Monolignols - Happy Arbor Day!

Happy Arbor Day from Nebraska! Today let's have a look at monolignols, simple ring molecules that when combined together make one of the strongest and most abundant biopolymers on Earth - wood.

Do you remember learning in school that the mass of a plant doesn't come from the soil, but from the air? Perhaps this tidbit is easy to forget, but it's true! Plants can suck carbon dioxide out the air and turn it into sugar. They use this sugar as energy to grow and reproduce. In order to stand up and reach for the sun, plants need to be able to build rigid structures can can hold weight. Trees are wizards at doing this.

Figure 1: The monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. In the background, an example of a lignin polymer.

To build rigid structures, woody plants break down the sugars they obtained from capturing sunlight and reassemble the carbon pieces into monolignols - ring structures made of carbon, oxygen, and hydrogen. Some of the most common monolignols are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Fig. 1). Scientists suspect that the trees then produce enzymes (small molecular-scale machines) that can assemble these monolignols use what's called (I kid you not) 'radical' chemistry. Radicals are the technical name for unpaired electrons. In an unpaired state, electrons are highly reactive and it seems that it is this energy that the tree's molecular machines use to link the monolignols together.

After radical polymerization, very large polymers are formed - see the background in Fig. 1. This process consumes an incredible amount of carbon. In fact, the Carboniferous Period (350 - 300 million years ago) is in part defined by plants' evolution of the ability to sequester carbon from the atmosphere and store it in a way that allows them to stand up tall. It is these huge, cross-linked polymers that give wood its strength, and it is these same structures that are oxidized during the burning of wood, returning the carbon to the atmosphere after potentially hundreds of years in storage.

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The periwinkle's deadly secret

Figure 1: The vinca alkaloid vincristine from the periwinkle Catharanthus roseus.

Periwinkle, the common ornamental plant, is highly toxic because of the chemical compounds it contains. One of these compounds is called vincristine - a compound that can prevent mammalian cells from dividing. This compound contains nitrogen atoms, making it an alkaloid. Vincristine and its sibling alkaloid, vinblastine, both inhibit cell division which makes them effective chemotheraputics. They are used to treat several types of cancer. For the plant, these compounds constitute a defense mechanism: if eaten in quantity, periwinkle leaves are highly toxic. So, even though the periwinkle is a pretty plant to look at - it's not good to eat!

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Limonene - oil from citrus peels

Figure 1: Orange peels and limonene. The peels of oranges contain oil. This oil is mostly made up of limonene. On the right: D- and L- limonene; mirror images of each other, but with very different properties.

Have you ever used a citrus degreaser? How does that stuff work? It turns out that the peels of citrus plants contain oil! Not the slimy black stuff that comes from million-year-old compressed algae, but a much fruitier substance. The majority (~90%) of orange peel oil is made up of a compound called limonene. This compound has a duality to its nature that is very important. Think about hands: there are left hands and right hands, which, though they are made of the same parts (four fingers, a thumb, and a palm), are not identical. Rather, hards are mirror images of each other. Limonene is just the same – there is a “left” limonene and a “right” limonene. In chemical language, these are called D-limonene and L-limonene. Our noses are particularly tuned to the difference between these two: D-limonene smells like citrus, while L-limonene smells like pine or turpentine. Limonene is used extensively in the perfume industry, as a component of degreasers, as a natural insect and pest deterrent, and even as a building block for renewable, biodegradable plastics [1]. Fantastic!

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[1] Alternating Copolymerization of Limonene Oxide and Carbon Dioxide. Christopher M. Byrne, Scott D. Allen, Emil B. Lobkovsky, and Geoffrey W. Coates*. Journal of the American Chemical Society 2004 126 (37), 11404-11405 https://pubs.acs.org/doi/abs/10.1021/ja0472580.

Nightshade

Figure 1: Mandragora roots (artist's rendition). The roots of Mandragora species contain hyoscyamine and scopolamine, tropane alkaloids with similar structures, but very different psychoactive effects.

Nightshade. The name conjures fear in many of us… but why? The nightshade family of plants is also called the Solanaceae. Some members of this family produce chemical compounds that are psychoactive in humans (though other members of the Solanaceae are important domesticated food crops, i.e. tomatoes, potatoes, eggplants, etc.). One member of the nightshade family is the Mandragora genus. These plants are also called mandrakes, and truly have roots that look like small humans. Alcoholic extracts from these plants contain hyoscyamine and scopolamine. Hyoscyamine is a central nervous system stimulant, but scopolamine (which differs only in that it contains an additional oxygen atom), is a central nervous system depressant. Plants containing these two compounds have been used for centuries as drugs, poisons, and aphrodisiacs - they appear in Homer’s Odyssey and works by Shakespeare. What would these icons have thought if they had known the uniqueness of these two compounds to the nightshade family and the subtleties of their structures and functions?