Impact of the solubility of phenolic compounds from Highland Barley (Hordeum vulgare L.) on their antioxidant property and protein binding affinity

The impact of the solubility of phenolic compounds (PCs) from highland barley (HB) on their antioxidant property and protein binding affinity was investigated. HB-PCs from the phenolic extract (HBE) were analyzed through the UPLC-QTOF-MS, and five fractions were further divided based on their water solubility. Representative compounds of proanthocyanidins, (+)-catechins, and (−)-epicatechin from water fraction (HB–W, 197.67 ± 4.8 μmol GAE/100 g) were significantly correlated with the antioxidant activity of the HBE. In contrast, the PCs enriched in the acetone fraction (HB-A, 60.2 ± 3.1 μmol GAE/100 g) with a structure of 2-phenyl chromogen ketone group (such as rutin, kaempferol, hesperidin, and quercetin) were positively correlated with the protein binding affinity. Further analysis showed that the water-insoluble HB-A fraction significantly lowered the starch digestibility (37.5%) and postprandial glycemia (17.8%) compared to HB-W fractions. Thus, the solubility of PCs is intimately associated with their biological functions as antioxidants or protein-binding ligands, indicating PCs with strong antioxidant properties or high protein binding affinity are structurally distinct from each other, and the physical properties of phenolic compounds might be an important factor to further the understanding of their health functions and mechanisms.