The Common Horse Chestnut: Surprisingly Extraordinary
| Polish version is here |
The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (4/2017):
Plants, as a vast kingdom of organisms, are no less diverse than animals. We can point to countless plants that are fascinating for a variety of reasons.
Many of them owe their fame to behaviors that stand out among other plants. I believe that's precisely why carnivorous plants, such as sundews Drosera, pitcher plants Nepenthes, or the Venus flytrap Dionaea, are so well-known.
Another interesting example is the sensitive plant Mimosa pudica, which, although not carnivorous, can perform relatively quick movements using its compound leaves [1].
Some plants may have particularly striking and ornate flowers or exhibit an unusual life cycle. However, we must admit that the importance of many plants is based on the presence of specific chemical compounds, which can be used in various ways—for instance, as medicinal agents or dyes.
In today’s experiment, I’d like to demonstrate that even a plant as commonly encountered as the horse chestnut (Photo 1) can help us achieve a truly intriguing and beautiful effect—one that allows us to explore energy transformations at a molecular level.
A Few Facts
I suspect it would be quite hard to find someone who couldn’t identify a horse chestnut—if only because of its characteristic leaves and fruits. Nevertheless, it’s useful to organize some essential information here.
The horse chestnut Aesculus is a genus of trees, and less frequently shrubs, belonging to the soapberry family Sapindaceae. Around 25 species of horse chestnut are known to exist in Europe, Asia, and North America.
It’s worth noting that all horse chestnut species found in Poland were introduced by humans.
Many species are cultivated as ornamental plants and are grown in parks, along avenues, and as roadside plantings. Despite some inconvenience—such as substantial litter from fallen leaves, seed capsules, and the seeds themselves—horse chestnuts remain popular.
Several species of horse chestnut can be found in Poland, including:
- red horse chestnut Aesculus carnea Hayne
- bottlebrush buckeye (small-flowered buckeye) Aesculus parviflora Walt.
- French horse chestnut Aesculus x plantierensis André
- Ohio buckeye Aesculus glabra Willd.
- Japanese horse chestnut Aesculus turbinata Blume
- red buckeye Aesculus pavia L.
- yellow buckeye Aesculus flava Sol. ex Hope
- common horse chestnut Aesculus hippocastanum L. [2]
The most widespread naturalized anthropophyte in Poland is the common horse chestnut. Its cultivation in Europe began in 1576 in the gardens of Emperor Maximilian II, after it was brought in from Istanbul. By the late 16th century, the first specimens of this plant had been brought to Poland from Austria. Interestingly, until the 19th century, there was a widespread but incorrect belief that the common horse chestnut originated in India.
The common horse chestnut can reach a height of about 25 meters (~82 ft). Its crown is dense and either domed or cylindrical, and the older bark peels off in patches.
The horse chestnut’s distinctive leaves are palmately compound, consisting of 5–7 leaflets (inverted-egg-shaped) up to about 25 cm (~9.8 in) long (Photo 2). Notably, mature leaf buds secrete a relatively large amount of sticky resin.
Its flowers form dense, erect panicles (Photo 3), which can reach lengths of up to 30 cm (~11.8 in).
The flower petals are white with yellow or red spots at the base (Photo 4). The common horse chestnut typically blooms in May [3].
The fruit of the horse chestnut is often referred to simply as the “chestnut.” Botanically, these are spiky capsules up to about 5 cm (~2 in) in diameter, containing one to three seeds. They appear in September and October. The seeds are rich in starch and are sometimes eaten by wild game such as deer, roe deer, and wild boar [4].
The horse chestnut is also considered a medicinal plant. It exhibits anti-inflammatory, antibacterial, antispasmodic, and hemostatic properties, and it improves the condition of blood vessels [5]. However, when used in herbal medicine, one must remember that the seeds of this plant are toxic because they contain saponins, which cause hemolysis of red blood cells.
Experiment and Observations
To observe the property of the horse chestnut that interests us here, you’ll need a small piece of this plant’s woody stem (Photo 5).
It’s best to use a section of a live twig, stripped of its leaves. Using a dry, dead branch often yields much poorer results.
Next, you need to shave or whittle the twig slightly, removing the outermost layers to obtain some small shavings (Photo 6).
Separately, prepare 50 cm3 (~1.7 fl oz) of an alkaline solution by dissolving a few pellets of sodium hydroxide (NaOH) in distilled water.
