Structural and antioxidant properties of hydroxytyrosol-pectin conjugates: Comparative analysis of adsorption and free radical methods and their impact on in vitro gastrointestinal process

Abstract

Limited data exist on the difference in polysaccharide-polyphenols conjugation using two methods: adsorption technique and free radical induction. This study investigates the structural and antioxidant properties of complexes formed between hydroxytyrosol (HT) and two commercial pectins (apple and citrus) using both approaches. The adsorption method generates non-covalent bonds, while the free radical method creates covalent bonds with the aid of ascorbic acid and hydrogen peroxide. Findings indicate that HT-pectin complexes exhibit distinct structural and antioxidant properties depending on the conjugation method. Proton nuclear magnetic resonance ( 1 H NMR) analysis of HT-pectin conjugates showed that the free radical method induced more significant changes in the chemical environment of HT and pectin protons compared to the adsorption method, suggesting stronger interactions.The retention of HT was higher in complexes formed via adsorption, with citrus pectin showing the highest HT retention (94.7 mg/g) compared to the free radical method (60.2 mg/g). Additionally, the antioxidant activity of each complex was directly related to the proportion of HT conjugated. In vitro antioxidant assays (DPPH and reducing power), revealed that adsorption method enhanced the antioxidant capacity of free HT more effectively than the free radical approach. Simulated gastrointestinal digestion showed substantial HT release in the gastric phase and partial degradation in the intestinal phase, with 27% to 35% of HT retained in the free radical method and 34.6% to 48.1% in the adsorption method. These results suggest that HT bound to pectin could serve as an effective antioxidant in the colon. Conjugating HT with pectin may provide specific benefits for gut health by facilitating the development of functional foods that support a balanced intestinal microbiota. • Citrus pectin retains more Hydroxytyrosol via adsorption than free radical method. • Free radical method forms stronger Hydroxytyrosol-pectin covalent bonds. • Hydroxytyrosol-pectin complexes show higher antioxidant activity via adsorption. • In vitro digestion releases 48-63% Hydroxytyrosol from pectin conjugates. • Hydroxytyrosol-pectin conjugates could improve antioxidant delivery to the colon.

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