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Chemistry


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Rhubarb (Rheum rhabarbarum)

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Rhubarb stem and leaf, viola, heuchera leaves and a molecule of chrysanthemin, one of the key chemicals giving the red colour of rhubarb stems.

A Finger in the Pie Plant

1. The picture above shows the beautiful shades of reds, pinks and purples you might find in a garden, for example in rhubarb, violas and heuchera leaves. The viola flowers are in a round bottomed flask, a vessel often used for doing chemical reactions. There is also a glass funnel, which is used for transferring liquids or for separating liquids from solids when used in combination with a filter paper. Inside the glass funnel sits an anthocyanin molecule, chrysanthemin (cyanidin 3-glucoside). Anthocyanins make up a large family of molecules which give rise to the reds, pinks and purples which we see in garden plants and flowers, notably in the stems of rhubarb.

Rhubarb is the gardeners friend; early to fruit and easy to maintain. Yet there is much more to rhubarb than crumble and pie. Cultivated by the Chinese for thousands of years as a medicinal herb, rhubarb has been revered for its medicinal as well as culinary uses. Rhubarb stems have long been used as a laxative and their purgative properties have also led to them being considered as dieting aids. More recently, specific molecules in rhubarb stems have been noted as reducing type 2 diabetes and as potential anti-cancer treatments, specifically for pancreatic cancer.

Rhubarb for the aesthetic gardener has two striking features: huge swaths of deep green leaves and a multitude of pink and red stalks. Interestingly, within these two features lies some very different chemistry.

pH paper strips, which are used to tell pH using a colour chart, sit on a rhubarb leaf.

pH paper strips, which are used to tell pH using a colour chart, sit on a rhubarb leaf.

Any good cook knows that the leaves are not suitable for eating and this is due to their high oxalic acid content, which causes the leaves to be toxic. However, rhubarb leaves contain approximately 0.5 % oxalic acid, which means you would have to consume about 5 kg of them for the dose to be lethal. Oxalic acid binds to calcium in the body and produces a solid called calcium oxalate in the kidneys – probably more familiar as kidney stones. On the plus side, soaking the leaves in your water butt creates a pesticide for protecting plants against red spider mite and black-spot on roses and bee keepers use it to kill bee mite.

The red colour of the stems varies between cultivated varieties of rhubarb, but the chemistry that creates the colour is the same, no matter how deep the colour. Contrary to popular belief, the redness of the stems does not indicate flavour or ripeness. The colour is due to the same chemistry as is responsible for many of the natural purples, pinks and reds seen in the garden. The purple of viola petals and the red of Japanese maples, as well as the red of autumnal leaves, are all due to the same family of molecules.

The molecule anthrocyanin has a delocalised, aromatic structure, which changes depending on the environment it is in.

The molecule anthocyanin has a delocalised, aromatic structure, which changes depending on the environment it is in.

2. On a molecular level, the red colour of rhubarb stalks is specifically due to a family of compounds known as anthocyanins. Anthocyanins are of particular interest in the food industry, where natural colourants are sought as safe alternatives to synthetic dyes. The anthocynanin responsible for the red pigment in rhubarb is known as Canada Red ( J. Food Sci., 1968, 33, 592 – 594), which is a mixture of compounds of which cyanidin 3-glucoside, or chrysanthemin, is one of the main components, although the exact composition varies with rhubarb cultivar (Int. J. Food Sci. Techn., 2013, 48, 172-178).

There are numerous anthocyanins, all with the same backbone structure. Key to the structure is the presence of aromatic rings linked together to give a delocalised system. This delocalisation of electron density throughout the molecule allows light to be absorbed and therefore the appearance of colour.

The interaction of anthocyanins with conditions, such as the level of acidity (also known as pH), causes different colours. For example, when in a strongly acidic solution, the positively-charged, or cationic, species is present, which is deep red. When in a mildly acidic solution, the molecule is neutral, it doesn’t have a formal charge, and is colourless. This is because the charged structure has a delocalised system of double bonds which extend throughout the structure, whereas the neutral species does not.

Emodin and rhein belong to a family of molecules known as anthroquinones

Emodin and rhein belong to a family of molecules known as anthroquinones

3. Yet rhubarb truly does deserve it’s title of “pie-plant”, since it really does have it’s finger in all the pies. Not only is it architecturally alluring, a tasty crop, a useful pesticide and a natural dye, its stems also contain another useful family of molecules known as anthraquinones.

The presence of anthraquinones in plant extracts is simply detected by extraction with a solvent, such as benzene, followed by the addition of ammonia. The presence of anthraquinones gives rise to a pink or violet colour.

Anthraquinones also have aromatic rings in their structures and thus are coloured. Both emodin and rhein, also known as Rhubarb Yellow, are orange crystalline compounds and have long been used as laxatives. However, more recently, it has been suggested that emodin may be effective in the treatment of type 2 diabetes, decreasing blood glucose levels (Br. J. Pharmacol., 2010, 161, 113-126). It has also been suggested as an anti-cancer treatment for human pancreatic cancer (Oncology Reports, 2011, 26, 81-89), whilst rhein may provide antibacterial resistance against MRSA, when used in combination with established antibiotics (Exp. Ther. Med., 2012, 3, 608-612).

Whilst the anthraquinones, emodin and rhein, are not soluble in water, the anthocyanin and oxalic acid molecules are. This is because only compounds which are polar, or charged, are soluble in water or in water-based solutions, referred to as aqueous solutions. Thus rock salt (ordinary table salt), made up of sodium, Na+, and chloride, Cl, ions, is very soluble in water, whereas olive oil, which doesn’t have charged or polar molecules, is not soluble in water and forms a separate layer.

Both oxalic acid and anthocyanins are able to form charged species and therefore are soluble in water.

In aqueous solution, oxalic acid exists in equilibrium with its singly and doubly charged ions, called anion and di-anion respectively. This not only makes oxalic acid very soluble in water but also allows it to bond so well to positively charged ions, cations, and in particular metals. The dianionic oxalate, is used in many cleaning fluids because it will bind metals such as iron. Rust is composed of iron oxides which contain iron as a cation, Fe3+. The iron cation binds to three oxalate molecules, thus creating a water-soluble complex which can be easily washed away.

Oxalic acid exists as an anion and a dianion, known as oxalate, in solution

Oxalic acid exists as an anion and a dianion, known as oxalate, in solution

The ferric oxalate complex and rhubarb leaves.

The ferric oxalate complex and rhubarb leaves.

However, this is not the case when the oxalate dianion meets calcium ions, Ca2+. The calcium ions are bigger than iron ions and so four oxalate ions can fit around a calcium ion. Because the oxalate ion has two areas of negative charge, another calcium ion can be attracted and thus a very stable lattice structure is created which is no longer soluble in water. Although this has dire consequences in the body, in the form of kidney stones, it has many technical applications in the field of science. For example, the rare earth metals or lanthanides, which are important for developing technologies, form insoluble complexes with oxalates,  just like calcium. So they can be isolated from their ores by precipitation with oxalate.

Oxalate (in red and black) and calcium ions (in green) bind to form a lattice structure.

Oxalate (in red and black) and calcium ions (in green) bind to form a lattice structure.

So whilst rhubarb may be a versatile plant for the gardener and budding chef, it also has many uses in the laboratory. The molecules contained in various parts of the plant have topical applications in medicine, agriculture and technology. Whether you enjoy rhubarb crumble or not, there is much more to this common garden plant than first meets the eye.

Contributors: Jenny Slaughter (photography, research, text), Natalie Fey (images, editing), Ben Mills (calcium oxalate).

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