Oxidation of peptides during electrospray ionization

Abstract

[M + H + 16]+ ions were observed in the electrospray ionization mass spectra of several synthetic and naturally occurring peptides. Initial results have shown that the appearance of the modification is dependent on the field strength at the electrospray needle and the flow rate of the solution passing through the capillary. Mass spectrometric experiments on peptides showing the + 16 Da modification attributed the change to the selective oxidation of either a methionyl, tryptophanyl, or tyrosyl residue present in the peptide. These results were further confirmed by tandem mass spectrometry experiments on peptides with a methionyl residue at either the N- or C-terminus, or within the peptide chain. This effect can occur under normal operating conditions and therefore care must be taken in the analysis of samples containing these oxidizable residues. Conversely, the selectivity of the oxidative process may be used to enhance the information obtained from the mass spectrometric analysis. For example, we show results for the analysis of a tryptic digest of the protein myoglobin where the occurrence of the [M + H + 16]+ ion is used, along with the molecular weight data, to correctly identify the trypic fragment.

Lysine 43 Is Trimethylated in Subunit c from Bovine Mitochondrial ATP Synthase and in Storage Bodies Associated with Batten Disease

“…These [MϩHϩ16] ϩ masses were observed in ESI-MS of mitochondrial and stored forms of subunit c (Table I). The intensity of the [MϩHϩ16] ϩ species differed from experiment to experiment with the same samples, suggesting that oxidation arose during either the sample preparation in chloroform and methanol solutions or during electrospray ionization (36). No methionine-containing peptides were isolated in these experiments, and so it was not possible to examine the state of methionine oxidation by MALDI mass spectrometry.…”

Oxidation Artifacts in the Electrospray Mass Spectrometry of Aβ Peptide

“…Oxygen molecules have positive electron affinity and can readily capture thermal free electrons in the discharge [20]. It was of interest to determine whether the oxygen atom inserted into Aβ derived from O 2 [5,16,17] (despite efforts to exclude room air), from other solutes, or from electrochemical oxidation of water.…”

“…Methionine is one of the most readily oxidized amino acid constituents of proteins [5,[11][12][13]. Oxidation of methionine residues can result in significant conformational and/or functional changes [14], and thus represents an important posttranslational modification under conditions of oxidative stress or aging [15].…”

Top-down quantitation and characterization of SILAC-labeled proteins

“…Online protein separation was performed by use of 75 m ϫ 150 mm IntegraFrit columns (New Objective, Berlin, Germany) packed in-house with 5 m RP-C18 beads (Reprosil-Pur Aq, 200 Å pore size, Dr. Maisch, Ammerbuch-Entringen, Germany). The column was connected to a short nanospray needle and spraying voltage was kept low (Ͻ2 kV) to prevent oxidation [18]. During loading and washing, the flow was set to 500 nL/min; whereas during the actual gradient the flow rate was 250 nL/min.…”