Article

Negative Ion Electrospray Ionization Mass Spectrometry and Computational Studies on Substituted 7-Hydroxycoumarins

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Abstract

Twenty-two substituted 7-hydroxycoumarins were studied by negative ion electrospray ionization collision- induced dissociation (CID) mass spectrometry. Fragmentation pathways were also investigated by computation method using the B3LYP density functional theory. In general, the most important fragmentations of the 7- hydroxycoumarin [M - H](-) ions were the elimination of CO(2) and CO which agreed with the calculated energies of the proposed fragmentation reactions. In most cases, methyl group elimination was also favorable. Methyl group elimination occurred in three different ways, the most interesting being hydrogen rearrangement from a neighboring alkyl group to a ring carbon, which led to a benzyl radical formation. In some cases, CH(2)CO elimination was observed as well. Isomeric compounds gave rise to different CID spectra.

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... [14] More recently, electrospray ionization (ESI) has been employed in the MS and MS n studies of coumarins. [15][16][17][18][19][20] Concannon and co-workers analyzed selected coumarin derivatives, in both positive-and negative-ion modes, by using an ion trap instrument. The observed dissociation characteristics in MS n experiments were surprisingly similar to those observed with EI-MS, although the techniques provide different types of precursor ions; that is, odd-electron (radical) ions ([M] +• ) in EI-MS, while even-electron ions (e.g., [M + H] + or [M -H] -) in ESI-MS n . ...
... Many of these compounds have shown promising anti-inflammatory activity in recent lipopolysaccharide-induced inflammation assays (Timonen et al., unpublished results). Following our earlier mass spectrometric study on 7-hydroxycoumarins, [18] we conducted a similar study with 7-acetonyloxycoumarins with the main emphasis on their dissociation characteristics in low-energy CID-MS n . The experimentally observed dissociation pathways were further investigated with density functional theory (DFT) calculations to confirm putative fragment ion structures and their energetics. ...
... The compounds bearing a 8-methoxy substituent had five instead of four oxygen atoms which increased possible dissociation channels. In comparison with 7-hydroxycoumarins studied earlier by negative-ion ESI, [18] the cleavages of CO and CO 2 were not as important as with 7-acetonyloxycoumarins. This is partly due to the differences between positive and negative ionization applied. ...
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Coumarins are naturally occurring, oxygen-containing heterocycles with considerable pharmaceutical potential. For structural elucidation of natural or synthetic coumarins, tandem mass spectrometry (MS(n) ) represents an essential tool. In this study, fragmentation characteristics of twenty-two 7-acetonyloxycoumarins, having promising anti-inflammatory properties, were investigated with low-energy collision-induced dissociation (CID). Accurate mass measurements were performed on a 12-T Fourier transform ion cyclotron resonance (FT-ICR) instrument. Most CID-MS(n) measurements were performed on a quadrupole ion trap (QIT) instrument, except some additional CID-MS(2) measurements performed on the FT-ICR instrument for further confirmation of some fragment ions. Positive-ion electrospray ionization (ESI) was employed throughout. Density functional theory (DFT) calculations (B3LYP) were carried out to analyze putative ion structures/fragmentation channels. The most favourable dissociation channel for [M + H](+) ions of 7-acetonyloxycoumarins was the elimination of a C3 H5 O(●) radical (57 Da) from the 7-acetonyloxy group via homolytic bond cleavage. The resulting phenolic radical ion was the primary fragment ion for the most compounds studied. Losses of even-electron neutrals, C3 H4 O and C3 H6 O (56 and 58 Da), were also observed. These primary eliminations were accompanied with other characteristic neutral losses from the coumarin skeleton, including H2 O, CO, CO2 , and C2 H2 O (ketene). In addition, propene (C3 H6 ) loss was also observed for 4-propyl or 3-ethyl-4-methyl-substituted compounds. The studied coumarins showed interesting characteristics in low-energy CID due to the presence of a 7-acetonyloxy group, leading to both even- and odd-electron product ions. The main dissociation channels observed for each compound were highly dependent on the substituents in the benzopyranone ring. The present results will advance our knowledge on the dissociation characteristics of both synthetic and natural coumarins. Copyright © 2013 John Wiley & Sons, Ltd.
... However, a mechanistic explanation of the fragmentation pathways for the formation of these product ions has not yet been provided. Deprotonated molecules of many important species were generated in the negative ion mass spectrometry, and they underwent charge induced and charge-remote fragmentations in the tandem mass spectrometry, which were much different from the fragmentation behaviors of the corresponding protonated species1415161718192021. Many negative ions were favorable to undergo the intramolecular nucleophilic displacement, which led to unusual fragment ions22232425262728. ...
... Many negative ions were favorable to undergo the intramolecular nucleophilic displacement, which led to unusual fragment ions22232425262728. The characteristic fragmentations provided useful information for structural elucidation2829303132 and differentiation of isomers [15,21]. Obviously, the title compounds can be easily ionized in the negative mode, due to the presence of the amide hydrogen function [18]. ...
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