Phenolic Compounds: Basic Principles

Phenolic compounds are among the most prevalent groups of phytochemicals in plants. These naturally occurring compounds as the prominent secondary metabolites, are of great value due to their physiological properties that contribute in promoting the food quality and human health. These include antioxidant, anti-atherosclerotic, anti-inflammatory, anti-carcinogenic and anti-microbial effects.

Fruits and vegetables are the rich sources of phenols. The concentration of total phenols is variable between different plants as well as being unevenly distributed in the tissues within the same groups of plants. The quantitative variation of phenols is reliant on numerous factors including genetic characteristic, cultivar, maturity level, growing region, harvesting time, handling, processing parameters and storage condition.

The structural feature of phenols defines the basic characteristics of each phenolic compound within the wide range of phenolic classes. These compounds vary from simple phenols (monophenols) through those of more complex polyphenols (high molecular-weight polymers). The biosynthetic mechanism of phenols results in the formation of an aromatic phenyl ring (hydrocarbon group) attached to one or more hydroxyl group/groups. The main precursors of most amino acid-rich phenols comprise tyrosine and phenylalanine that are generated in shikimic acid pathway. The amino acids are converted to cinnamic acids via reactions involving deamination.

There is an extensive range of phenolic compounds that naturally exist in nature. This is basically reflected by the structural diversity of these compounds. There are various category designs of phenolic components that highlights the major components of dietary phenols. They are commonly classified based on the amount of carbons (e.g., C6–C1 denoting hydroxybenzoic acids, (C6–C3–C6)2 representing bioflavonoids, etc.). The largest proportion of phenolic constituents is composed of flavonoids that are abundantly found in plants. Based on the variations in the molecular structure, there is a sizeable classification of flavonoid compounds that include flavonols, flavones, flavanols (catechins), flavanones, flavanonols, isoflavones and anthocyanidins.

The antioxidant attributes of biophenols specifies their bioactive ability against oxidation. The actual mechanism involves the hydrogen-donating of the hydroxyl group to favourably scavenge most free radicals. Free radicals are characterised as reactive oxygen species (ROS). They are adversely responsible for damaging the prominent components of the cell (including oxidation of amino acids, lipid peroxidation and DNA damage). The inhibition of free radicals brings about increasing stability and decreasing reactivity of the resultant phenolic radicals due to having lower energy to be catalysed and therefore the incidence of oxidation is minimised/suppressed.

Furthermore, the antioxidant efficiency of phenols can be developed by the chelation activity of metal chelating agents. This refers to chelating transition metals that is influential on limiting the generation of ROS. This results in curtailing the redox-catalytic ability of metal ions in the liberation of ROS and hence the reduction of oxidative damage is expected.  The redox-active metals can be inactivated by endogenous or exogenous chelating agents. Some examples of organic chelating agents are gluconic acids, citric acid and homocitric acid. Some natural phenols such as 2,3-Dihydroxybenzoic acid are effectively accountable for the chelation role as well. 

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