Antioxidant Activity of Phenolic Acids: Correlation with Chemical Structure and in Vitro Assays for Analytical Determination

Phenolic acids are a subclass of a larger category of plant metabolites commonly referred to as “phenolics” possessing a carboxylic acid functionality. In general, the term phenolics encompasses approximately 8000 naturally occurring compounds, all of which possess one common structural feature, a phenol (an aromatic ring bearing at least one hydroxyl substituent). The naturally occurring phenolic acids contain two distinguishing constitutive carbon frameworks: the hydroxycinnamic and hydroxybenzoic structures. Although the basic skeleton remains the same, the numbers and positions of the hydroxyl groups on the aromatic ring create the variety. Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases due to their antioxidant activity (AOA). This property is related with the ability to scavenge free radicals, donate hydrogen atoms or electron, or chelate metal cations. The molecular structure, referred to as structure–activity relationships SAR (including substituents on the aromatic ring, numbers and positions of the hydroxyl groups in relation to the carboxyl functional group, esterification, glycosylation) affects the antioxidant properties.With the current upsurge of interest in the function, measurement of efficacy and use of natural antioxidants has received much attention. There is a great multiplicity of the methods used for the determination of AOA, which can be broadly classified two groups. In the first group, the degree of inhibition of lipid peroxidation is measured by using lipid or lipoprotein substrate under standard conditions; while in the second the radical scavenging ability is determined. Moreover, a number of assays have been introduced for determining the total antioxidant activity, intended as the cumulative capacity of food compounds to scavenge free radicals. In the discussion, phenolic acids structure as well as SAR will be considered. The various in vitro methods used for the determination of antioxidant activity, with their merits and limitations, will be also presented.