Oxidative polymerization process of hydroxytyrosol catalysed by polyphenol oxidases or peroxidase: Characterization, kinetics and thermodynamics.

Hydroxytyrosol oligomer prepared by bioenzyme shows stronger health-promoting properties than its monomer. However, the polymerization process carried out by laccase, tyrosinase or horseradish peroxidase is still lacking in term of product characterization, kinetics and thermodynamics. To achieve these aspects, ATR-FT-IR, NMR, the Michaelis-Menten equation and isothermal titration calorimetry were explored. The results showed that the identified polymers presented a CC bond and a degree of polymerization less than six. Laccase showed the greatest affinity to hydroxytyrosol via comparison of Km and Vm. All of these polymerization processes were spontaneous and exothermic behaviuors ranging from 30 to 50 °C, and were driven by hydrogen bonds, van der Waals interactions and hydrophobic interactions. Furthermore, circular dichroism spectroscopy was used to reveal the enzymatic structural changes during the catalysis, which showed that β-sheet levels for laccase, α-helix levels for tyrosinase, and the α-helix and random coil levels for horseradish peroxidase were dramatically decreased.