Electron transfer reduction of the diazirine ring in gas-phase peptide ions. On the peculiar loss of [NH4O] from photoleucine.

Electron transfer to gas-phase peptide ions with diazirine-containing amino acid residue photoleucine (L*) triggers diazirine ring reduction followed by cascades of residue-specific radical reactions. Upon electron transfer, substantial fractions of (GL*GGR +2H)(+[Symbol: see text]) cation-radicals undergo elimination of [NH(4)O] radicals and N(2)H(2) molecules from the side chain. The side-chain dissociations are particularly prominent on collisional activation of long-lived (GL*GGR +2H)(+[Symbol: see text]) cation-radicals formed by electron transfer dissociation of noncovalent peptide-18-crown-6-ether ion complexes. The ion dissociation products were characterized by multistage tandem mass spectrometry (MS(n)) and ion mobility measurements. The elimination of [NH(4)O] was elucidated with the help of (2)H, (15) N, and (18)O-labeled peptide ions and found to specifically involve the amide oxygen of the N-terminal residue. The structures, energies, and electronic states of the peptide radical species were elucidated by a combination of near-UV photodissociation experiments and electron structure calculations combining ab initio and density functional theory methods. Electron transfer reaching the ground electronic states of charge reduced (GL*GGR +2H)(+[Symbol: see text]) cation-radicals was found to reduce the diazirine ring. In contrast, backbone N - Cα bond dissociations that represent a 60%-75% majority of all dissociations because of electron transfer are predicted to occur from excited electronic states.