Hydroxycinnamic acid antioxidants: an electrochemical overview

Abstract

Hydroxycinnamic acids (such as ferulic, caffeic, sinapic, and p-coumaric acids) are a group of compounds highly abundant in food that may account for about one-third of the phenolic compounds in our diet. Hydroxycinnamic acids have gained an increasing interest in health because they are known to be potent antioxidants. These compounds have been described as chain-breaking antioxidants acting through radical scavenging activity, that is related to their hydrogen or electron donating capacity and to the ability to delocalize/stabilize the resulting phenoxyl radical within their structure. The free radical scavenger ability of antioxidants can be predicted from standard one-electron potentials. Thus, voltammetric methods have often been applied to characterize a diversity of natural and synthetic antioxidants essentially to get an insight into their mechanism and also as an important tool for the rational design of new and potent antioxidants. The structure-property-activity relationships (SPARs) correlations already established for this type of compounds suggest that redox potentials could be considered a good measure of antioxidant activity and an accurate guideline on the drug discovery and development process. Due to its magnitude in the antioxidant field, the electrochemistry of hydroxycinnamic acid-based antioxidants is reviewed highlighting the structure-property-activity relationships (SPARs) obtained so far.

Figure 1

Differential pulse voltammograms for 0.1 mM solutions of (a) (black line) caffeic acid, (blue line) 5-bromocaffeic acid, (green line) 3,4,5-trihydroxycinnamic acid, and (red line) hydrocaffeic acid and (b) (black line) ferulic acid, (blue line) 5-bromoferulic acid, (green line) sinapic acid, and (red line) hydroferulic acid, in physiological pH 7.3 supporting electrolyte. Scan rate: 5 mV s⁻¹.

Figure 2

Cyclic voltammograms for 0.1 mM solutions of (a) (black line) caffeic acid, (blue line) 5-bromocaffeic acid, (green line) 3,4,5-trihydroxycinnamic acid, and (red line) hydrocaffeic acid and (b) (black line) ferulic acid, (blue line) 5-bromoferulic acid, (green line) sinapic acid, and (red line) hydroferulic acid, in physiological pH 7.3 supporting electrolyte. Scan rate: 50 mV s⁻¹.

Figure 3

Differential pulse voltammograms for 0.1 mM solutions of (a) (black line) ethyl caffeate, (blue line) ethyl 5-bromocaffeate, (green line) ethyl 3,4,5-trihydroxycinnamate acid; (b) (black line) ethyl ferulate, (blue line) ethyl 5-bromoferulate, (green line) ethyl sinapate; (c) (black line) caffeoylhexylamide, (red line) hydrocaffeoylhexylamide; and (d) (black line) feruloylhexylamide, (red line) hydroferuloylhexylamide, in physiological pH 7.3 supporting electrolyte. Scan rate: 5 mV s⁻¹.

Scheme 1

Principle of DPPH radical scavenging capacity assay.