Polyphenol

Polyphenols are a massive family of naturally occurring chemical compounds. They are found abundantly in plants and are structurally diverse. The name comes from Ancient Greek, where "poly" means many or much. The term "phenol" refers to a specific chemical structure. This structure involves an aromatic benzenoid, or phenyl, ring attached to a hydroxyl group. This hydroxyl group is a single oxygen atom bonded to a hydrogen atom.

Scientists have used the term "polyphenol" since at least 1894. These compounds are essential to many biological systems. They play roles in how plants grow and how they interact with their environment.

There are different ways to define what makes a molecule a polyphenol. One method is the White–Bate-Smith–Swain–Haslam, or WBSSH, definition. This group focuses on the structural traits of plant phenolics used in tanning, known as tannins. According to WBSSH, these are moderately water-soluble compounds. They typically have a molecular weight between 500 and 4000 Daltons. They also feature more than 12 phenolic hydroxyl groups and 5 to 7 aromatic rings per 1000 Daltons.

Another scientist, Stéphane Quideau, uses a different definition. He defines them as compounds derived from the shikimate/phenylpropanoid or polyketide pathways. He notes they must have more than one phenolic unit and lack nitrogen-based functions. Some molecules, like ellagic acid, fit Quideau's definition but not the WBSSH definition. However, raspberry ellagitannin meets the criteria for both.

Polyphenols can be categorized into several principal classes. These include phenolic acids, flavonoids, stilbenes, and lignans. Flavonoids are a particularly large group with many sub-types. These include flavones, flavonols, flavanols, flavanones, isoflavones, proanthocyanidins, and anthocyanins. Many of these are found in the foods we eat every day. For example, catechin is in tea and fruits. Hesperetin is found in citrus fruits. Cyanidin provides color to red fruits and berries. Daidzein is found in soybeans, and quercetin is in onions and apples.

Proanthocyanidins are often found in apples, grapes, and cocoa.

At a molecular level, the structure of a polyphenol determines how it behaves. Some are small molecules that can move through cell membranes easily. Others are macromolecules, which are very large molecules. Many larger polyphenols are built inside the plant from smaller units. These units can be connected by different types of chemical bonds. Hydrolyzable tannins use ester linkages to connect their parts. Nonhydrolyzable condensed tannins use more stable carbon-carbon bonds.

Many polyphenols also contain repeating units like pyrocatechol or resorcinol. This complexity allows them to serve many different functions within a living organism.

In the wild, polyphenols serve vital ecological purposes for plants. They act as UV screens to protect against ionizing radiation. They also provide coloration through plant pigments. Some polyphenols act as phytoalexins, which help prevent microbial infections. They can even act as deterrents to herbivores by changing how a plant tastes or feels. In some woods, they help preserve the wood against rot.

They can even affect other plants through allelopathic interactions. For instance, the aquatic plant Myriophyllum spicatum secretes them to interact with its surroundings. These molecules are essential for a plant's survival and its role in the forest nutrient cycle.

Scientists use many specialized tools to study these compounds. To find them, they use extraction methods with solvents like water, methanol, or ethanol. They can also use high-pressure liquid extraction or even microwave-assisted extraction. Once extracted, they use High Performance Liquid Chromatography, or HPLC, to separate the different compounds.

To measure how much is present, they might use volumetric titration or colorimetric measurements. Some tests, like the Folin–Ciocalteu reaction, measure total phenol content. Others, like the TEAC assay, measure antioxidant capacity. These scientific methods help us understand exactly how much of a specific polyphenol is in a sample.

Humans have used the properties of polyphenols for centuries. Historically, they were used as dyes for tanning garments and fabrics. In the Indian subcontinent, people used pomegranate peel to dye non-synthetic fabrics. In the early days of silver-based photography, pyrogallol and pyrocatechin were used as photographic developers.

Today, there is interest in using them as "green chemicals." Scientists want to use plant residues, like grape or olive scraps, to create renewable plastics or resins. This could also lead to new ways to make adhesives for particleboards. By turning waste into useful materials, we can use the power of polyphenols to help the environment.