SO₂^−· electron transfer ion/ion reactions with disulfide linked polypeptide ions

Abstract

Multiply-charged peptide cations comprised of two polypeptide chains (designated A and B) bound via a disulfide linkage have been reacted with SO 2 Ϫ· in an electrodynamic ion trap mass spectrometer. These reactions proceed through both proton transfer (without dissociation) and electron transfer (with and without dissociation). Electron transfer reactions are shown to give rise to cleavage along the peptide backbone, loss of neutral molecules, and cleavage of the cystine bond. Disulfide bond cleavage is the preferred dissociation channel and both Chain A (or B)OS · and Chain A (or B)OSH fragment ions are observed, similar to those observed with electron capture dissociation (ECD) of disulfide-bound peptides. Electron transfer without dissociation produces [M ϩ 2H] ϩ· ions, which appear to be less kinetically stable than the proton transfer [M ϩ H] ϩ product. When subjected to collision-induced dissociation (CID), the [M ϩ 2H] ϩ· ions fragment to give products that were also observed as dissociation products during the electron transfer reaction. However, not all dissociation channels noted in the electron transfer reaction were observed in the CID of the [M ϩ 2H] ϩ· ions. The charge state of the peptide has a significant effect on both the extent of electron transfer dissociation observed and the variety of dissociation products, with higher charge states giving more of each. ( T andem mass spectrometry currently plays a major role in the identification and characterization of proteins [1][2][3]. This application is enabled by the ability to form gaseous ions from peptides and proteins, typically via electrospray ionization [4,5] or matrix-assisted laser desorption ionization [6,7], the ability to form fragments from peptides and proteins of interest that reveal primary structure information, and the ability to measure and detect the fragments. Amino acid sequence information is usually sought for the identification of a protein. However, the identities and locations of post-translational modifications arising from, for example, phosphorylation, glycosylation, and disulfide bonding are also of interest for the complete characterization of the protein of interest. Unimolecular dissociation is the predominant chemical means for deriving primary structure information from a peptide or protein in the gas phase. The extent to which structural information can be derived from a posttranslationally modified peptide or protein depends upon many factors including, for example, charge state and nature of the ion (e.g., protonated versus metal cationized), nature of the modification, and ion activation conditions. The nature of the modification itself can play a major role in directing dissociation chemistry. For this reason, it is of interest to explore various means for both forming and activating post-translationally modified ions. In this study, we focus on the behavior of polypeptide chains bound by a cystine bridge. The formation of such bridges takes place as a protein folds into its native conformation, and the bri...

Electron Transfer versus Proton Transfer in Gas-Phase Ion/Ion Reactions of Polyprotonated Peptides

“…The square of the impact parameter for a hard-sphere collision, b h−s 2 , is given by 31 Proton transfer and electron transfer become possible when the energy surfaces of the reactants and products cross. Hence, r PT for ground state reactants and products can be estimated from r PT ≈ Z 1 Z 2 e 2 ΔH PT (6) and r ET can be estimated from…”

Pulsed dual electrospray ionization for In/In reactions

“…Applications of charge manipulation via ion/ ion reactions include: mixture analysis [5][6][7][8], especially with the application of the "ion parking" technique for gas-phase concentration and charge state purification [9], the formation of ions that cannot be directly produced by ESI for subsequent tandem mass spectrometry studies [10], and the reduction of the product ion charge states to singly and doubly charged so as to simplify the interpretation of product ion spectra [11][12][13][14][15][16]. Electron-transfer ion/ion reactions have recently been described as a means of inducing structurally informative dissociation, giving rise to cleavages analogous to those noted in electron capture dissociation [17][18][19][20][21]. In this regard, ion/ion reactions play a role as a probe of primary structure.…”

A pulsed triple ionization source for sequential ion/ion reactions in an electrodynamic ion trap

“…. Electron-transfer reactions appear to be a highly useful means for deriving the primary structure information of peptides and proteins [7][8][9][10][11].Most ion/ion reaction studies involving biomolecules are conducted in one of two ways. The first approach is to mix the oppositely charged reactant ions at or near atmospheric pressure before sampling ions into a mass spectrometer.…”

“…. Electron-transfer reactions appear to be a highly useful means for deriving the primary structure information of peptides and proteins [7][8][9][10][11].…”

“…Application of proton-transfer reactions include charge reduction of ions derived from electrospray of protein mixtures [3], charge state reduction of product ions following dissociation for simplification of spectral interpretation [4], ion parking [5], and parallel ion parking [6]. Electron-transfer reactions appear to be a highly useful means for deriving the primary structure information of peptides and proteins [7][8][9][10][11].…”