Dissociation of N-terminal C_α-C_β bond induced by electron transfer between N-terminus and tyrosine side chain in molecular peptide radical cation

Dissociation of molecular radical cationic peptides displays rich gas-phase ion chemistry, providing useful structural information for protein sequencing.^1,2 Low-energy collision-induced dissociation (CID) of tyrosine-containing peptide molecular radical cations [M]^•+ led to dissociation of C_α-C_β bond at the side chain of the N-terminal phenylalanine residue forming [M – 91]⁺ associated with a loss of odd-electron neutral benzyl side chain (91 Da).² Herein, we demonstrate that the dissociation of N-terminal C_α‒C_β bond of [FYGG]^•+ induced by electron transfer from N-terminus to the tyrosine residue electron hole. Interestingly, infrared multiple-photon dissociation (IRMPD) action spectroscopy of [FYGG]^•+ shows a characteristic band at 1573 cm^-1 assigned to the bending of a NH₂ ^•+ moiety resulting from the n-π interaction between the non-bonding electron of the N-terminal NH₂ and the π-cationic radical of tyrosine residue. This electron transfer also exhibits distance dependency; increasing the number of spacer (glycine residue) between N-terminal phenylalanine and tyrosine will decrease the abundance of [M – 91]⁺ .