Plant Poisons: Alkaloids and How to Detect Them
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The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (2/2018):
I would argue that plants form the very foundation of life on Earth. Without them most other organisms could not exist. Virtually all energy available to us originates, directly or indirectly, from the Sun, and photosynthesis is the primary source of energy and organic matter in most ecosystems. For many animals, plants are the main, and often the only, source of food.
Plants provide more than oxygen and nourishment; they also supply indispensable raw materials such as wood, fibers, and countless other substances. Among the many compounds they produce are the alkaloids.
This umbrella term refers to naturally occurring, predominantly plant-derived basic (alkaline) organic molecules that almost always contain heterocyclic nitrogen. Amino acids, peptides, proteins, nucleotides, nucleic acids, antibiotics, and amino sugars are generally excluded from the alkaloid family, though some neutral compounds related to true basic alkaloids are often grouped under the same label [1].
Because alkaloids are nitrogen-rich, it is no surprise that their biosynthesis begins with amino acids. Under standard conditions nearly all alkaloids are crystalline solids; liquids are rare. They dissolve poorly in water but readily in many organic solvents.
A defining feature of these compounds is their pronounced, often toxic, physiological activity in animals. Humanity takes advantage of this property: administered at the right dose, many alkaloids are effective medicines against a host of ailments; quinine and codeine are classic examples. Alkaloids also appear in legal stimulants (caffeine, theobromine, nicotine), in illicit drugs such as cocaine, and in highly toxic rodenticides like strychnine.
Given the regulatory bans on producing or possessing certain psychoactive alkaloids, chemists have devised many detection methods that see regular use in forensic work. In this article, I present relatively simple, classroom-friendly procedures capable of revealing both controlled substances and more commonplace alkaloids.
Experiment
Among many detection techniques I chose the classic Dragendorff reagent, invented in the 19th century by the German chemist Johann Georg Noël Dragendorff, professor of pharmacy at the University of Dorpat (Tartu). The reagent is neither highly toxic nor strongly caustic.
To prepare it you need the following reagents [2]:
- bismuth(III) nitrate, Bi(NO3)3,
- potassium iodide, KI,
- concentrated hydrochloric acid, HCl 36%.
Tartaric acid is often included in the traditional recipe, but we can omit it here.
Bismuth(III) nitrate is the nitrate salt of bismuth in the +3 oxidation state. From aqueous solution it crystallizes as a pentahydrate (Photo 1). When heated it decomposes to bismuthyl nitrate, BiONO3. The solid is irritating and a strong oxidizer, so handle it with care.
Potassium iodide is best known as part of Lugol’s iodine solution and is added to table salt in regions that lack dietary iodine.
Concentrated hydrochloric acid is highly corrosive and releases pungent hydrogen chloride gas, which may be is toxic, so appropriate safety precautions are essential.
The Dragendorff reagent is prepared from two stock solutions:
- A – 0.4 g (≈0.014 oz) of bismuth nitrate dissolved in 40 cm3 (≈1.4 fl oz) of distilled water plus 1.5 cm3 (≈0.05 fl oz) of concentrated HCl (add a little more acid if the solution remains cloudy),
- B – 3 g (≈0.11 oz) of potassium iodide dissolved in 50 cm3 (≈1.7 fl oz) of distilled water.
Both stock solutions are colorless and clear (Photo 2).
Combine A and B, add distilled water to a final volume of 100 cm3 (≈3.4 fl oz), and, if a precipitate forms, filter after about 24 h. The resulting Dragendorff reagent is an orange liquid (Photo 3) that keeps well when stored in a dark, tightly sealed bottle.
To test the reagent we need an alkaloid-rich plant. The European yew, Taxus baccata, an evergreen conifer native to Europe, western Asia, and North Africa, fits the bill. Yews are extremely long-lived; Poland’s famous Henryków Yew is estimated to be more than 1,200 years old [3]. The wood is dense and prized for ornamental and medicinal uses [4].
All parts of the yew except the fleshy seed aril are poisonous because they accumulate alkaloids, so exercise caution when handling the plant.
Leaves (actually needles) serve as the raw material. They are elongated, 2–3 cm (0.8–1.2 in) long and about 0.3 cm (≈0.12 in) wide, slightly pointed, and arranged spirally on the stem (Photo 4).
We need to cut the leaves into small pieces (Photo 5).
Transfer the pieces to a ceramic mortar, add a little water and a splash of ethanol (optional), and sprinkle in clean quartz sand (Photo 6) to help grind the tough tissue.
After thorough grinding, filter the slurry. For comparison, prepare an extract from an alkaloid-free plant such as spinach (Spinacia oleracea) using the same method. Both the yew extract (Photo 7A) and the spinach extract (Photo 7B) are pale green.
Transfer the pieces to a ceramic mortar, add a little water and a splash of ethanol (optional), and sprinkle in clean quartz sand (Photo 6) to help grind the tough tissue.
After thorough grinding, filter the slurry. For comparison, prepare an extract from an alkaloid-free plant such as spinach Spinacia oleracea using the same method. Both the yew extract (Photo 7A) and the spinach extract (Photo 7B) are pale green.
