Formation, isomerization, and dissociation of tyrosine-containing tripeptide radical cations

Two series of tripeptide radical cations ([AYG] · +, [AYG · ]+; [GYG] · +, [G · YG]+ and [GYG · ]+) were successfully prepared via multistage collision-induced dissociation (CID) of a ternary complex of copper-ligand-tripeptide [Cu(L)Peptide]2+ (L=4′-chloro-2,2′:6′,2′′terpyridine (4Cl-tpy); Peptide=alanyl-tyrosyl-glycine (AYG) or glycyl-tyrosyl-glycine (GYG) tripeptide). The gas-phase isomerization and dissociation mechanism of isomeric AYG radical ions, and the different dissociation behaviors between AYG and GYG radical ions, have been obtained by observing their gas-phase dissociation reactions combined with density functional theory (DFT) calculation. Studies have revealed that: (1) [AYG · ]+ and [AYG] · + produced almost identical CID spectra (their main product ions are [x2+H] · + ions), suggesting that interconversion occurs prior to dissociation. (2) The dissociation paths of [A · YG]+ and [G · YG]+ are the same (both of which produces [b2−H] · + fragment ions by cleavage of amide C−N bond, indicating that [A · YG]+ cannot isomerize to [AYG · ]+ and [AYG] · +. DFT calculation gives the gas-phase stability order of these three peptide radical ions ([A · YG]+ > [AYG · ]+ > [AYG] · +) and their isomerization/dissociation energy barriers. Isomerization energy barrier (47.4 kcal/mol) of [AYG · ]+ ® [AYG] · + is lower than dissociation energy barrier (72.7 kcal/mol) of [AYG · ]+, but higher than that (44.3 kcal/mol) of [AYG] · +, thereby the gas-phase dissociation mechanism of [AYG · ]+ undergoes a two-step reaction: the first step is the isomerization process of [AYG · ]+ ® [AYG] · +, and the second step is the dissociation reaction of [AYG] · + ® [x2+H] · +, which produces [x2+H] · + fragment ions by cleavage of N-terminal Cα−C bond in [AYG] · +. (4) The gas-phase dissociation characteristics of [GYG] · + and [AYG] · + are dramatically different. [GYG] · + mainly produces [z2−H] · + by cleavage of the second N−Cα bond (counting from the N-terminal amino acid residue).