This month we’re looking at a whole class of molecules – polyphenols – because it seems like they are in everything and can do anything! From vegetables, fruits, tea, and other healthy foods through to wine and chocolate, these complicated ring-filled molecules pop up in all sorts of places. So let’s dive in and ask what are polyphenols and is there is anything they can’t do?

Starting simple – phenol

Polyphenol means that we have lots of phenols joined together. So before we look at what a polyphenol is, we should probably know what phenol itself is!

A phenol structure made from atom models. The molecule is a hexagon of black carbon atoms. Five of those carbons have a tiny white hydrogen atom on them. The remaining one has an alcohol group - one big red oxygen with a small white hydrogen on it.
The structure of phenol. The molecule is totally flat!

This molecule has a 6 membered carbon ring – a very nice, flat hexagon thanks to all those double bonds – and a hydroxy (otherwise known as an alcohol) group sticking off from one carbon. These types of molecules are incredibly common in chemistry so if ever you need someone to draw you a really good hexagon, find a chemist.

corrosive sign

Phenol is not a nice molecule.

Phenol on its own is a really toxic molecule. It can cause chemical burns and as little as 1 gram can be fatal. However, it is also completely naturally formed in your body and is excreted normally too when in small amounts.

So if it is so toxic in large amounts, why have creatured evolved to make it and use it in the first place?

A ruby

Shine On You Crazy Diamond

As Pink Floyd said – if it’s shininess your’re after you definitely want a diamond. As we saw from April’s molecule of the month – aluminium oxide – ruby and sapphire are pretty shiny too!

But what if we want to absorb light instead of reflecting it?

To absorb that light instead, we need to rearrange all the carbon atoms from that diamond into a different shape. What we’re going to need is to break all the single bonds between the carbons in diamond and make plenty of double bonds instead. The real trick is to alternate single and double bonds around a molecule. Chemists call this conjugation and whenever you see that alternating single-double bond structure, you know there is some interesting chemistry coming up.

One property of conjugated molecules is that all the electrons that make up those bonds can bounce around between different states. In some states they might have more energy than in others – think of it like your moods, some are more excitable than others. In your moods you might flip between them when you absorb sugar, or caffeine, for example. Electrons though swap these energy states when they absorb their own stimulant – energy from light. The light has to be just the right energy to match the gaps between these electron energy states.

So changing the shape of a molecule, its structure, changes the gaps between the electron energy states and so changes the colour of light that molecule can absorb to excite its electrons. The balance between which light is absorbed, reflected or even emitted helps to determine the colour a substance has – at least as we see it!

the structure of retinol - it has alternating single and double bonds throughout.

Seeing is believing

A great example of these molcules is found in your eyes – and it’s because of all those double bonds that you can see at all!

Back to Phenols.

Phenols have conjugation a-plenty so can absorb light. In fact, phenol can do more then just undergo conjugation when it absorbs light. Phenol is known as a photo-acid. When exposed to light, the excitation of the electrons can actually result in the hydrogen on the hydroxy group coming off. Because of this, phenol can be described as a photo-acid.

The structure of phenol shown in two forms - one with an alcohol group (OH) and one with the oxygen double bonded into the carbon ring and the hydrogen from the OH moved to one carbon instead.
Phenol has two forms – or tautomers – and both have plenty of conjugation.

The colour of light that can be absorbed though is so incredibly purple its beyond what human eyes can see. We call it ultra-violet or UV light. UV can be really harmful to living things, or really useful if you can absorb that energy and use it to grow. Plants spend all their time in the sun and actually use phenol-based molecules to help protect them from the damage that UV can cause.

colour wavelengths
Ultraviolet light is just outside the range of wavelengths we can see. Find out more about what we can see and what colour even is here!

But one phenol can only absorb so much light – and only a narrow range of colours of UV! To get around that, and have a wide range of light absorbed and protected against, plants make loads of molecules that are similar but a bit different. The thing that is the same between all those molecules that we’re looking at today? There are a lot of phenol groups in them. Hence, polyphenols.

Adding rings – polyphenols

These molecules can be really complex. There’s a lot of them. Forgive us for not covering every single detail of every molecule in this massive class of chemicals, this is going to be a whistle-stop tour instead.

Here are some particularly interesting polyphenols, and what they might be useful for.

Flavonoids

Flavonoids are often found in plants. In fact, over 5000 different flavonoids have been found in plants! That’s a lot of flavonoids.

a molecular structure made from atomic model kits. There are three hexagons. Two of the hexagons share an edge and are fused together. The third hexagon is joined by a single bond.
The structure of an example flavonoid – flavone

Most flavonoids have a structure which consists of a 15 carbon skeleton, 2 phenyl rings and a heterocyclic ring. A heterocyclic ring is a ring structure where one of the atoms that makes up the ring is not carbon. It could be oxygen, nitrogen or even sulfur! In the flavonoids, the heterocyclic ring contains an atom of oxygen!

Keen eyes readers will notice that there isn’t a phenol in this molecule – the rings are PHENYL rings instead. Very closely related. However, flavonoids are still classed as polyphenols and do often contain phenols!

In plants, the flavonoids can have many uses. The most common use is as a pigment to give the flowers the wonderful array of colours they have. This is especially important in attracting bees, moths and other creatures that helps to pollinate the plant.

