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.

1280px-Ripe_jalapeno_pepper — 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.

Nordihydrocapsaicin — nnordihydrocapsaicin 9 100 000

Dihydrocapsaicin — dihydrocapsaicin 16 000 000

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.

Sources:

Molecular basis for species-specific sensitivity to “hot” chili peppers. Cell. 2002 Feb 8;108(3):421-30.
Comparative Study of Capsaicinoid Composition in Capsicum Peppers Grown in Brazil
http://www.tandfonline.com/doi/full/10.1080/10942912.2015.1072210
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 .

November 16, 2018 0

From Playing Cards to Clover

If we keep looking at the same thing over a period of time with an uncritical, non-curious or distracted mind, attenuation sets in. And not only do our nervous systems tune out what’s commonplace, we also develop a false sense of history. We act as if things always were as they currently appear to be, and as if they will continue to be immune from flux.

Take playing cards as an example. If one was raised in North America and was not in contact with recent Spanish, Moroccan or Italian immigrants, it’s likely that the person has a fixed image of playing cards: 52 cards, 13 in each of four suits: clubs (clover), hearts, spades and diamonds with three of the cards being a king, queen and a jack.

But these familiar cards are just one version, one that originated under the rule of King Charles VI of France between 1380 and 1422. They continued to evolve a little later in the 15th century into their current forms which are neither unique nor static. Take a tour of cafes in Italy and you will see people playing with variations upon the theme of Italian-Spanish cards: the king is still present, but there is a cavalier and a servant instead of the queen and jack, respectively. And there are only 40 cards with suits of clubs (large sticks), cups, swords and coins. These strike more of a resemblance to the first cards that were imported into Europe by Italian merchants around 1360. They discovered them while trading with Mamluks in Egypt. The cards had the same suits like those of contemporary Spanish and Italian cards, except that they featured a wand instead of a club. Regional cards in Northeastern Italy still include a wand as a suit.

I often wondered why the word “club” is used for the clover leaf, one of the four suits of French cards. One hypothesis is that the suit was associated with commoners who grew clover alternately with their crops for fodder and in order to enrich the soil. But wooden clubs were also commoners’ weapon of choice, so the name “clubs” stuck to clover.

Clover is a legume whose roots include a mutualistic relationship between bacteria and plant cells.

cloverroots — The arrows point to white clover’s root- nodules, the site of nitrogen fixation by mutualistic bacteria. (The scale is in inches. 1 inch = 2.54 cm) Picture from NC State Extension Publications – NC State University

Thanks to bacterial enzymatic action, valuable nitrogen compounds are made from otherwise unusable nitrogen air molecules. In exchange, the bacteria receive shelter and sugars. Before many people developed the bad habit of adding herbicide to lawn grass, white clover seed was often mixed with grass seed. Clover, when coexistent, helps grass by adding nitrogen compounds to the soil.

Like playing cards, not all clovers are alike. White clover is adapted to moist conditions. It should not be the only companion of lawn grass. Especially where grass grows on slopes where water drains easily, it makes more sense to grow bird’s foot trefoil. Scientifically known as Lotus corniculatus , it is another leguminous plant which does better in arid conditions and which will help abate dandelions. These unpopular relatives of the lettuce in turn out-compete grass when less water is available. A smaller legume with smaller yellow flowers, black medic, can do well in both moist and more arid soils. And of course there is a common origin to the genera of the bird’s foot (Lotus), to medic (Medicago) and to that of at least 238 Trifolium species, including red and white clover—-we see it in the similarity of leaflet shape, small pea-like flowers and to the all-important and welcome infection in their productive roots.

Their family Leguminosae evolved about 56 million years ago, 9 million years after large dinosaurs went extinct. A few million years later, their most important clades separated. From genetic analyses, it’s been determined that their evolution occurred quickly, reminiscent of the way playing cards quickly branched out in Europe. And like the latter, the 18000 species of legumes are now spread all over the planet. They are being used for food, oil, lumber, fiber, medicines, aesthetic purposes and not least of all, for a vital role in the Earth’s nitrogen cycle.

SOURCES:

Lavin, et al. 2004. Evolutionary Rates Analysis of Leguminosae Implicates a Rapid Diversification of Lineages during the Tertiary. Systematic Biology 54 (4): 530-549 http://sysbio.oxfordjournals.org/content/54/4/575.full

The Botanical Garden. Phillips and Rix

The Mediterranean and Mediterranean World. Francois Braudel

The four suits of a pack of cards www.theguardian.com/books/booksblog/2013/dec/04/books-advent-calendar-4-four-card-suits

Have you ever wondered why the symbols on playing cards are called…. http://www.brierfieldbridgeclub.org.uk/themes/Have%20you%20ever%20wondered.htm

April 18, 2018 0

Perfectly Natural: Gardeners’ Love of Chemistry and Chemists’ Love of Plants

When citrus trees are grown from seed, they revert to a wild state and they can take decades to flower. About 15 years after germination I’m still waiting to see grapefruit blossoms in our kitchen, and of course the short days of long winters at high latitudes don’t help the matter. But a few years ago I was lucky enough to visit an organic citrus farm in the Orlando, Florida region, which turned out to be a far more magical treat than any ride at nearby DisneyWorld.

