability of the eliminated benzylic carbocation. This work effectively demonstrates the applicability of σ (and ΔE(int)) as substituent selection parameters for the design of benzyl-based peptide-reactive reagents which tune CCE product formation as desired for specific applications.