Effect of Hydroxyl or Methoxy Group in B-Ring of Flavonols on Structural Change and Antiseptic Ability of Soy Protein Isolate.

Soy protein isolate (SPI) enhances the functionality of flavonols, with their interaction and bioactivity critically dependent on B-ring substitution. Multispectral analysis and molecular docking were used to investigate the interaction between SPI and flavonols (Kaempferide (Kaem), Galangin (Gal), Kaempferol (Kae), Myricetin (Myr)) and their resulting antiseptic/antifungal performance. The results indicated that hydroxylation on the B-ring of flavonols significantly enhanced their binding affinity to SPI through static quenching, primarily driven by hydrogen bonds and van der Waals forces, whereas methylation reduced it. Consequently, the binding affinity followed the order of Myr, Kae, Gal, and Kaem. Key functional findings demonstrated a structure-activity relationship: The hydroxyl or methoxy group in B-ring substantially enhanced antibacterial activity in the determination of colony counts. However, for antifungal activity against Gibberella zeae (GZ) and Physalospora piricola (PP), B-ring hydroxylation was unfavorable, whereas methylation was beneficial. Consequently, Gal shows the best antifungal ability, followed by Kaem, Kae, and Myr. A notable discovery was that while pure flavonols or SPI alone promoted GZ growth, their formed flavonol-SPI complexes reversed this effect, exerting inhibitory activity. These insights provide a significant reference for designing natural, green preservative systems by strategically combining specific flavonols with protein carriers like SPI.