P1080404 — On a single citrus tree we not only see fruits at various stages of ripening but also new flowers. Picture by the author.

Unlike most plants, those of the genus Citrus can simultaneously bloom while fruit is ripening elsewhere on the same tree. And the blossoms include some of the most delightful fragrances one will ever experience. And understanding how some of the compounds are synthesised by tangerines, lemons, grapefruits and plants in general makes one appreciate them even more.

At the level of citrus tissues, we see among different species variations upon a theme, the theme being what all plants of the genus have in common: bee-pollinated, bisexual flowers with usually 5 petals and 5 sepals and superior ovaries which become the fruit’s rind and flesh. Their leaves are evergreen and being part of subtropical trees, even the hardiest among them, the Meyer’s lemon, cannot survive temperatures below -5 °C. At the biochemical level, in order to produce their special fragrance, there are variations upon themes as well. Like many other plants they use a common precursor, the molecule isopentenyl pyrophosphate, and with subtle changes they produce their beautiful characteristic bouquets which diffuse into our nasal receptors.

Using a process relying on thiamine(vitamin B1), among others, plants make isopentenyl pyrophosphate from acetyl coenzyme A, a metabolic byproduct of glucose known as pyruvate. Isopentenyl pyrophosphate is a useful and ubiquitous molecule containing a 5-carbon atom-building block known as an isoprene unit. Some isopentenyl pyrophosphate molecules isomerize, meaning they rearrange with the same atoms, essentially shifting the double bond from the tail end over to the next pair of carbon atoms. The stable pyrophosphate group then leaves the molecule, leaving behind a reactive positive charge on a carbon atom at the head of the isopentenyl molecule. This is incidentally one of the many reasons plants need to absorb phosphorus from their environment.

Squalenesynthesis21 — Simplified reaction mechanism(enzyme is not shown): electron flow reveals how isopentenyl pyrophosphate bonds to a charged molecule derived from isopentenyl pyrophosphate itself. The product, geranyl pyrophosphate is then used by lemons to make geraniol. Many other fragrant molecules are also derived from geranyl pyrophosphate. See diagram below. Diagram by the author.

geraniolderiv — The backbone of geranyl pyrophosphate is highlighted in red and is found in three citrus scents.

Notice that I emphasized that only some isopentenyl pyrophosphate isomerize and become ionized. The rest then attack and stick to the positively charged molecules, creating geranyl pyrophosphate (C₁₀H₂₀O₇P₂). This molecule can eventually be used to make an important component of cell membranes known as cholesterol, but lemons and some non-citrus plants also use the right enzyme to react it with water to produce geraniol (C₁₀H₁₈O). Although roses produce far more of the scent, it is also a minor component of citrus peels. In addition, lemon plants can oxidize the alcohol group of geraniol to an aldehyde to produce two isomers of citral, A and B. The former has a strong lemon scent and the latter, which has the same formula but with an aldehyde group on the other side of the carbon-double-bond, has a less intense but sweeter odor.

One 2009 study by Citrus Research and Education Center in Florida analyzed the flower scents of 15 species of citrus plants including lemons, limes, sweet oranges and mandarins. Using a relatively new solvent-less technique known as solid phase microextraction (SPME) along with mass spec-gas chromatagrophy, they identified 70 compounds, of which 29 were identified for the first time. The compounds belonged to four different groups of terpenes, compounds that are all derived from previously mentioned isoprene units. One of those oxygenated terpenes, linalool, is an alcohol derivative of geranyl pyrophosphate. Of the two possible isomers of the compound, oranges produce the R-version of linalool, which smells like lavender blended with citrus. It attracts bees and me.

P1080415cropped — A bee in a lemon flower. Linalool is one of the compounds that attracts the pollinator. Picture by the author.

geranyl pHosp deriv — More variation upon a theme at the molecular level: geranyl pyrophosphate, which is itself built up from two isoprene units, goes on to be the precursor of four other key compounds found in the floral essence of several citrus plants. Structures by the author.

Twenty four and forty five percent of the blossom-volatiles of sweet oranges and mandarins, respectively, consist of ß -myrcene. This compound is yet another variation upon the theme of geranyl pyrophosphate. Instead of having an allylic alcoholic head, it has a pair of conjugated double-bonded carbons and a pleasant fragrance. The same fruit blossoms and those of certain limes and lemons also produce of yet another geranyl pyrophosphate-derivative known as E-ocimene. Its aroma has been described as woody, green and tropical, an indication of how difficult it is for humans to describe smells. The difficulty becomes more pronounced when the isolated compounds of the labs force us to perceive “solo performances” while the reality of nature’s citrus blossoms present us with a symphony.

Sources:

The Botanical Garden. Ryx and Phillips. Firefly.

A comparison of citrus blossom volatiles. Phytochemistry 70 (2009) 1428–1434
https://www.ncbi.nlm.nih.gov/pubmed/19747702

Principles of Biochemistry. Lehninger.

The Merck Index. Twelfth Edition

Myrcene as a Natural Base Chemical in Sustainable Chemistry: A Critical Review

https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.200900186

March 23, 2018 0

Category: Plant Chemistry

Hot Chili Pepper Chemistry

From Playing Cards to Clover

Perfectly Natural: Gardeners’ Love of Chemistry and Chemists’ Love of Plants

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