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Correct identification of oxidized histidine residues using electron-transfer dissociation.

ABSTRACT: Oxidative modification to the side chain of histidine can noticeably change the collision-induced dissociation (CID) pathways of peptides containing this oxidized residue. In cases where an oxidized peptide consists two or more isomers differing only in the site of modification, oxidation to histidine usually causes the other oxidized sites to be mis-assigned in CID spectra. These spectral misassignments can sometimes be avoided by using multiple stages of MS/MS (MS(n)) or via specially optimized liquid chromatographic separation conditions. In this manuscript, we demonstrate that these misassignments can be more readily and easily avoided by using electron-transfer dissociation (ETD) to dissociate the oxidized peptides. Furthermore, we find that the relative insensitivity of ETD to side-chain chemistry allows the extent of oxidative modification to be determined readily for peptide isomers having more than one site of oxidation. The current results along with previous studies of oxidized peptides suggest that ETD is probably a better technique than CID for obtaining correct sequence and modification information for oxidized peptides.

SUBMITTER: Srikanth R

PROVIDER: S-EPMC2737336 | biostudies-literature | 2009 May

REPOSITORIES: biostudies-literature

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Correct identification of oxidized histidine residues using electron-transfer dissociation.

Srikanth Rapole R Wilson Jonathan J Vachet Richard W RW

Journal of mass spectrometry : JMS 20090501 5

Oxidative modification to the side chain of histidine can noticeably change the collision-induced dissociation (CID) pathways of peptides containing this oxidized residue. In cases where an oxidized peptide consists two or more isomers differing only in the site of modification, oxidation to histidine usually causes the other oxidized sites to be mis-assigned in CID spectra. These spectral misassignments can sometimes be avoided by using multiple stages of MS/MS (MS(n)) or via specially optimize ...[more]

PMID: 19160434

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Project description:Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of doubly protonated electron affinity (EA)-tuned peptides were studied to further illuminate the mechanism of these processes. The model peptide FQpSEEQQQTEDELQDK, containing a phosphoserine residue, was converted to EA-tuned peptides via beta-elimination and Michael addition of various thiol compounds. These include propanyl, benzyl, 4-cyanobenzyl, perfluorobenzyl, 3,5-dicyanobenzyl, 3-nitrobenzyl, and 3,5-dinitrobenzyl structural moieties, having a range of EA from -1.15 to +1.65 eV, excluding the propanyl group. Typical ECD or ETD backbone fragmentations are completely inhibited in peptides with substituent tags having EA over 1.00 eV, which are referred to as electron predators in this work. Nearly identical rates of electron capture by the dications substituted by the benzyl (EA = -1.15 eV) and 3-nitrobenzyl (EA = 1.00 eV) moieties are observed, which indicates the similarity of electron capture cross sections for the two derivatized peptides. This observation leads to the inference that electron capture kinetics are governed by the long-range electron-dication interaction and are not affected by side chain derivatives with positive EA. Once an electron is captured to high-n Rydberg states, however, through-space or through-bond electron transfer to the EA-tuning tags or low-n Rydberg states via potential curve crossing occurs in competition with transfer to the amide pi* orbital. The energetics of these processes are evaluated using time-dependent density functional theory with a series of reduced model systems. The intramolecular electron transfer process is modulated by structure-dependent hydrogen bonds and is heavily affected by the presence and type of electron-withdrawing groups in the EA-tuning tag. The anion radicals formed by electron predators have high proton affinities (approximately 1400 kJ/mol for the 3-nitrobenzyl anion radical) in comparison to other basic sites in the model peptide dication, facilitating exothermic proton transfer from one of the two sites of protonation. This interrupts the normal sequence of events in ECD or ETD, leading to backbone fragmentation by forming a stable radical intermediate. The implications which these results have for previously proposed ECD and ETD mechanisms are discussed.

Dataset Information

Correct identification of oxidized histidine residues using electron-transfer dissociation.

Publications

Correct identification of oxidized histidine residues using electron-transfer dissociation.

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