Science: Metabolic Poison, Cyanide

Cyanide! Most people would have heard of its name, but never actually seen it. So let me show it to you. The cyan- of cyanide is actually a dye called Prussian blue, which was once used to develop blueprint photographs. Cyanide was made from Prussian blue by the famous Sweedish chemist Carl Wilhelm Scheele in 1782. More than two centuries later, cyanide still instill fear in humanity, to the point of being synonymous with poison itself. Today we are going to explore the deadly secrets of cyanide. 

A blueprint I've made with Prussian blue. This is the colour that gives cyanide its name.

Cyanide refers to any chemical compounds that contain the moiety C triple bonded to N (Figure 1). The simplest compound containing cyanide is hydrogen cyanide (HCN), and it is a volatile liquid that boils above room temperature. Cyanide can also exist as ionic salts, which are white coloured solids such as potassium cyanide (KCN). When cyanide is bonded to organic compounds, it is often called nitrile. The cyanide anion (CN-) is rich in electrons, making it a powerful ligand for transition metals. In other words, cyanide can form strong bonds with metals like iron, making complex ions such as Prussian blue (Ferric ferrocyanide). This is also the reason why cyanide salt is used to extract gold because it can form a water soluble complex ion (potassium auro(gold)cyanide), dissolving the gold out of ores. In organic chemistry, cyanide is a powerful nucleophile that can be used to extend carbon units during synthesis. It is a valuable reagent in the production of many pharmaceutical products. On the other hand, hydrogen cyanide is produced on an industrial scale to serve as an intermediate to various plastics and polymers such as polymethyl methacrylate and nitrile rubber. In plants, cyanide is produced as a toxin called cyanogenic glycoside to deter herbivores, and it is always incorporated with sugar molecules.

 

Figure 1: Cyanide and its bonding to heme.

Hydrogen cyanide is the most toxic form of cyanide. At temperature above 25 degrees Celsius, HCN is a gas with a pungent smell. It  partially ionises in water to produce a weak acid called prussic acid. In human blood (pH 7.2), less than 1% of HCN is ionised into H+ and CN-, but that's all it takes to be fatal. It is the free cyanide ion (CN-) that exerts tremendous toxicity, and for this reason cyanide ions that are tightly bound, such as Prussian blue, or many covalent nitriles (that cannot ionise) are not nearly as toxic. Cyanide salts like sodium and potassium cyanide are very toxic because they convert into HCN in the presence of stomach acid. HCN is a small molecule that is easily absorbed into the blood. At high concentrations of more than 200 parts per million, HCN is fatal to humans within seconds, making it one of the most rapidly acting of all poisons. HCN is used today as a fumigant to exterminate rodent pests. The Nazis used HCN (Zyklon B) to murder countless lives in concentration camps, and it is the most lethal chemical warfare agent ever used. In fact, HCN is still used as a method of execution via gas chambers nowadays. Humans are also exposed to HCN in modern house fires, because polymers degrade into HCN under high temperature. It is accountable for most if not all of the smoke-inhalation deaths.

The cyanide anion is highly toxic because it has a strong tendency to form a bond with transition metals like iron (Fe). The electron pair in CN- induce iron ion (Fe3+ or Fe2+) to reorganise its electron shell (orbital), forming a strong coordinate bond, which is indicated by an arrow pointing from a ligand. The resulting compound is called a coordinate complex where cyanide is the ligand. Cyanide is a potent non-competitive inhibitor of complex IV in the electron transport chain (ETC). This is because complex IV relies on heme proteins to transport electrons. Examine Figure 1 carefully, the heme molecule contains an iron atom, which is coordinated by a macrocyclic ring called porphyrin. Under normal conditions, the heme iron forms another coordinate bond with the endogenous amino acid histidine, leaving a room (hybridised d orbital) to accept or donate an extra electron. In complex IV cytochrome c oxidase, the heme iron in its ferric state (Fe3+) accepts an electron from complex III, and gets reduced into the ferrous state (Fe2+). The Fe2+ heme iron then passes its electron to an oxygen atom to make water, and is itself oxidised back to Fe3+. Cyanide anion forms a strong coordinate bond with heme's ferric iron, causing it to be unable to accept and transfer electrons. As a result, the flow of electrons in ETC is halted, electrons block up (repel each other), and the entire ETC is terminated. Without a proton gradient, no ATP can be made and the victim dies of energy deprivation by internal suffocation (histotoxic hypoxia). All these can happen in just a few seconds! Besides, being a non-competitive inhibitor, cyanide toxicity cannot be reversed by giving more oxygen or more electrons because the cyanide coordinate bond is too strong to be displaced. Organs with high energy demand such as the brain and heart are most affected. Victims experience a sense of suffocation, followed by strong convulsion, lactic acidosis, multi-organ failure, and death. Contrary to the movies where heros take a rapidly acting suicide pill and die peacefully, death by cyanide poisoning is most unpleasant!

