Radical-driven dissociation of odd-electron peptide radical ions produced in 157 nm photodissociation

Abstract

Odd-electron a ϩ 1 radical ions generated in the 157 nm photodissociation of peptide ions were investigated in an ion trap mass spectrometer. To localize the radical, peptide backbone amide hydrogens were replaced with deuterium. When the resulting radical ions underwent hydrogen elimination, no H/D scrambling was obvious, suggesting that without collisional activation, the radical resides on the terminal ␣-carbon. Upon collisional excitation, oddelectron radical ions dissociate through two favored pathways: the production of a-type ions at aromatic amino acids via homolytic cleavage of backbone C ␣ -C(O) bonds and side-chain losses at nonaromatic amino acids. When aromatic residues are not present, nonaromatic residues can also lead to a-type ions. In addition to a-type ions, serine and threonine yield c n-1 and a n-1 ϩ 1 ions where n denotes the position of the serine or threonine. All of these fragments appear to be directed by the radical and they strongly depend on the amino acid side-chain structure. In addition, thermal fragments are also occasionally observed following cleavage of labile Xxx-Pro bonds and their formation appears to be kinetically competitive with radical migration. (J Am Soc Mass

Fragmentation of Singly, Doubly, and Triply Charged Hydrogen Deficient Peptide Radical Cations in Infrared Multiphoton Dissociation and Electron Induced Dissociation

“…For ACTH 1-10, in addition to side chain losses and formation of a-type product ions, b-, y-,c-, and xtype product ions were also detected with relatively high yields, suggesting that amino acid composition and charge location may also influence the dissociation outcome. These results are analogous to those previously reported in MS 3 CID of radical cations [16,36,40,44,71].…”

“…Formation of a 9 and a 6 product ions, resulting from cleavages next to the two histidine residues was abundant ( Figure 2a). Enhanced cleavage at aromatic residues was also observed in CID of the same species ( Figure S1) and is in accordance with previous reports [44,70,71]. In the IRMPD spectrum, the a 5 and a 6 product ions were observed as [a 5 +· type product ions has been previously observed in CID [40] and SID [35] spectra of hydrogen deficient peptide radical cations.…”

“…It has been previously shown that fragmentation of singly charged radical cations featuring an arginine residue close to the N-terminus (and therefore having the charge localized at the N-terminus) results in facile formation of a-type product ions [36,40,70,71]. For peptides featuring an arginine residue close to the C-terminus, formation of a wide range of product ions including a-, y-, x-, and z-type product ions has been reported in the literature [16,40].…”

Fragmentation ofα-radical cations of arginine-containing peptides

“…As a result, CID spectra of the three triglycine ␣-radicals are distinctly different. Julian and coworkers observed differences in fragmentation behavior of several more complex ␣-radical peptides [41], while Zhang and Reilly reported similar fragmentation behavior for a number of [a n ϩ H] ϩ· ions, in which the radical is initially positioned at the C-terminal ␣-carbon [42]. Recently, we examined the energetics and dynamics of dissociation of an ␣-radical produced by the loss of p-quinomethide from DRVYIHPF ϩ· [43].…”

“…As a result, fragmentation of relatively large peptide radicals with arginine at the N-terminus is often dominated by the formation of a n ions [41]. In addition, c-ions are formed at serine and threonine residues [41,42] and c/z · ions are formed at tryptophan and tyrosine [46]. While the formation of a n ions has received significant attention in the literature, other dissociation pathways of peptide radical cations have been generally described only for di-and tripeptides [34 -36, 40, 46].…”