Hot Chili Pepper Chemistry

The English word pepper is ambiguous. Botanically it could be refer to plants that produce black powdered pepper. The angiosperms (flowering plants) who serve as a source of that spice belong to the order Piperales and to the 3600-member family Piperaceae. (Kings play chess only for goodness’ sake is a mnemonic for remembering the order of biological classification: kingdom, a group of related phyla, which in turn, at least in singular form, phylum, is a group of related classes, and so on with orders, families, genera and species. ) Piperine is the compound mainly responsible for black pepper’s pungency, but as expected it does not appear in unrelated plants also dubbed as peppers.

In jalapeños,  dihydrocapsaicin and capsaicin, are the two  amides mainly responsible for their hotness.

Pepper can also refer to Capsicum chinense, the Red Savino haanero chili, a member of the tomato family, Solanacea.  That fruit scores in the hundreds of thousands of units on the Scoville scale of “hotness”.  Other hot  peppers and milder ones are found among the many varieties of another Capsicum species, known as annum, which includes sweet peppers, jalapeños, and New Mexico chili.


There are four different amides (specifically, capsaicioids) found in hot peppers of the Capsicum genus. A Brazilian study revealed that in these fruits capsaicin makes up anywhere from 24% (in jalapeños) to 95% (pimenta preta) of capsaicioids present. These in turn combine to account anywhere from 0.2 mg to 7 mg per gram of fresh pepper, in other words, less than 1%. The percentage of course soars when the pepper is dried. Except in jalapeños and fatalis, the equally potent compound, dihydrocapsaicin, is the second most abundant amide. The greatest concentration (9.2%) of nordihydrocapsaicin, a substance about half as hot, was found in pimenta de mesa. Finally another related compound, homocapsaicin, the least potent of the quartet, is entirely absent from most spicy peppers, but it makes up 12% of dedo-de-moça pequena’s capsaicioids.

nnordihydrocapsaicin 9 100 000
dihydrocapsaicin 16 000 000
homocapsaicin 8 600 000
capsaicin 16 000 000
pimenta de mesa
The hot amides in pimenta de mesa consist of about 53% capsaicin, 37% dihydrocapsaicin, 1% homocapsaicin and about 9% nordihydrocapsaicin . The latter’s concentration is not surpassed in 20 other species of Capsicum.

From examining the above structural diagrams notice that in comparison to capsaicin,  homocapsaicin’s double bond is slightly further away from the tail end of the molecule. On the other hand, nordihydrocapsaicin’s strength is compromised by being one CH2 shorter than its hotter counterpart, dihydrocapsaicin.

How do these compounds exert such a powerful reaction in our mouths? In all mammals they cause tingling and burning sensations by activating  a non-selective cation channel, called VR1, on nerve endings. It’s not a coincidence that the same channel also interacts with compounds released by inflammation from actual intense heat sources or acidic protons. Birds, however, have a variant of VR1, which is still sensitive to heat and acid but which does not interact with capsaicin or its analogues.  It’s likely an example of co-evolution between Capsicum plants and animals who can eat their fruits without suffering deterring consequences. They then fly to other destinations to spread ingested seed.
We get more evidence of coevolution thanks to Jordi Altimiras of  Linköping University who made me aware of a study revealing that chilli seed germination is decreased in the gastrointestinal tract of mammals but not by the passage through the tract of birds.

Finally why are peppers producing capsaicioids in the first place? Any biosynthesis is catabolic and thus consumes energy. But making capsaicinoids in fruits is a worthwhile investment; it reduces fungal infection and seed mortality. From that narrow perspective we mammals have more in common with fungi than birds.


  • Molecular basis for species-specific sensitivity to “hot” chili peppers.  2002 Feb 8;108(3):421-30.
  • Comparative Study of Capsaicinoid Composition in Capsicum Peppers Grown in Brazil
  • Directed deterrence by capsaicin in chillies. Nature 412: 403-404 and Tewksbury JJ, Reagan KM, Machnicki NJ, Carlo TA, Haak DC, et al. (2008)
  • Evolutionary ecology of pungency in wild chilies. ProcNatAcadSciUSA 105: 11808-11811 .

Marcel Laurin Woodland Park

p1130047The Marcel Laurin Woodland Park is all too easy to under-appreciate. Drive or cycle by it too quickly, and it could easily go unnoticed because of its size–it’s not one of Montreal’s largest regional wooded areas. But once inside the woods, one realises that it is a special part of Montreal’s parks and protected woodlands, which combine to hold 75% of the city’s approximately 1.2 million trees. It gives citizens easy access to wildlife, which is key since not all can afford to travel far to experience it. Still a long way to becoming a mature forest, the woodland  is in part a wetland . Especially in the springtime,  the water that it collects allows dead leaves and other lingering organic material to decompose, increasing the availability of nutrients. These lead to a diversity of fauna and flora.

