Preparation of Quercetin Mono-Esters and Their Antioxidant Activity as Affected by the Acyl Chain Length

Objective: Due to the potential adverse effects of synthetic lipophilic antioxidants, research on natural sources of antioxidants such as quercetin has intensified. However, quercetin is a relatively polar molecule and may not be easily incorporated into food lipids. Therefore, investigating novel quercetin derivatives via enzymatic modification and evaluation of the antioxidant activity of the esterified products is of interest in this study.
Materials and

Methods: To achieve our objectives, we used a green enzymatic, ultrasonically-assisted process. Various saturated fatty acids with 2 to 18 carbon atoms were used as acyl donors in order to increase the hydrophobicity of quercetin. Nineteen different lipases and twelve different solvent systems were screened. Products prepared under most effective conditions were then tested for their antioxidant activity using the ABTS, DPPH, and metal-chelation methodologies.

Results: The quercetin monoesters exhibited a lower ABTS radical scavenging ability (48.04-98.39% equivalents of Trolox, n/n) than the parent quercetin molecule (185.40% equivalents of Trolox). Similar results were found for the DPPH scavenging and ferrous chelation test. Overall, quercetin esters with short-medium chain length showed the highest antioxidant ability in both ABTS and DPPH as well as metal-chelation assays.

Conclusion: The ultrasonication enhanced the esterification yield. The quercetin monoesters displayed a significantly higher chelation ability, but a lower radical scavenging capacity than quercetin itself. However, the monoesters so prepared were as effective as the traditional synthetic antioxidants (BHA, BHT, and TBHQ). Thus, acylated quercetins could serve as viable alternatives to synthetic antioxidants. However, preparation conditions may require further improvements.