A conformational study of hydroxylated isoflavones by vibrational spectroscopy coupled with DFT calculations

•Full vibrational assignment of a series of dietary hydroxylated isoflavones.•Complete conformational analysis.•Use of vibrational spectroscopy for establishing reliable structure–activity relationships (SARs).•SARs will allow to understand the health-promoting ability of dietary compounds (phytochemicals).

The conformational preferences of a series of hydroxylated isoflavones were studied by optical vibrational spectroscopy (FTIR and Raman) coupled with density functional theory (DFT) calculations. Special attention was paid to the effect of the hydroxyl substitution, due to the importance of this group in the biological activity of these systems. The isoflavones investigated – daidzein, genistein and formononetin – were shown to exist in distinct conformations in the solid state, namely regarding the orientation of the hydroxylic groups at C7 and within the catechol moiety, that are determinant factors for their conformational behaviour and antioxidant ability. In the light of the most stable conformers obtained for each molecule, a complete assignment of their experimental vibrational spectra was performed.