Figure 2: Cyanogenic glycosides and their mechanism of action.

 

Cyanide is a deadly poison, but how can plants fashion it as a toxin without harming themselves in the first place?  Plants also rely on iron and transition metal enzymes for metabolism. Besides, HCN is lighter than air, and it will quickly dissipate in open environment. That's why it failed to be an effective war gas when compared to other agents like phosgene. However, plants have found a tricky way of turning cyanide into a toxin called cyanogenic glycoside, which is stable and non-volatile. When the plant is injured or eaten, enzymes in the herbivore (or plant) break down the cyanogenic glycoside into an unstable compound called cyanohydrin, which is the condensation product between HCN and a ketone or aldehyde (Figure 2). The cyanohydrin then decomposes back into HCN and ketone, releasing the deadly gas inside the victim's body. As with the case with many other plant toxins, it is the victim's body that spells its own doom. Cyanogenic glycosides are wide spread in the plant kingdom, but they can be found predominantly in the families Euphorbiaceae, Fabaceae, Lauraceae, and Passifloraceae. Prominent examples include amygdalin (bitter almond), prunasin (cherry laurel), linamarin (cassava), tetraphylin (Passiflora foetida), and gynocardin (Pangium edule). Interestingly, many cyanogenic glycoside containing plants are valued as food crops such as almond, cassava, lima beans, kepayang fruit, and even sorghum. Some cultivars of cassava and kepayang are so toxic, they cannot be eaten without careful preparation to remove the HCN content. It is even speculated that increasing carbon dioxide level in the air (due to global warming) can lead to increased levels of cyanogenic glycoside in plants!

 

Figure 3: Biosynthesis pathway of cyanogenic glycoside

Cyanogenic glycosides appear to be an ancient plant toxin, which can be found in about 5% of all living plant species from grasses, to ferns, to beans and trees. The biosynthesis of cyanogenic glycoside is highly conserved and it starts from amino acids. I'll take prunasin as an example in Figure 3, but different amino acids give rise to different cyanogenic glycosides (the cyanide pathway remains the same). Firstly, an amino acid is doubly oxidised at its amine nitrogen atom to become an N,N-dihydroxy amino acid, which undergoes a decarboxylation-elimination step to produce an aldoxime intermediate. The aldoxime gets protonated at its –OH group to eliminate a molecule of water (as leaving group) while generating a nitrile (organic cyanide). The nitrile then is oxidised into a cyanohydrin, which is quickly combined with sugar to render it stable as a cyanogenic glycoside. Thus, plants circumvent the deadly cyanide ion and HCN all together, smart isn't it? 

A sample of potassium cyanide (KCN). It is definitely something that demands great caution, but give the respect it deserves, KCN is a very useful reagent in organic synthesis.

Before I end, I'll talk about the antidotes of cyanide poisoning. After all, this is one of the few poisons that we have successfully developed effective antidotes. Believe it or not, our body has evolved an enzyme to detoxify cyanide, and it's called rhodanase. Since cyanide craves to form coordinate bond, our body gives it a sulfur atom, turning the cyanide anion into far less toxic thiocyanate (SCN-), which is easily excreted via the kidneys. In fact, we can tolerate low levels of cyanide indefinitely thanks to this enzyme, and mild cyanide poisoning can be treated by giving a sodium thiosulfate injection, which boosts our rhodanase enzyme. However, rhodanase is overwhelmed by severe cyanide overdose, and that calls for more effective antidotes. The key to detoxify cyanide is to displace it out of the heme iron in complex IV. We can give a more reactive transition metal like cobalt (hydroxocobalamine) to persuade cyanide to give up heme for the cobalt, making harmless vitamin B12 in the process. Historically, cyanide poisoning used to be treated with another poison called nitrite, which oxidises our red blood cells. Red blood cells also contain heme, and cyanide will prefer the oxidised heme in red blood cells over the heme in ETC. However, nitrite has to be given together with thiosulfate, and they are seldom used nowadays to treat cyanide poisoning due to dangerous side effects.

 

A perfect end to a 'perfect' poison? We will meet cyanide again in the section [Experiment] next time. In my last article on metabolic poisons, we will leave the ETC and focus on a poison that targets ADP-ATP translocase. Mind you, it is at least as toxic than hydrogen cyanide, but unlike cyanide, no antidote exists for it!