Sunflower (Helianthus anuus ‘Velvet Queen’), in a shallow beaker with a short strand of cis-polyisoprene (20 monomers).
Originating in the Americas, sunflowers have joined the potato and tomato in becoming part of cuisine and culture across Europe. Whether they feature in growing contests in the British children’s programme Blue Peter, or are grown to provide a cooking oil from their seeds, sunflowers are a common part of our daily lives. Vincent van Gogh famously painted a series called simply ‘The Sunflowers’, originally intended to decorate the room of his friend Paul Gaugin. While one can read many things into such works of art, we do know several rather fascinating scientific facts about some of the molecules in these plants which are not subject to such interpretation!
1) The main image at the top shows an open flower of the sunflower, Helianthus annuus ‘Velvet Queen’, resting in a shallow glass vessel often used to contain an oil bath (perhaps more accurately called a “heating bath”). Inside the flower, you can see a short strand of the polymer cis-polyisoprene (20 monomers), one of the key components of natural rubber and latex. In the laboratory, heating baths can be used to maintain a chemical reaction at an elevated temperature by immersing the reaction vessel into a silicone oil with good temperature stability and heat-transfer properties. This can provide even heat over relatively long time periods.
It was thought for a time that sunflowers were capable of tracking the sun across the sky, likely leading to their common name. So-called “heliotropism” exists in other plants, but sunflowers in bloom are actually fixed in the orientation which provides most sunlight. It is now known that sunflowers are phytoremediators – simply put, they are capable of removing toxic metals from polluted soil. When heavy industry (such as smelting plants) pollute the surrounding land, there are several options for cleaning up the pollutants from the soil. Intriguingly, plants such as sunflowers have been shown to be useful in this process. They have even been planted on contaminated soil near Fukushima to help with the clean-up as well as morale. Such “hyperaccumulators” may have evolved to take up metals as a defence mechanism, in order to be protected from herbivores (Plant Science (2011), 180, 169-181).
Cultivars such as the American Giant normally grow up to 4 m tall, with the tallest (according to the Guinness Book of World Records) measuring in at 8.23 m. Sunflowers are not just grown for records and decoration, though, the oil produced by the plant is worth an estimated USD 1 billion to the US economy per year. In the UK, sunflowers are mainly grown for their flowers and their seeds (used for their oil and as bird food), as our wet and windy climate prevents more widespread commercial cultivation. Sunflower fields are a more common sight in many parts of Europe, such as the south of France.
Surprisingly, sunflowers can also produce natural latex in their leaves. Currently nearly all natural rubber comes from the Brazilia rubber tree (Hevea Brasiliensis). The U.S.A. is dependent on imports of rubber as the rubber tree requires a tropical climate to grow successfully. Thus there is a strong commercial incentive to develop sunflower latex production as an alternative. Although there are many plants which produce latex, including the Euphorbias covered in an earlier post, sunflowers have an advantage in that they are already commercially grown. It is easy to look at the sunflower as simply a pretty flower, but the science involved could prove to be just as bright and uplifting.
Sunflowers (Helianthus annuus ‘Velvet Queen’) with a range of beakers containing coloured solutions and a small chain of cis-polyisoprene (20 monomers) at the bottom.
2) Although we tend to associate latex with the stretchy substance that gloves are made of, natural latex is harvested as a milky fluid which consists of tiny particles (around a millionth of a metre across!) floating in water. These latices are a subset of “colloids”, which include everything from the blood in your body to the paint on your walls and the screen of your e-reader. A colloid is a dispersion of one substance in a second substance, for example milk is a dispersion of fat in water. The latex is extracted as small particles of polymer surrounded by water, and these small particles of polymer scatter light, making the colloidal latex seem white to the human eye.
The latex particles themselves are negatively charged and so repel each other, and this prevents coagulation. Although the latex producing nature of the sunflower plant has been known for many years, each plant is only able to produce very small amounts of low molecular weight latex. Scientists are currently working on genetic modification to improve the yield of natural rubber from the plants (currently only 5% of the latex produced in the sunflower is of high molecular weight (~600,000 gmol-1), in order to decrease the U.S.A.’s dependence on imported rubber and synthetic rubber, made from petroleum (Ind. Crop. and Prod. 31 (2010) 481–491).
Fig.1. Cis-polyisoprene
Natural rubber (briefly mentioned in a previous post on Euphorbia) is a polymer of cis-isoprene (Fig. 1), which means it contains a long chain of covalently linked isoprene monomers. Colloidal latex extracted from sunflowers consists of roughly 30% cis-polyisoprene and around 55% water, with proteins, sugars, resins, ash and sterol glycosides also present in small quantities (Ind. Crop. Prod. (2010), 31 (3), 481–491).
Fig. 2 Formic acid
Once the latex has been extracted from the sunflower plant, it needs to undergo treatment in order to be turned from the milk-like substance to the stretchy material which we are familiar with. The latex can either be made into a more concentrated colloid, which is useful for dipping products in the rubber and allowing them to dry, or formic acid (Fig. 2) can be used to coagulate it. The formic acid acts by neutralising the negative charge on the surface of the polymer particles, and therefore stops the particles repelling. Once the latex has coagulated, it can be dried in the form of sheets.
3) Although cis-polyisoprene is synthesised through complex biological pathways in plants such as Helianthus annuus, we have synthetic pathways to make this polymer ourselves.
Fig. 3 Isoprene polmerisation
A sunflower (Helianthus annuus) photographed in the Bristol Botanic Garden last summer.
Isoprene itself can polymerise into 4 different regio- and stereoisomers depending on the conditions used, as shown in Fig. 3. Polymer A is cis-1,4 polyisoprene, B is trans-1,4 polyisoprene, C is 3,4-polyisoprene , D is 1,2-polyisoprene.
In order to obtain regio- and stereospecificity in the polymerisation of isoprene, an effective catalyst must be used (see our post on perennial grass Miscanthus for some general information about catalysts). The synthesis of cis-polyisoprene uses Ziegler-Natta catalysts and a possible mechanism is shown in Fig. 4.
Fig. 4 The polymerisation of isoprene uses Ziegler Natta catalysts to give cis-polyisoprene. (Note that some mechanistic studies show the diene coordinated via two eta2 bonds.)
Different catalysts will give different isomers of the synthetic polymer, for example cobalt complexes (Macromolecules, (2009) 42 (23), 9263-9267), lanthanide complexes (Rubber Chem. and Technol. (1985) 58 (1), 117-145) or metallocenes (Macromolecules (2004), 37, 5860-5862) each give different polymerisation results.
The synthesis of cis-polyisoprene in sunflowers follows a different mechanism from the route used to produce it in an industrial setting, and natural rubber exhibits superior properties to synthetic rubber, as synthetic rubber generally contains some trans-polyisoprene which can degrade the properties. However, latex from sunflowers consists of lower molecular weight latex, which is currently less useful for applications in rubber production (Helia, (2007), 30(46), 157-166). Nevertheless, research continues in this area and we might yet see more fields of sunflower in cultivation to provide latex in the future.
Contributors – Emma Kastrisianaki-Guyton (research, ideas and words), Sam Finlayson (research, ideas and words); Natalie Fey (molecular structures and editing), Jenny Slaughter and Ben Mills (photography).
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