Be very cautious, as sodium hydroxide and its solutions are highly caustic and can cause severe skin and eye damage.
Add the wood shavings to the colorless alkaline solution and stir. The liquid will quickly take on a brown hue that darkens over time (Photo 7).
You should separate the resulting extract from the solid residue fairly quickly and proceed with further observations because it may degrade.
Treat this substance cautiously as a potentially hazardous chemical reagent, since some components of the horse chestnut wood can be harmful.
After darkening the room and illuminating the alkaline extract of horse chestnut wood with ultraviolet light, you’ll notice it glows with a striking, bright blue light! The brightness of this light is fairly strong—Photo 8 shows the effect of dissolving just 1–2 drops of the extract in about 60 cm3 (~2 fl oz) of distilled water.
Interestingly, the phenomenon observed is not the only surprising property of the extracted substance. It turns out you can manipulate the intensity of its glow by altering the pH of the environment. With a mildly alkaline character, the solution emits a very bright light under UV illumination (Photo 9A). However, if you gently acidify it, for instance with a small addition of acetic acid CH3COOH (e.g., household vinegar) or another acid, the intensity of the glow drops dramatically (Photo 9B).
The observations presented here do not fully explore the phenomenon, and many additional experiments can be designed. For instance, I encourage you to investigate how the wavelength of the exciting light influences the glow and intensity of the horse chestnut extract. Experimenting with materials and substances that block UV radiation—such as protective filters—may also yield intriguing results.
Explanation
We know that every object with a temperature above absolute zero emits so-called thermal radiation [6]. At room temperature, the peak of this emission lies in the infrared region of the electromagnetic spectrum.
However, other mechanisms allow for emission of radiation triggered by causes unrelated to heating the object to a sufficiently high temperature. Such phenomena are collectively called luminescence—“cold light.”
Luminescence can be categorized according to the factor that induces the emission of radiation. For instance, chemiluminescence occurs when light is produced through certain chemical reactions. A common example is the widely demonstrated oxidation of luminol C8H7N3O2. Less well-known are the chemiluminescent properties of polyphenols naturally found in tea, lophine C21H16N2 (easy to synthesize), Wöhler’s siloxene Si6O3H6 (an example of an organosilicon compound), singlet oxygen, and other substances [7] [8] [9]. Another type of luminescence is triboluminescence, caused by mechanical factors—strong light emission can occur when, for example, copper complex crystals [Cu(NCS)(py)2(PPh3)] are crushed, or even just ordinary table sugar (sucrose C12H22O11) [10] [11].
In photoluminescence, the emission of light is triggered by radiant energy. Under photoluminescence, we differentiate between fluorescence and phosphorescence. Fluorescence ceases almost immediately when the exciting radiation stops, whereas phosphorescence can last for a while. Phosphorescent properties are used in the manufacture of glow-in-the-dark paints, which emit light after being illuminated [12].
We can now assert that in the case of the extract obtained from the horse chestnut twig, we observed fluorescence. This happens because the tissues of this plant contain relatively large amounts of a particular chemical compound—esculin C15H16O9. It’s a glycoside, meaning it consists of a sugar component and an aglycone part. Figure 1 shows the structural formula of esculin.
Many glycosides exhibit biological activity and have pharmacological significance.
Esculin is a coumarin glycoside, meaning its aglycone (esculetin) is based on the coumarin skeleton. This structural feature is why esculin can fluoresce—indeed, many coumarin derivatives share this property. Besides the horse chestnut, this compound also appears in daphnin, the dark green resin of the spurge laurel Daphne mezereum. However, be warned against using this plant in experiments: spurge laurel is highly poisonous. Eating about a dozen ripe berries can kill an adult; even 1–2 berries can be fatal to a child. The entire plant is dangerous—direct contact of its leaves with skin can cause painful swelling and blisters [13]. Additionally, the plant is under partial protection [14].
The mechanism behind this phenomenon can be explained by the energy changes that occur on a molecular scale. Under normal conditions, the esculin molecule remains in the ground state, its lowest energy level, most of the time. However, it can be excited by absorbing radiation of a specific wavelength, and thus a certain energy. In this way, the molecule transitions to an excited state with higher energy. This excited condition is unstable and decays very quickly back to the ground state. The energy difference must be released as visible light.
