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. 2023 Mar 8;71(9):4005-4015.
doi: 10.1021/acs.jafc.2c07453. Epub 2023 Feb 27.

Protonated Forms of Naringenin and Naringenin Chalcone: Proteiform Bioactive Species Elucidated by IRMPD Spectroscopy, IMS, CID-MS, and Computational Approaches

Davide Corinti  1 , Lucretia Rotari  1 , Maria Elisa Crestoni  1 , Simonetta Fornarini  1 , Jos Oomens  2 , Giel Berden  2 , Aura Tintaru  3 , Barbara Chiavarino  1
Affiliations

Protonated Forms of Naringenin and Naringenin Chalcone: Proteiform Bioactive Species Elucidated by IRMPD Spectroscopy, IMS, CID-MS, and Computational Approaches

Davide Corinti et al. J Agric Food Chem. .

Abstract

Naringenin (Nar) and its structural isomer, naringenin chalcone (ChNar), are two natural phytophenols with beneficial health effects belonging to the flavonoids family. A direct discrimination and structural characterization of the protonated forms of Nar and ChNar, delivered into the gas phase by electrospray ionization (ESI), was performed by mass spectrometry-based methods. In this study, we exploit a combination of electrospray ionization coupled to (high-resolution) mass spectrometry (HR-MS), collision-induced dissociation (CID) measurements, IR multiple-photon dissociation (IRMPD) action spectroscopy, density functional theory (DFT) calculations, and ion mobility-mass spectrometry (IMS). While IMS and variable collision-energy CID experiments hardly differentiate the two isomers, IRMPD spectroscopy appears to be an efficient method to distinguish naringenin from its related chalcone. In particular, the spectral range between 1400 and 1700 cm-1 is highly specific in discriminating between the two protonated isomers. Selected vibrational signatures in the IRMPD spectra have allowed us to identify the nature of the metabolite present in methanolic extracts of commercial tomatoes and grapefruits. Furthermore, comparisons between experimental IRMPD and calculated IR spectra have clarified the geometries adopted by the two protonated isomers, allowing a conformational analysis of the probed species.

Keywords: IRMPD action spectroscopy; conformational analysis; flavanones; isomeric discrimination; naringenin; structural elucidation; tandem mass spectrometry.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic representation and numbering schemes of (A) naringenin and (B) naringenin chalcone. The numbering of the atoms is reported according to convention.
Figure 2
Figure 2
Comparison between the IRMPD spectra of protonated naringenin (light blue trace) and naringenin chalcone (red trace, magnified by a factor of 2, as indicated, in the yellow trace).
Figure 3
Figure 3
IRMPD spectrum of [Nar + H]+ (bottom panel) compared with calculated IR spectra of Nar_1, Nar_C1a, Nar_C2, Nar_2a, and Nar_3, whose optimized structures are on the right. Relative free energies at 298 K are reported in kJ mol–1. Distances are given in Å. An open diamond indicates the calculated C4=O stretching mode at 1676 cm–1 for Nar_C1a and Nar_C2, and a black diamond indicates the C5–OH stretching mode expected at 1490 cm–1 for Nar_2a and Nar_3. The y-scale of calculated IR intensity ranges up to 1200 km mol–1.
Scheme 1
Scheme 1. Exemplary Notation of Conformers
Figure 4
Figure 4
IRMPD spectrum of [Nar + H]+ (light blue profile) compared with calculated IR spectra of Nar_1, Nar_1a, and Nar_1_Ax (gray, red, and violet profiles, respectively), whose optimized structures are reported on the right. Distances are given in Å. Relative free energies at 298 K are reported in kJ mol–1. The y-scale of calculated IR intensity ranges up to 1200 km mol–1.
Figure 5
Figure 5
IRMPD spectrum of [ChNar + H]+ (bottom panel, in red) compared with calculated IR spectra of ChNar_1, ChNar_1a_sT, ChNar_2a, and ChNar_3a, whose optimized structures are reported on the right. Distances are given in Å. Relative free energies at 298 K are reported in kJ mol–1. The y-scale of calculated IR intensity ranges up to 3500 km mol–1.
Figure 6
Figure 6
Comparison between the IRMPD spectra of mass-selected ions at m/z 273 from (1) the methanolic extract of tomato peel (upper panel (a and b), black trace) and (2) the methanolic extract of grapefruit albedo (lower panel (c and d), green trace) with standard solution of naringenin chalcone (on the left (a and c), red trace) and of naringenin (on the right (b and d) light blue trace) recorded at higher energy.
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