Because so many flavonoids are found in plants and because we eat so many plant based foods, then the flavonoids are the most common polyphenols found in our diet. Studies suggest that a human can consume between 100 and 200 milligrams of flavonoids per day.

We can divide the flavonoids into smaller categories, each found within a specific food or type of food. Let’s start with the anthocyanidins.

Anthocyanidins

The anthocyanidins are a type of flavonoids responsible for the bright color of many foods. One example is cyanidin, which is common in blueberries. Cyanidin is responsible for the red colour seen in many other berries including grapes, cranberries , raspberries and blackberries.

Blueberries owe their rich colour to anthocyanidins
the structure of cyanidin
Cyanidin is a brightly-coloured molecule that gives blueberries their distinctive colour.

The structure of cyanidin is shown above. However, changing the groups attached to the rings will change the color of the molecule. Delphinidin has very similar structure to cyanidin, but contains just one extra -OH group. This molecule gives a blue colour to the flowers of plants such as the delphinium and viola.

The other interesting feature of the anthocyanidins is that they are pH sensitive. In order words, their colour will change depending on how acidic or basic the environment around them is. This is why red cabbage can be used as a pH-indicator – it contains many different anthocyanidins.

In highly acidic solutions, the red cabbage indicator turns red. As the pH increases and the solution becomes weakly acidic, neutral and than basic, the colour changes from violet, to blue to green and finally yellow.

Flavanols

Another group of flavonoids are the flavanols. Similar in name, but slightly different in structure. You are most likely to find the flavanols in black tea.

The structure of flavan-3-ol

A common flavanols is known as catechin. This molecule is found in black tea too, but also in chocolate and even barley. Early research suggests there MIGHT be link between catechins are positive health benefits such as blood pressure. But a lot more research is needed!

As we know a cup of tea contains a lot of atoms – 11 septillion to be more exact. As a result, we consume many, many molecules of flavanols every time we have a cup of tea!

Flavonols

Unfortunately, the flavonols have a very similar spelling to the flavanols. So what’s the difference chemically?

Well the flavonols (with an ‘o’) contain a ketone group (C=O) in the molecule. This group is not present in the flavanols.

Many flavonols molecules are able to fluoresce in two different ways. This is because the molecule is able to move a hydrogen atom around its structure from excited. As a result, the structure of the molecule can change ever so slightly. However, this is enough to change the colour it fluoresces.

Research has suggested that flavonols along with proanthoycyanins (shown below) might have health benefits. A study of cranberry juice, which contains both types of molecules, showed they might play an important role in prevent urinary tract infections (UTIs).

Tannins

So far, the polyphenols we have looked at have been relatively small. However, let’s have a look at the tannins!

Truly the king of polyphenols – these molecules are huge.

The king of the polyphenols. Tannic acid and its derivates are huge molecules.

Even though tannins are associated with food, the name tannin originally refers to oak bark. This is because oak bark was used in the tanning of animal skin into leather. Sometimes you will hear tannins being used to treat leather.

However, when it comes to food, tannins can be found in lots of different kinds! Berries, nuts, herbs, and legumes are contain tannings. However, most of the tannins we consume come from tea, coffee and wine. Some people drink one of the other, some drink all three, although usually not at the same time.

Tannins are what give black tea and wine that bitter, dry taste in your mouth. They also affect the color and texture of the wine.

wine

Wine is usually prepared by the fermentation of grapes. Tannins can be found in the skin, stem and seeds of the grapes using the make wine. They can also be introduced if the wine is stored or aged in oak barrels.

The longer the skin, stem and seeds are allowed to soak in the juices, the more tannin the wine will contain. In general, red wines will contain more tannins than white wines.

Tannins are a natural anti-oxidant. Anti-oxidants are important substances that prevent oxidation from occurring. Oxidation can produce very reactive chemical species that can spoil food or damage cells. It is also believe that anti-oxidants can be beneficial to our health.

Tannins are also found in dark chocolate alongside theobromine

Health benefits? Polyphenols as medicines

We have seen how polyphenols are responsible for the color and taste of many flowers and foods. Because of this, we consume a lot of different polyphenols. We have already seen how some of the polyphenols found in foods can be beneficial to our health. So could polyphenols can be used as medicines? Or even just help us to maintain a healthy. lifestyle?

Both theaflavin and catechins in green tea are good antioxidants. We’ve already covered the benefits of tannins as anti-oxidants, but other polyphenols could also be beneficial.

Theaflavin, a type of polyphenol made from flavanols, could be used to stave off Alzheimer’s disease.

Studies have also shown that polyphenols can be good for skin as sunscreens. Furthermore, they could also be used to help reduce or even prevent wrinkles. Although, this is probably not a reasonable excuse to drink more wine or eat more chocolate!

As well as being antioxidant, polyphenols may also be anti-inflammatory

Many phenols, many uses

It’s impossible to escape polyphenols. If you eat, chances are you’re consuming some polyphenols. However, polyphenols in food and the colour of flowers only covers a small fraction of this class of molecules.

Polyphenols can also be used as dyes and in production of plastics. The word ‘poly’ means ‘many’ and so it’s obvious that polyphenols and poly-useful!

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