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

Abstract

, 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.

w‐Type ions formed by electron transfer dissociation of Cys‐containing peptides investigated by infrared ion spectroscopy

“…Other z • ‐type ions were observed to expel a thiol radical only upon applying additional CID activation after ETD, in line with reported higher rates of formation of w ‐ions compared with u ‐ions . Also in accordance with previous studies, loss of CH 2 S (46 Da) after collisional activation was only observed for z • ‐ions that contain a Cys residue, but not at the cleavage site. Spectroscopic probing of the structure of these fragment ions is challenging because of the more involved ETD/CID MS n scheme required to produce them, and the inherent lower ion yields as compared with the w ‐type ions; such experiments are reserved for future studies.…”

“…Cys‐specific ExD pathways have been discussed in several studies . For z • ‐type fragments from Cys‐containing peptides, secondary fragmentation by loss of a 33‐Da neutral fragment is common and is attributed to loss of a thiol radical (SH • ).…”

“…For z 6 ‐ and larger z • ‐type ions, neutral loss of CH 2 S (46 Da) has also been observed . ETD on [AACAR+2H] 2+ generated the z 4 ‐ion ( m / z 404) and the z 3 ‐ion ( m / z 333), where the intensity of the z 3 ‐ion was significantly decreased by spontaneous loss of the SH radical, forming a secondary product at m / z 300 . ECD studies on peptides containing modified cysteine residues, such as carboxymethylated or carbamidomethylated Cys residues, report neutral losses corresponding to the modified Cys side chain .…”

“…8,24,27,28 Cys-specific ExD pathways have been discussed in several studies. 23,27,29 For z • -type fragments from Cys-containing peptides, secondary fragmentation by loss of a 33-Da neutral fragment is common and is attributed to loss of a thiol radical (SH • ). For z 6and larger z • -type ions, neutral loss of CH 2 S (46 Da) has also been observed.…”

“…27 ETD on [AACAR+2H] 2+ generated the z 4 -ion (m/z 404) and the z 3 -ion (m/z 333), where the intensity of the z 3 -ion was significantly decreased by spontaneous loss of the SH radical, forming a secondary product at m/z 300. 29 ECD studies on peptides containing modified cysteine residues, such as carboxymethylated or carbamidomethylated Cys residues, report neutral losses corresponding to the modified Cys side chain. 30 The odd-electron neutral loss of C 2 H 4 NOS • (90 Da) has also been observed in a negative electron-transfer dissociation process on a doubly negatively charged ion containing carbamidomethylated Cys residues.…”