Transfer small portions of each extract into test tubes; use distilled water as a blank (Photo 8).
Add a few drops of Dragendorff reagent to each sample and mix (Photo 9).
A cloudy precipitate indicates a positive Dragendorff test and confirms the presence of alkaloids [5].
The yew contains numerous toxic compounds, the most important of which is taxine, C35H47NO10. Its structural formula appears in Fig. 1.
Taxine disrupts cardiac, gastric, and intestinal function and, in sufficient doses, paralyzes the respiratory center, causing sudden death [6]. It also interferes with cell division by interfering spindle formation.
Tonic water provides another readily available alkaloid source because its characteristic bitterness comes from quinine. Let a sample stand uncovered until the CO2 escapes, then test it with Dragendorff reagent. Photo 10 shows tonic (A), tonic diluted ten-fold (B), and distilled water (C) before and after adding the reagent; the tonic samples turn cloudy.
Quinine, C20H24N2O2 (Fig. 2), occurs naturally in the bark of Cinchona trees and was the first effective antimalarial drug; it also acts as an antipyretic, anti-inflammatory, and analgesic.
European Union regulations cap quinine in food products (including tonic) at 100 mg kg⁻¹ (≈0.01%). Even at a ten-fold dilution (≈0.001%) Dragendorff reagent readily detects it. Further dilutions still yield a faint precipitate, underscoring the method’s sensitivity.
Quinine’s presence is even more dramatic under ultraviolet light: tonic fluoresces bright blue, as shown in Photo 11.
Pursuing alkaloids in everyday products, we can also test an energy drink. After degassing, the Dragendorff test (Photo 12) produces a heavy precipitate.
Energy drinks contain large amounts of caffeine, C8H10N4O2, a purine alkaloid (Fig. 3).
Natural sources include coffee beans Coffea spp., tea leaves Camellia sinensis, guarana seeds Paullinia cupana, and yerba mate Ilex paraguariensis. Caffeine is generally safe, but doses above 0.5 g (≈0.018 oz) can cause severe agitation, arrhythmia, nausea, and weakness, and about 10 g (≈0.35 oz) may be fatal [7].
In our sample, so much precipitate forms that it settles into a thick layer (Photo 13).
If only a tiny sample is available, the Dragendorff test can be run on a spot plate or glass slide (Photo 14A). Place a drop of sample (a) next to a drop of reagent (b), then merge them; a colored precipitate (c) appears instantly (Photo 14B).
The sample shown contained caffeine.
Similar experiments can be performed with many other substances, but remember that numerous alkaloids are extremely poisonous and that possessing some of them is illegal.
Explanation
Dragendorff reagent works because most alkaloids are basic; the very term derives from Arabic alkali (“potash”) and Greek eidos (“form”). Most alkaloids are tertiary amines that react with potassium tetraiodobismuthate(III), forming insoluble, colored complexes that range from yellow to red-brown.
Other classic alkaloid testing reagents include the Mandelin reagent (Photo 15A) and the Marquis reagent (Photo 15B). Interestingly, both Karl Friedrich Mandelin and Eduard Marquis, the chemists who developed these reagents, were affiliated with the University of Dorpat, just like Dragendorff.
Both reagents can identify not only classes of compounds but specific chemicals, yet they require handling concentrated sulfuric acid and are less suitable for classroom use.
Physiologically, alkaloids do not participate directly in primary metabolism and have long been dismissed as waste products. This view is controversial because synthesizing them is energetically expensive and consumes scarce nitrogen. Notably, the highest alkaloid concentrations occur in tissues most vulnerable to herbivores, supporting the hypothesis that alkaloids function as chemical defenses, nature’s deterrents against plant-eating organisms.
References:
- [1] Moss G. P., P. Smith A. S., Tavernier D., Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995), Pure and Applied Chemistry, 67 (8-9), 1995, pp. 1313 back
- [2] Ples M., Odczynnik Dragendorffa (eng. Dragendorff reagent), online: https://weirdscience.eu/Odczynnik%20Dragendorffa%20-%20wykrywanie%20alkaloid%C3%B3w.html, [01.02.2018] back
- [3] Seneta W., Dendrologia, Państwowe Wydawnictwo Naukowe, Warszawa, 1987, pp. 38 back
- [4] Barański M. J., Cis, w: Harnaś, 7, wyd. PTTK, Gliwice, 1981 back
- [5] Nayeem A. A., Khatun A., Rahman M. S., Rahman M., Evaluation of phytochemical and pharmacological properties of Mikania cordata (Asteraceae) leaves, Journal of Pharmacognosy and Phytotherapy, 3 (8), 2011, pp. 118-123 back
- [6] Perju-Dumbravă D., Morar S., Chiroban O., Lechintan E., Cioca A., Suicidal poisoning by ingestion of Taxus Baccata leaves. Case report and literature review, Romanian Journal of Legal Medicine, 2 (21), 2013, pp. 115-118 back
- [7] Mutschler E., Farmakologia i toksykologia, MedPharm Polska, Wrocław, 2010 back
All photographs and illustrations were created by the author.
Marek Ples