Iron ions as mediators in pectin-flavonols interactions

Flavonols are natural compounds that are widely distributed in fruits and vegetables and suggested to have beneficial health effects. During processing of plant-based food products as well as during mastication, the flavonols that usually located within the cell can be released and interact with other plant compounds such as pectin - a part of the cell wall material. Such interactions might affect the flavonols stability, functionality, bio-accessibility, and bioavailability. The aim of this work was to explore the interactions, in pH 4.5 buffered solution, between water-soluble commercial pectin (from apple pomace) and flavonols that possess low aqueous solubility, and to present a novel possible interaction mechanism mediated, at least partially, by pectin bound iron ions. We report the preparation method of iron enriched pectin (IRP) and reveal its interactions with quercetin and rutin, compared to the interactions with commercial pectin, having a lower iron content, analyzed by UV-VIS spectroscopy, membrane centrifugation, HPLC and SEC-MALLS-RI-UV system. Our results point towards high involvement of iron ions in the interaction between flavonols and pectin and dependence of these iron-mediated interactions on: duration of interaction, iron content and flavonol structure. This conclusion was farther verified by the use of rutin (the glycoside of quercetin) that resulted in a significantly lower tendency to bind pectin, this difference might be explained by the presence of sugar moiety that reduces the number of possible iron binding sites and also can origin in a steric hindrance.