A sticky plaster capable of self-defence, as well as a source of natural rubber, fish poison and anticancer drugs, forget the multi-tool and make room for Euphorbias in your affections.
1. The main picture shows the herbaceous Euphorbia cyparissias ‘Fens Ruby’ standing in for other members of the Euphorbiaceae family in the water bowl of a rotary evaporator. This is a really useful part of the laboratory kit, which can be used to evaporate solvents from reaction mixtures at reasonably low temperatures. Removing the solvent is achieved by lowering the pressure in the apparatus with the help of a vacuum pump, which you can just see on the left hand side of the picture. This lowers the boiling point of solvents compared to that at atmospheric pressure, so it evaporates from the reaction mixture, usually in a round bottomed flask attached where the foliage of the Euphorbia sits. The solvent then condenses again on the water coils in the condenser, which is the glass spiral seen at the top of the kit, and drips into the round collection vessel below the condenser, from where it can be removed and disposed off. If the boiling point under vacuum is still quite high, the water bowl can be filled with water and heated to aid the evaporation of the solvent.
A number of Euphorbias are popular garden plants in the gardens of Europe and the UK because they have an architectural quality and flower, depending on species and cultivar, in spring or during early to mid summer, providing interest to humans and nectar to pollinators. Although many originate from a Mediterranean climate, they can be hardy in all but the harshest winters, and they are often very drought resistant, meaning they will grow in tricky spots in the garden. In milder climates, they can actually turn into invasive weeds, but while they self-seed or spread underground quite merrily in the UK, they can generally be kept in check with regular weeding.
However, when it comes to pruning them, or indeed the introduction of children or animals to the garden, they can prove a little bit tricky. When the stems and leaves of Euphorbiae in the garden are injured, be it intentionally while pruning or unintentionally during play or gardening work, the plants start to exude a white sticky sap, which can gum up secateurs, cause burn-like allergic reactions when it comes into contact with skin and is poisonous to aquatic species. Indeed, in both Africa and South America, Euphorbia plants are used as a source of poison which is dripped into water to stun fish, making them very easy to collect at the water surface. Some people are very allergic to the sap, and ingestion can cause poisoning in both humans and animals. If you are sensible about this, they are perfectly fine as garden plants (indeed, I have a reasonable collection myself), but if there’s any doubt about this, or you find yourself reacting strongly to the sap, they might have to go.
The sap is likely to be a defence mechanism shared with many other plants, and two of its most useful components will be discussed in greater detail below – latex, likely having the same function as a sticky plaster, i.e. to help stop oozing and heal the wound, and a family of molecules called diterpenes, which have shown biological activity including against some human cancers, and may help to defend the plant against hungry animals and insects, but also microbes which might otherwise infect the wound.
2. A number of so-called diterpenes have been found in the sap of Euphorbia characias which is common in the Mediterranean (in UK gardens, you are quite likely to find Euphorbia characias subsp. wulfenii, which is particularly architectural). The family of compounds derived from E. characias have been named characiols, and they all share the same basic structure, but with different groups attached to the framework (see e.g. J. Org. Chem. 2009, 74, 1698-1708 which shows a good selection). As discussed in this link, they are made up of the isoprene unit C5H8 (Fig. 1) and in diterpenes, four of these units show up. Fig. 2 shows a member of the characiol family (also shown as a space-filling molecular structure in the main picture), and counting up the connected C atoms in this fairly complicated molecule should give you 20 carbons (+ some extra from the peripheral substituents in this example).
Like most of the so-called natural products (i.e. useful molecules first found in nature), these diterpenes have several chiral centres, which means they have a very distinctive shape in three-dimensions and they interact in very specific ways with organisms which have many chiral receptors and so provide a chiral environment. As discussed in a previous post, chirality, in its most simple form, describes that in some molecules there are different ways of arranging atoms around a centre which turn them into non-superimposable mirror images. For molecules with multiple chiral centres, that becomes more complicated and additional stereoisomers become possible, arising from all possible combinations of different configurations at each centre. This can make their synthesis very difficult (a few more links for this in the next section), but it can also make them useful drug molecules or precursors, so extracting them from plants or producing them synthetically in sufficient quantities for further testing is an important area of organic chemistry.
Natural rubber can be produced from plant latex, and perhaps the most obvious example is the so-called rubber plant, Hevea brasiliensis, which is also a member of the Euphorbiaceae family. Latex is an emulsion of polymers, which are large molecules with a (fairly) regular pattern of repeating units which form a long chain. One of the main repeating units is indeed isoprene (Fig. 1), which we have already met in the structures of diterpenes above. Latex can be refined or extracted to get mainly these isoprene polymers, some of which can be processed into useful natural rubber. This type of processing is sufficiently interesting to save this for a future post.
3. The total synthesis of diterpenes, including the characiols, can help to verify their proposed structures and give access to pure materials for testing of biological activity ( J. Org. Chem. 2009, 74, 1698-1708), but it can also prove a testing ground for relatively new synthetic approaches and catalysts, testing whether these are suitable for the control of sterochemistry (Org. Lett. 2009, 11(12), 2555-2558). In addition, Euphorbia characias latex has been explored as a source of natural rubber (Biopolymers 2012, 97(8), 589-594) and some of the interactions of molecules and enzymes in plant latex has been investigated in greater detail (Plant Biosystems2010, 144(2), 381-391), suggesting that there is more to this mixture of compounds than variations on isoprene polymers and diterpenes.
So next time you see a Euphorbia strutting its stuff in a dry spot of the garden, remember the potency and potential of its sap and give it an appreciative nod.
(Contributors: Natalie Fey (text and images), Jenny Slaughter (photography)).