Serine effects on collision-induced dissociation and photodissociation of peptide cation radicals of the +●-type

Abstract

The serine residue displays specific effects on the dissociations of peptide fragment cation-radicals of the z+• type which are produced by electron transfer dissociation. Energy-resolved collision-induced dissociation (ER-CID), time-resolved infrared multiphoton dissociation (TR-IRMPD), and single-photon UV photodissociation at 355 nm revealed several competitive dissociation pathways consisting of loss of OH radical, water, and backbone cleavages occurring at N-terminal and C-terminal positions relative to the serine residue. The activation modes using slow-heating and UV photon absorption resulted in different relative intensities of fragment ions. This indicated that the dissociations proceeded through several channels with different energy-dependent kinetics. The experimental data were interpreted with the help of electron structure calculations that provided fully optimized structures and relative energies for cis and trans amide isomers of the z4+• ions as well as isomerization, dissociation, and transition state energies. UV photon absorption by the z4+• ions was due to Cα-radical amide groups created by ETD that provided a new chromophore absorbing at 355 nm.

Near-UV Photodissociation of Tryptic Peptide Cation Radicals. Scope and Effects of Amino Acid Residues and Radical Sites

“…At the same time, the enolimine mechanism is unlikely to operate in the ground electronic state of the ion because intermediates 2 and 3 are high-energy isomers of the more conventional amides [31]. The ground-state reactions, as observed in CID, then can be formulated by conventional radical chemistry involving C α and side-chain hydrogen migrations combined with amide trans-cis isomerizations [23,25,31,41].…”

“…Equation 1 applies when the ion population is homogeneous and every photon absorption is dissociative on the time scale of the experiment, which is often justified by the low activation energies for radical dissociations [23,25,31,40,41] and the substantial excitation energy delivered by the photon (3.49 eV or 337 kJ mol -1 at 355 nm). , where I(n) converges to I 0 x 0 (M 2 ).…”

“…Conversion to the Roepstorff-Fohlman-Biemann system [35,36] (Table 1). With the other residues in this group, photo-induced hydrogen migrations among the C α positions [23] can deplete the initial N-terminal chromophore and affect absorption at 355 nm. Residues possessing π-electron systems (H, N, D, F, W) or lone pairs (C, M) may undergo electron transitions involving these orbitals; a detailed description of these interactions would require a substantial effort in analyzing the pertinent ion conformers and their electronic excitations.…”

“…Infrared multiphoton dissociation of ETD-produced peptide fragment ions has also been implemented to provide useful information of the structure and conformation of c- [20] and z-type [21] ions. Whereas UV photo-excitation (UVPD) at 193 nm results in nonspecific dissociation of many bonds in ETD fragment ions, UVPD at 355 nm has been shown to trigger backbone and side-chain bond cleavage that were specific for amino acid residues in the peptide ion [22][23][24][25][26]. When combined with action spectroscopy [9], UV-VIS photodissociation provides information on mass-resolved photodissociation channels pertinent to different excited electronic states of the ion [27,28].…”

Combining Near-UV Photodissociation with Electron Transfer. Reduction of the Diazirine Ring in a Photomethionine-Labeled Peptide Ion

“…In short, electron transfer can trigger hydrogen atom migration from the N-terminal ammonium to the olefin side chain [36], which is followed by a rearrangement to a photoactive M* C α radical whose optimized structure and absorption spectrum are shown in Figure S4 (Supplementary Data) [5,37].…”