Effect of flavonoid structure and pH on iron-mediated pectin interaction

Flavonoids are polyphenolic natural compounds with suggested health-promoting properties and impact on sensory aspects. Processing, as well as mastication of plant-based food, can allow interactions between flavonoids and plant cell wall material (CWM). Pectin is a cell wall polysaccharide, present in all plant-based products, and used in a wide range of foods. Recently, a novel iron-mediated mechanism for pectin-flavonoid interactions was suggested. The current work aimed to study the impact of flavonoid structure and pH on the iron-mediated interaction by using 7 aglycones (belonging to the flavonol, flavanonol, and flavone sub-groups) differing by specific small structural moieties resulting in different binding sites. By using commercial water-soluble pectin from apple pomace and an identical (except a higher content of iron) iron enriched pectin, we have qualitatively and quantitatively presented the importance of the C2–C3 double bond for such interactions. Without such a bond the aglycones do not bind to the pectin, likely due to the loss of molecular planarity. This was studied using UV-VIS spectroscopy and membrane centrifugation. Also, the binding dissociation constants (K_d) of the flavonoids presenting a specific wavelength for the ternary complexes were determined using UV-VIS spectroscopy, showing that the lack of a hydroxyl group on C3 only slightly reduces the affinity (K_d increased from 13.4 × 10⁻⁶ M for quercetin to 29.2 × 10⁻⁶ M for luteolin), while a covalently bound disaccharide at this position practically nullifies the binding. pH 2.5 significantly reduced such binding while the difference between 4.5 and 6.5 was small and structure dependent.