IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid

Abstract

Genistein is a naturally occurring polyphenol belonging to the family of flavonoids with estrogenic properties and proven antioxidant, anti-inflammatory, and hormonal effects. Genistein and its derivatives are involved in radical scavenging activity by way of mechanisms based on sequential proton-loss electron transfer. In view of this role, a detailed structural characterization of its bare deprotonated form, [geni-H]^-, generated by electrospray ionization, has been performed by tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy in the 800-1800 cm^-1 spectral range. Quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory were carried out to determine geometries, thermochemical data, and anharmonic vibrational properties of low-lying isomers, enabling to interpret the experimental spectrum. Evidence is gathered that the conjugate base of genistein exists as a single isomeric form, which is deprotonated at the most acidic site (7-OH) and benefits from a strong intramolecular H-bond interaction between 5-OH and the adjacent carbonyl oxygen in the most stable arrangement.

Figure 1

Structure of genistein. The numbering of carbon atoms defining the dihedral angle τ1 (formed by the C2–C3–C1′–C2′ atoms) is reported in red.

Scheme 1. Reaction Scheme for Antioxidant Action in Water via Deprotonated Genistein

Figure 2

Optimized geometries of the lowest-energy isomers and conformers of deprotonated genistein computed in the gas phase at the B3LYP/6-311+G(d,p) level of theory. Relative free energies are reported in parentheses in kJ mol^–1, and intramolecular hydrogen-bond distances (green dashed lines) are reported in Å.

Figure 3

Optimized geometries of isomers and conformers of deprotonated genistein computed in the gas phase at the B3LYP/6-311+G(d,p) level of theory. Relative free energies are reported in parentheses in kJ mol^–1, and intramolecular hydrogen-bond distances (green dashed lines) are reported in Å.

Figure 4

IRMPD spectrum (orange and magenta profiles) and the calculated anharmonic IR spectra (blue profiles) of the lowest-lying conformers and isomers of deprotonated genistein computed at the B3LYP/6-311+G(d,p) level of theory in the gas phase. Free energies relative to G-H_O7_a are reported in parentheses (kJ mol^–1).