Effect of enzymatic polymerization on the thermal stability of flavonoids

The mechanisms of the enzymatic polymerization of flavonoids, including quercetin, rutin and catechins, have been fairly well researched, but the properties of polymeric forms of flavonoids still require in-depth analysis. The products of enzymatic polymerization are oligomeric flavonoids. The antioxidant and antimicrobial properties of many oligomeric flavonoids have been described in the literature. However, data on the thermal properties of oligomeric flavonoids are lacking and the supplementing of these deficiencies is a scientific novelty of this work. The aim of this study is to determine the effect of enzymatic polymerization on thermal stability of oligomeric flavonoids. As part of the work, oligomeric quercetin, rutin and catechin were prepared by the enzymatic polymerization reaction with laccase and horseradish peroxidase enzymes. The oligomeric structure of the flavonoids was confirmed by spectroscopic (Fourier transform infrared spectroscopy (FTIR) and UV–visible spectroscopy (UV–Vis)) as well as chromatographic (ultra-performance liquid chromatography (UPLC) and gel permeation chromatography (GPC)) methods. Thermal properties of oligomeric flavonoids were investigated using differential scanning calorimetry (DSC) and thermogravimetry (TG). Moreover, the antioxidant and antimicrobial properties of oligomeric forms of selected polyphenols were investigated. Based on DSC analysis, it was found that the common feature of all oligomeric flavonoids was a higher final oxidation temperature and a higher oxidation enthalpy than the reference flavonoids. Thermogravimetric analysis showed that oligomeric poly(flavonoids)-laccase had better thermal stability, which correlated with higher molar mass.