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The Right Chemistry: The mysterious case of cyanide death and almonds

Agatha Christie trained as a pharmacist and generally got the chemistry of poisons right. But what about this time?

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“We can rule out cyanide poisoning,” declares the detective upon sniffing the mouth of a man who died under mysterious circumstances. “Not even a trace of almond smell,” he proclaims. Was he right? If you are a fan of mystery stories, you would certainly think so. Agatha Christie often refers to the smell of bitter almonds to indicate that cyanide has been used to commit a murder. There is even a Jeopardy question, “Agatha Christie taught us it smells like bitter almonds,” with the answer being “what is cyanide?” Christie had trained as a pharmacist and in general she got the chemistry of poisons right, but not this time. Although she was not totally wrong, either.

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Almonds are the seeds of the almond fruit and have evolved an effective mechanism to deter predators that choose to dine on them. Starting with the common amino acid phenylalanine, a series of chemical reactions leads to the formation of mandelonitrile, some of which in turn gets incorporated into amygdalin, a more complex chemical that serves as a storage form of mandelonitrile. When a predator chomps into a seed, an enzyme that breaks mandelonitrile down into benzaldehyde and cyanide is released. The cyanide dissuades consumption with help from the bitter taste of benzaldehyde, which is also responsible for the classic smell of almonds. As far as the almond fruit is concerned, humans are just giant predators, meaning that they merit a dose of cyanide. Of course, whether this presents a problem depends on the dose.

Almond trees are thought to have originated in Asia with their seeds being very bitter due to high levels of amygdalin. Then along the way a chance mutation resulted is seeds with lower levels of mandelonitrile and amygdalin and consequently a sweeter flavour. These are the trees that have been domesticated and while their seeds do yield some cyanide, the amounts are way too small to worry about. Since benzaldehyde has a potent smell, it is detectable even in small doses and consequently we still have the characteristic smell and taste of almonds.

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Now for the “cyanide smelling like bitter almonds” issue. Bitter almond trees still exist, and a handful of their seeds can contain enough amygdalin to release a potentially lethal amount of cyanide. A murder could indeed be committed if ground seeds, or their extract, were somehow slipped into a food or beverage. In this case, there would be a tell-tale smell, but it would be that of benzaldehyde, not of cyanide. By all accounts, hydrogen cyanide has no smell, but obviously nobody is going to carry out a dose-response experiment to put that claim to a test.

The fact that bitter almonds are a source of amygdalin has given rise to a rumour that the Food and Drug Administration in the U.S. has banned the growing of bitter almond trees. Why? In order to protect the pharmaceutical industry from suffering financially if a cheap cancer cure were readily available. What nonsense! First, it is not true that growing bitter almond trees has been banned, although it is true that the sale of bitter almonds is not allowed. This has nothing to do with protecting the interests of pharmaceutical companies; it has to do with protecting the public from cyanide poisoning.

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How about protecting the public from false claims that amygdalin or one of its breakdown products, levo-mandelonitrile-beta-glucuronoside, both of which are commercially available as “Laetrile,” are effective cures for cancer?

Amygdalin, which is also present in apricot, peach and plum pits, was tried as a cancer treatment as early as 1892 by German doctors, theorizing that the cyanide released would impair the multiplication of rapidly dividing cancer cells more than normal cells. Later, proponents of amygdalin as a cancer treatment claimed that cancer cells are richer in the enzyme that leads to the release of cyanide. This is not so, as numerous trials of various versions of “Laetrile” have demonstrated.

Because of the lack of evidence for any benefit, Laetrile was eventually made illegal. That of course has not stopped it from being sold on the internet, or from being used by clinics in Mexico. Proponents are still filling websites with misleading information, giving false hope to unfortunate cancer victims. Some even claim there are populations in the world that are free of cancer and have extreme longevity, such as inhabitants of the Hunza Valley of Pakistan, thanks to a diet that includes lots of apricots, and consequently amygdalin. The claim of being cancer-free is not valid, and neither is the claim of extreme longevity in the absence of birth certificates. While it is true that apricots are prominently featured in the diet, there is no reason to believe that the Hunzakuts eat the pits of the fruit. Neither should you. Unless you want some doctor or detective sniffing your mouth for the smell of benzaldehyde. And getting back to our detective, he was wrong to rule out cyanide poisoning. If potassium cyanide were used instead of bitter almonds, there would be no smell.

joe.schwarcz@mcgill.ca

Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.

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