Effects of different ester chains on the antioxidant activity of caffeic acid

Caffeic acid ester derivatives have been widely found in propolis extract and plants. In this work, the effect of ester groups with different aromatic and alkyl chains on the antioxidant activity of caffeic acid was performed on the double H⁺/e^- process using DFT calculations. We found that 1) O3-H3⋯O4 intramolecular hydrogen-bonds exist in the catechol moiety of the investigated compounds, which have the same strength and are closed shell interactions, weak-strength and electrostatic in nature, making the 4-OH more favourable than 3-OH to trap free radicals. 2) In weak polarity phases, caffeic acid and its derivatives prefer to perform the double H⁺/e^- processes via the dHAT mechanism. In the polar phases, the SdPLdET mechanism is more favoured. The first step of these mechanisms is more possible in 4-OH groups. 3) The ester group with different aromatic and alkyl chains would enhance the antioxidant capacities of caffeic acid.