In vitro Antioxidant Activity of 5-caffeoylquinic Acid and Ester Analogues
Chlorogenic acid, an ester of caffeic acid with quinic acid, also known as 5- O-caffeoylquinic acid (5-CQA), is a ubiquitous plant constituent that is an important intermediate in lignin biosynthesis. In some cases, it occurs at surprisingly high levels in the leaves and fruits of certain higher plants, such as coffee beans. Due to its catechol moiety and an extended side chain conjugation, it easily forms a resonance-stabilised phenoxy radical, accounting for its powerful antioxidant potential. The objective of this work was to determine if the esterification and methylation of 5- CQA would enhance its antioxidant activity. Two 5-CQA derivatives were prepared for this study. Chlorogenic acid was esterified with methanol over Amberlite IR120-H to obtain methyl chlorogenate, while methyl 3',4´-dimethyl chlorogenate was prepared from 5-CQA by treatment with diazomethane. Spectroscopic methods confirmed the structure of these derivatives. Their antioxidant properties were tested to establish a relationship between structure and antioxidant activity. Antioxidant activity results were generated for 5-CQA and its ester analogues using eight different methods. Depending on the method applied, results were expressed as IC50/MCE50 values or as equivalents of the applied standard (ascorbic acid and Trolox). In most of these tests, 5-CQA showed the highest antioxidant activity compared to its derivatives. Nevertheless, due to their hydrophobic characteristics, their ester analogues remain promising antioxidant candidates in emulsifying systems.