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

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Abstract. Noncovalent complexes of hydrophobic peptides GLLLG and GLLLK with photoleucine (L*) tagged peptides G(L* n L m )K (n = 1,3, m = 2,0) were generated as singly charged ions in the gas phase and probed by photodissociation at 355 nm. Carbene intermediates produced by photodissociative loss of N 2 from the L* diazirine rings underwent insertion into X−H bonds of the target peptide moiety, forming covalent adducts with yields reaching 30%. Gas-phase sequencing of the covalent adducts revealed preferred bond formation at the C-terminal residue of the target peptide. Site-selective carbene insertion was achieved by placing the L* residue in different positions along the photopeptide chain, and the residues in the target peptide undergoing carbene insertion were identified by gas-phase ion sequencing that was aided by specific 13 C labeling. Density functional theory calculations indicated that noncovalent binding to GL*L*L*K resulted in substantial changes of the (GLLLK + H) + ground state conformation. The peptide moieties in [GL*L*LK + GLLLK + H] + ion complexes were held together by hydrogen bonds, whereas dispersion interactions of the nonpolar groups were only secondary in ground-state 0 K structures. Born-Oppenheimer molecular dynamics for 100 ps trajectories of several different conformers at the 310 K laboratory temperature showed that noncovalent complexes developed multiple, residue-specific contacts between the diazirine carbons and GLLLK residues. The calculations pointed to the substantial fluidity of the nonpolar side chains in the complexes. Diazirine photochemistry in combination with Born-Oppenheimer molecular dynamics is a promising tool for investigations of peptide-peptide ion interactions in the gas phase.

Section: Photodissociationmentioning

confidence: 99%

Efficient Covalent Bond Formation in Gas-Phase Peptide–Peptide Ion Complexes with the Photoleucine Stapler

Shaffer

Andrikopoulos

Řezáč

et al. 2016

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“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Nguyen

Tureček

2017

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, where X = A, C, D, E, F, G, H, K, L, M, N, P, Y, and W, were generated by electron transfer dissociation of peptide dications and investigated by MS 3 -nearultraviolet photodissociation (UVPD) at 355 nm. Laser-pulse dependence measurements indicated that the ion populations were homogeneous for most X residues except phenylalanine. UVPD resulted in dissociations of backbone CO─NH bonds that were accompanied by hydrogen atom transfer, producing fragment ions of the [y n ] + type. Compared with collision-induced dissociation, UVPD yielded less sidechain dissociations even for residues that are sensitive to radical-induced side-chain bond cleavages. The backbone dissociations are triggered by transitions to second (B) excited electronic states in the peptide ion R-CH • -CONH-chromophores that are resonant with the 355-nm photon energy. Electron promotion increases the polarity of the B excited states, R-CH + -C • (O -)NH-, and steers the reaction to proceed by transfer of protons from proximate acidic C α and amide nitrogen positions.

“…Collisioninduced dissociation under slow heating conditions of singly and doubly charged L*-and M*-tagged peptide ions proceeds by competing loss of N 2 and backbone dissociations [3]. Electron transfer dissociation (ETD) [7] of L*-and M*-tagged peptide ions results in competing backbone dissociations and radical reactions involving the diazirine ring [8,9]. The L* residue has been calculated to have a substantial electron affinity that facilitates electron attachment in the diazirine ring and triggers proton and hydrogen atom migrations in the charge-reduced peptide ions [8].…”

mentioning

confidence: 99%

Photoleucine Survives Backbone Cleavage by Electron Transfer Dissociation. A Near-UV Photodissociation and Infrared Multiphoton Dissociation Action Spectroscopy Study

Shaffer

Martens

Marek

et al. 2016

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Abstract. We report a combined experimental and computational study aimed at elucidating the structure of N-terminal fragment ions of the c type produced by electron transfer dissociation of photo-leucine (L*) peptide ions GL*GGKX. The c 4 ion from GL*GGK is found to retain an intact diazirine ring that undergoes selective photodissociation at 355 nm, followed by backbone cleavage. Infrared multiphoton dissociation action spectra point to the absence in the c 4 ion of a diazoalkane group that could be produced by thermal isomerization of vibrationally hot ions. The c 4 ion from ETD of GL*GGK is assigned an amide structure by a close match of the IRMPD action spectrum and calculated IR absorption. The energetics and kinetics of c 4 ion dissociations are discussed.