Since no real process is 100% efficient, part of the excitation energy disperses—for example, through thermal vibrations—so the emitted light has lower energy (longer wavelength) than the excitation light. This is known as the Stokes shift [15]. Notice that in this case, the Stokes shift is relatively small because both the exciting (near ultraviolet) and the emitted (blue) wavelengths lie at the same end of the visible spectrum.
Plenty of other substances fluoresce as well, such as ethacridine lactate (Rivanol) C18H21N3O4, fluorescein C20H12O5, rhodamine B C28H31N2O3Cl, and many others.
It may be of interest that coumarin derivatives are used as the working medium in modern dye lasers [16].
Esculin also finds use in microbiology. It can be used to identify certain types of bacteria, such as enterococci. This method relies on the ability of these microorganisms to hydrolyze the compound, producing glucose and esculetin. The latter is then detected by its reaction with trivalent iron ions Fe3+, forming a dark olive to black complex.
As you can see, even a seemingly ordinary plant like the horse chestnut can be a source of very engaging experiments, observations, and insights.
References:
- [1] Ples M., Wstydliwa roślina (eng. Sensitive plant), Biologia w Szkole (eng. Biology in School), 6 (2015), Forum Media Polska Sp. z o.o., pp. 52-56 back
- [2] Mirek Z., Piękoś-Mirkowa H., Zając A., Zając M., Flowering plants and pteridophytes of Poland: a checklist. Krytyczna lista roślin naczyniowych Polski, Instytut Botaniki PAN im. Władysława Szafera w Krakowie, 2002 back
- [3] Podbielkowski Z., Słownik roślin użytkowych, Powszechne Wydawnictwo Rolnicze i Leśne, Warszawa, 1989 back
- [4] Mayer J., Schwegler H.-W., Wielki atlas drzew i krzewów, Oficyna Wydawnicza Delta W-Z, Warszawa, 2007 back
- [5] Wielgosz T., Wielka księga ziół polskich, Publicat S.A., Poznań, 2008 back
- [6] Sawieliew I.W., Wykłady z fizyki 3 (Wyd. II), Wydawnictwo Naukowe PWN, Warszawa, 1994 back
- [7] Ples M., Światło z retorty (eng. Light from the Chemist’s Retort), Chemia w Szkole (eng. Chemistry in School), 5 (2014), Agencja AS Józef Szewczyk, pp. 33-34. back
- [8] Ples M., Całkiem niezwykła herbatka (eng. A Rather Unusual Tea), Chemia w Szkole (eng. Chemistry in School), 4 (2015), Agencja AS Józef Szewczyk, pp. 6-9 back
- [9] Co i jak można otrzymać z piasku? Nieznane oblicze krzemu (eng. What and How Can Be Obtained from Sand? The Unknown Face of Silicon), Chemia w Szkole (eng. Chemistry in School), 6 (2016), Agencja AS Józef Szewczyk, pp. 38-43 back
- [10] Ples M., Fiat lux! Tryboluminescencja związku kompleksowego miedzi(I) (eng. Fiat lux! Exploring Triboluminescence in a Copper(I) Complex), Chemia w Szkole (eng. Chemistry in School), 2 (2015), Agencja AS Józef Szewczyk, pp. 10-11 back
- [11] Ples M., Świecący cukier, czyli o tryboluminescencji sacharozy (eng. Glowing Sugar: How Sucrose Exhibits Triboluminescence), Fizyka w Szkole (eng. Physics in School), 3 (2015), Agencja AS Józef Szewczyk, pp. 45-46 back
- [12] Ples M., Jak uwięzić światło? O skutkach domieszkowania siarczku cynku (ang. Trapping Light: Exploring the Effects of Zinc Sulfide Doping), Chemia w Szkole (eng. Chemistry in School), 1 (2017), Agencja AS Józef Szewczyk, pp. 12-18 back
- [13] Bohne B., Dietze P., Rośliny trujące: 170 gatunków roślin ozdobnych i dziko rosnących, Wydawnictwo Bellona, Warszawa, 2008 back
- [14] Dz.U. 2014 nr 0 poz. 1409 – Rozporządzenie Ministra Środowiska z dnia 9 października 2014 r. w sprawie ochrony gatunkowej roślin back
- [15] Gispert J.R., Coordination Chemistry, Wiley-VCH, 2008, pp. 483 back
- [16] Duarte F. J., Appendix of Laser Dyes. Tunable Laser Optics, Elsevier-Academic, Nowy Jork, 2003 back
All photographs and illustrations were created by the author.
Marek Ples