Some of its Fauna

Source Wikipedia

The common garter snake (Thamnophis sirtalis), like all reptiles in Quebec, is not poisonous to humans. It’s a good thing because after ignoring my advice, a student of mine once picked up a baby garter and was lightly bitten on the thumb. When defiant students are not around 🙂 ,  the snakes feed off the woodland’s slugs and toad-eggs. The adults form a mating ball consisting of several males competing for a single female, which eventually gives birth to live offspring. They were considerably widespread on the island as far back as the 1960s when I remember seeing garter snakes on a weekly basis during my childhood in St. Leonard. But that suburb of Montreal, like the rest of the city, has lost most of its wetlands. A stream that once ran through the former St-Leonard site has totally disappeared and though memories of some citizens have not yet faded, the biological diversity in many areas is long gone.

This 1832 map shows two areas of Montreal including (1) the current site of Marcel Laurin Woodland and (2) St-Leonard described in the text. Large sections of the island had already been deforested for the sake of agriculture. But we can see persistent streams(highlighted) and associated wetlands that no longer exist but which influenced the type of vegetation and animal life on our island. Source: James Wyld (1812-1887), David Rumsey collection

The availability of nutrients in the wetland makes the area rich in insect life which attract a variety of birds. In the Marcel Laurin Woodland, I’ve seen cardinals throughout the year along with wood peckers, birds that are rarely seen by citizens in the more densely urbanised sections of the island. Thanks to the efforts of amateur ornithologists who reported their sightings to Regroupement Québec Oiseaux, the organisation was able to include them in the ÉPOQ database.  Here are some of the ones people and I have spotted in the woodland. From left to right are the the black-capped chickadee (Poecile atricapillus); the eastern kingbird (Tyran tritri); magnolia warbler(Setophaga magnolia); hairy woodpecker (Picoides villosus); and the least flycatcher (Empidonax minimus). (The first pic is mine and was taken in Marcel Laurin; the rest are from Wiki)

The city’s website reports that

the woodland holds a certificate from the University of Kansas’ Monarch Waystation Program, and in 2009 it received significant support numerous partners for the planting of native grasses and milkweed necessary for the Monarch butterfly’s feeding and reproductive needs.

I have yet to see monarchs in the park, which does not necessarily mean they don’t come by, but I also noticed some replanting of milkweed late in the summer of 2016.  Thus the project described above is still a work in progress.

Some of its Flora and an Unwelcome Guest

Drawn to the borders of the Marcel Laurin’s stream are two dominant species, the silver maple and the red ash. One of the drier areas also has a small strand of at least 20 scattered beeches. A few years ago, invasive species such as the European and adler buckthorn were removed to help indigenous species such as the ash and maple, when suddenly the former was invaded by the Emerald Ash Borer (EAB), an insect whose larvae can girdle and kill trees. The threat is serious because 1 in every 6 trees in Montreal (and the majority of trees in Marcel Laurin Woodland) is an ash. In the past two years the city of Montreal has been forced to cut almost 8000 thousand severely infected ashes on the island. In that same time period, 37,000 infected trees have been treated with TreeAzin

(A) egg (B) larva (C) the EAB adult. Source: Phil Geib, Chicago Tribune

The insecticide consists of a 5% solution of azadirachtin, a compound found in neem seeds.

azadirachtin from chemSpider

It is biodegradable and shows very low toxicity to mammals. It actually doesn’t kill the insects; it prevents the destructive larvae from developing. The reason that the larvae are so destructive is that until the late 1990s EAB was only found in eastern Russia and China, where the insect and ashes coevolved. Continuous election created a balance where EAB could complete its life cycle without killing trees. But in North America, in only about a decade, EAB has spread like wildfire from Michigan to Quebec and all the way to Texas. Out continent’s ash trees lack defensive mechanisms and compounds that are used by trees in Area where the insect is indigenous. In addition, our ash trees on this continent release compounds like hexanal, linalool and 13 other volatiles whose signals are picked up by the antennae of adult EAB , facilitating insect-movement from one host to another. This happens when they are chewing on the leaves. As the larvae develop they  feed off the phloem, tissues that transport sugars from the leaves to the roots. The bark of the ash tree releases sequiterpene-compounds like α-cubebene, which also helps EAB adults locate a tree. And with more guests invited, enough tissue can be damaged to kill the tree.

What has not helped Montreal ash trees in the face of this EAB epidemic is climate change. Drought weakens trees, making them more vulnerable to insects. Winter temperatures below -33 º C are needed to kill dormant eggs. Montreal’s winters have been milder in recent decades. Environment Canada records indicate that we have not seen -33 º C in February since 1994; December  temperatures have been above -33 º C since 1980, and in the last four years, the coldest January temperature was only -24.6 º C.

Other Sources:

Review of the emerald ash borer (Coleoptera: Buprestidae), life history, mating behaviours, host plant selection, and host resistance
Therese M. Poland, Yigen Chen, Jennifer Koch, Deepa Pureswaran.  The Canadian Entomologist. 147(03): 252-262.

Life is hazardous and short for Montreal's downtown trees

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