Detection of oxygen addition peaks for terpendole E and related indole-diterpene alkaloids in a positive-mode ESI-MS

Abstract

This report describes that a regular positive electrospray ionization mass spectrometry (MS) analysis of terpendoles often causes unexpected oxygen additions to form [M + H + O]+ and [M + H + 2O]+, which might be a troublesome in the characterization of new natural analogues. The intensities of [M + H + O]+ and [M + H + 2O]+ among terpendoles were unpredictable and fluctuated largely. Simple electrochemical oxidation in electrospray ionization was insufficient to explain the phenomenon. So we studied factors to form [M + H + O]+ and [M + H + 2O]+ using terpendole E and natural terpendoles together with some model indole alkaloids. Similar oxygen addition was observed for 1,2,3,4-tetrahydrocyclopent[b]indole, which is corresponding to the substructure of terpendole E. In tandem MS experiments, a major fragment ion at m/z 130 from protonated terpendole E was assigned to the substructure containing indole. When the [M + H + O]+ was selected as a precursor ion, the ion shifted to m/z 146. The same 16 Da shift of fragments was also observed for 1,2,3,4-tetrahydrocyclopent[b]indole, indicating that the oxygen addition of terpendole E took place at the indole portion. However, the oxygen addition was absent for some terpendoles, even whose structure resembles terpendole E. The breakdown curves characterized the tandem MS features of terpendoles. Preferential dissociation into m/z 130 suggested the protonation tendency at the indole site. Terpendoles that are preferentially protonated at indole tend to form oxygen addition peaks, suggesting that the protonation feature contributes to the oxygen additions in some degrees. © 2014 The Authors. Journal of Mass Spectrometry published by John Wiley & Sons, Ltd.

Loss of atomic nitrogen from even‐electron ions? A study on benzodiazepines

“…(Note that in both cases, oxygen attaches to distonic ions.) Lemr et al described the attachment of O 2 to metal complexes, while Hongo et al observed oxygen attachment to terpendole derivatives; in the latter study, the water in the solvent was identified as the source of the attached oxygen, while our results clearly show that the O 2 source is the residual gas in the mass spectrometer.…”

“…While the formation of solvent adducts to ions present in an ion trap mass spectrometer has been reported several times; attachment of molecular oxygen has rarely been observed [29‐32] . In the present case of benzodiazepines, some prerequisites for its occurrence, most notably the unique structure of the distonic ion generated by the cleavage of the nitro group, have been determined.…”

Gas phase reaction between chromones and solvent in an electrospray ionization source

“…7 The [M + 15] + ion was observed as the most abundant ion when measuring aromatic aldehydes due to the gas phase in-source aldolization reaction. 8,9 Other side reactions, occurred during the ionization process, such as redox, 10,11 fast dimerization reaction, 12,13 and oxygen addition, 14 were also observed using ESI as the ionization method.…”

“…The [M + 15] + ion was observed as the most abundant ion when measuring aromatic aldehydes due to the gas phase in‐source aldolization reaction . Other side reactions, occurred during the ionization process, such as redox, fast dimerization reaction, and oxygen addition, were also observed using ESI as the ionization method. The in‐source reaction have also been reported in other types of soft ionization processes, such as hydrolysis of neonicotinoid pesticides in the atmospheric pressure chemical ionization source and the Birch reduction of benzene in a low temperature plasma ionization source .…”