The iodide-catalyzed decomposition of hydrogen peroxide: mechanistic details of an old reaction as revealed by electrospray ionization mass spectrometry monitoring

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Abstract

Electrospray ionization mass spectrometry in the negative ion mode, ESI(-)-MS, was used to investigate the iodide-catalyzed decomposition of H2O2 in aqueous medium. ESI(-)-MS monitoring revealed the presence of an intense anion of m/z 287, suggested to be the [I-IOOH]- anion, which was proposed to be formed in solution (and then transferred to the gas phase by the ESI process) via an interaction between iodide and the neutral and short-lived species IOOH. Evidences for the proposed structure were obtained by CID (collision-induced dissociation) experiments, which yielded exclusively a product ion of m/z 254 (I2•-) via a peroxide radical (HOO , 33 Da) loss. High level ab initio calculations revealed that the formation of the [I-IOOH]- anion from IOOH and I- is a thermodynamically-favored process whereas its fragmentation leading to I2•- and HOO is the most favorable dissociation channel. Hence, an unprecedented mechanism for the iodide-catalyzed decomposition of H2O2 to H2O and O2 based on the assumption of the participation of the key intermediate IOOH was proposed.

Translated abstract

A espectrometria de massas com ionização electrospray no modo negativo, ESI(-)-MS, foi usada para investigar a decomposição da água oxigenada catalisada por iodeto em meio aquoso. O monitoramento por ESI(-)-MS revelou a presença de um ânion intenso de m/z 287, o qual foi sugerido possuir a estrutura [I-IOOH]- em solução aquosa, através de uma interação entre o íon iodeto e a espécie neutra e instável IOOH. Evidências da estrutura proposta para tal ânion foram obtidas através de sua fragmentação por colisão com hélio, a qual forneceu, exclusivamente, um íon-produto de m/z 254 (I2•-), através de uma perda de radical peroxila (HOO , 33 Da). Cálculos teóricos ab initio revelaram que a formação do ânion [I-IOOH]-, a partir da interação entre IOOH e I-, é um processo termodinamicamente favorável, enquanto que sua fragmentação levando a I2 e HOO , é o canal de dissociação mais provável. Deste modo, foi proposto um novo mecanismo para a decomposição da H2O2 catalisada por iodeto para gerar H2O e O2 tendo como base a participação do intermediário-chave IOOH.

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Rapid Commun. Mass Spectrom.

H. Leis, W. Windischhofer (2008)

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Comparative analysis of glycosylinositol phosphorylceramides from fungi by electrospray tandem mass spectrometry with low-energy collision-induced dissociation of Li(+) adduct ions.

S Levery, M. S. Toledo, A Straus … (2001)

Glycosylinositol phosphorylceramides (GIPCs) are a class of acidic glycosphingolipids (GSLs) expressed by fungi, plants, and certain parasitic organisms, but not found in cells or tissues of mammals or other higher animals. Recent characterizations of fungal GIPCs point to an emerging diversity which could rival that already known for mammalian GSLs, and which can be expected to present a multitude of challenges for the analytical chemist. Previously, the use of Li(+) cationization, in conjunction with electrospray ionization mass spectrometry (ESI-MS) and low-energy collision-induced dissociation tandem mass spectrometry (ESI-MS/CID-MS), was found to be particularly effective for detailed structural analysis of monohexosylceramides (cerebrosides) from a variety of sources, including fungi, especially minor components present in mixtures at extremely low abundance. In applying Li(+) cationization to characterization of GIPCs, a substantial increase in both sensitivity and fragmentation was observed on collision-induced dissociation of [M + Li](+) versus [M + Na](+) for the same components analyzed under similar conditions, similar to results obtained previously with cerebrosides. Molecular adduct fragmentation patterns were found to be systematic and characteristic for both the glycosylinositol and ceramide moieties with or without phosphate. Interestingly, significant differences were observed in fragmentation patterns when comparing GIPCs having Manalpha1 --> 2 versus Manalpha1 --> 6Ins core linkages. In addition, it was useful to perform tandem product ion scans on primary fragments generated in the orifice region, equivalent to ESI-(CID-MS)(2) mode. Finally, precursor ion scanning from appropriate glycosylinositol phosphate product ions yielded clean molecular ion profiles in the presence of obscuring impurity peaks. The methods were applied to detailed characterization of GIPC fractions of increasing structural complexity from a variety of fungi, including a non-pathogenic Basidiomycete (mushroom), Agaricus blazei, and pathogenic Euascomycete species such as Aspergillus fumigatus, Histoplasma capsulatum, and Sporothrix schenckii. The analysis confirmed a remarkable diversity of GIPC structures synthesized by the dimorphic S. schenckii, as well as differential expression of both glycosylinositol and ceramide structures in the mycelium and yeast forms of this mycopathogen. Mass spectrometry also established that the ceramides of some A. fumigatus GIPC fractions contain very little 2-hydroxylation of the long-chain fatty-N-acyl moiety, a feature that is not generally observed with fungal GIPCs. Copyright 2001 John Wiley & Sons, Ltd.