The enzymatic preparation of novel antioxidants and the oxidative mechanism of the parent compound thereof

Han Peng, Ph.D. Candidate

Monday 22^nd Nov 2021

1-2 PM

Direct link to WebEx:

https://mun.webex.com/mun/j.php?MTID=m90d0d089e343ae981cb1f6320220f934

Abstract:

Antioxidants retard oxidative processes in food and pharmaceuticals as well as in the body. However, potential adverse effects of synthetic antioxidants and insufficient efficacy of current natural/natural-derived antioxidant substitutes necessitate developing and diversifying the novel antioxidants. Several most widely distributed dietary flavonoids and stilbenes, including EGC (epigallocatechin), EGCG (epigallocatechin gallate), resveratrol, and quercetin, serve as efficient natural antioxidants with numerous health effects, but they are relatively poorly soluble in lipophilic media. In this study, novel lipophilic esters of these phenolic compounds were prepared enzymatically. Meanwhile, related synthesis conditions were optimized, and their antioxidant capabilities evaluated and compared using different antioxidant assays. Overall, the antioxidant ability of phenolic esters as natural antioxidants was retained or even enhanced after modification. Thus, certain acylated flavonoids and stilbenoids may serve as viable alternatives of current commercial antioxidants.

On the other hand, apart from the shared highly reactive ortho-polyhydroxy aromatic moieties, some flavonoids, mainly flavonols and anthocyanidins, also possess a relatively unstable C3 unit that is vulnerable to certain physicochemical conditions including oxygen, pH level, and temperature. Consequently, during the alkaline esterification process, oxidative degradation of flavonoids may occur. This phenomenon stimulates a further investigation of oxidative mechanisms happening during the alkaline treatment of natural flavonoids. Herein, HPLC–ESI-ToF-MS technique was employed to identify the oxidation products of flavanols generated by atmospheric oxygen oxidation and various base-catalyzed reaction. The strong basic condition led to accelerated hydrolysis and oxidation of EGCG/GCG and ECG/CG and yielded gallic acid, flavanol aglycones and corresponding o-quinone derivatives. Meanwhile, peroxidation or base-catalyzed cleavage and rearrangement occurred extensively on C- and B-rings of flavanol aglycones and generated various aldehydes and/or acids as well as the formation of dimers/trimers. Details of the reaction mechanisms of C-/B-ring degradation and dimerization/polymerization phenomena are proposed to help better understanding structural changes of flavanols under alkaline conditions employed for esterification modification or to extract the insoluble-bound phenolics from food matrices.