Mechanistic and Kinetic Study of Singlet O₂ Oxidation of Methionine by On-Line Electrospray Ionization Mass Spectrometry

Abstract

Abstract. We report a reaction apparatus developed to monitor singlet oxygen ( 1 O 2 ) reactions in solution using on-line ESI mass spectrometry and spectroscopy measurements. 1O 2 was generated in the gas phase by the reaction of H 2 O 2 with Cl 2 , detected by its emission at 1270 nm, and bubbled into aqueous solution continuously. O 2 oxidation were monitored by UV-Vis absorption and positive/negative ESI mass spectra, and product structures were elucidated using collision-induced dissociation-tandem mass spectrometry. To suppress electrical discharge in negative ESI of aqueous solution, methanol was added to electrospray via in-spray solution mixing using theta-glass ESI emitters. Capitalizing on this apparatus, the reaction of 1 O 2 with methionine was investigated. We have identified methionine oxidation intermediates and products at different pH, and measured reaction rate constants.1 O 2 oxidation of methionine is mediated by persulfoxide in both acidic and basic solutions. Persulfoxide continues to react with another methionine, yielding methionine sulfoxide as end-product albeit with a much lower reaction rate in basic solution. Density functional theory was used to explore reaction potential energy surfaces and establish kinetic models, with solvation effects simulated using the polarized continuum model. Combined with our previous study of gas-phase methionine ions with 1 O 2 , evolution of methionine oxidation pathways at different ionization states and in different media is described.

Type I Photosensitized Oxidation of Methionine†

“…Since Met is oxidized by 1 O 2 (33) and pterins are relatively good 1 O 2 ‐photosensitizers (12), we investigated the role of this ROS in the oxidation of Met photosensitized by Mep in aerobic conditions. Therefore, experiments using D 2 O as solvent were carried out and the results were compared to those obtained in H 2 O.…”

“…3b), thus indicating that P1 is this Met oxidation product. According to previous studies, MetO is a common product of thermal and photochemical oxidation of Met (8,9,32,33,34).…”

Lysozyme oxidation by singlet molecular oxygen: Peptide characterization using [18O]‐labeling oxygen and nLC‐MS/MS

“…24 Met oxidation by singlet molecular oxygen in aqueous solutions was monitored in positive and negative mode, showing the formation of a Met persulfoxide intermediated in both acid and basic solutions. 25 Lysozyme has two methionine, the Met 12 and Met 105 residues (Supplementary Figure S7), which were both identified as sulfoxides 112. MS/MS analyses of I 98 VSDGNGM 105 NAW 108 VAWR 112 will be described in the oxidation of the residue Trp 108 .…”

“…Recently, amino acid oxidation, in gas-phase, has been investigated using ESI mass spectrometry and could be an interesting methodology to study this reaction in a protein gas phase methodology. [12][13][14][15][16] Protein oxidation by 1 O 2 has been extensively investigated using different photosensitizers, 17 although oxidized residues have not been widely mapped using a clean chemical 1 O 2 source. In the present work, a detailed study has been made using collision-induced dissociation (CID), nLC-MS/MS, and [ 18 O]-labeling experiments.…”

“…and b12 to b13 ions presenting Trp 111 without any modification. The red ions y 6 to y 7 , y9 , y 11 to y14 and b 8 to b10 , b 12 to b 14 are modified ions with addition of +36 amu, referring to 2 [ 18 O]-labeling atoms in the residue of Met 105 and Trp 108 .…”