Chiral: asymmetric in such a way that the structure and its mirror image are not superimposable.
Chirality, also known as “handedness”, since hands are the most obvious example, is derived from the ancient Greek word for hand. It is a concept found in mathematics and physics, and in physics, an example related to the theme above is the left-hand, and right-hand rule:
This concept, however, finds its greatest variety and is perhaps the most intriguing when looked at, in the fields of chemistry, especially, and biology where its consequences are both interesting, but as I will show at the end, it can be dangerous, and sometimes deadly.
Before We Continue, However, I would like to first try to explain chirality more clearly than in the definitions above. Then in the main part of the answer, We will use examples we are more familiar with in our day-to-day lives, then move on to the examples from chemistry and biology.
Explanation of Chirality
Above is a picture of the front and back of a human right hand. If we were to flip the hand on the right, it would overlay perfectly with the other. Therefore, these are not mirror images of each other.
The left hand, um…on the other hand, is a mirror image of the right hand, which we can confirm by holding it up to a mirror.
However, unlike two right hands, we can not turn or twist our left hand and perfectly overlap it with our right hand no matter how hard we try.
Despite both hands being made of the exact same flesh, having the exact same number of fingers, knuckles, and lines, they have one property that seperates them. Therefore, each hand is chiral, or exhibits “chirality.” This same property is found with our feet and ears, for example.
One of the most common examples in day-to-day life are screws. For comparison, a nail is not chiral. If you have ever used a screw, you may have heard of the phrase “lefty loosey, and righty tighty”, meaning that to move a screw into a piece of wood or metal, you must turn your hand to the right, or clockwise, and to remove a screw you must turn your hand to the left, or counter-clockwise.
Interestingly enough, it does not matter if you use your left hand or right hand, your hand must rotate clockwise and counter-clockwise to tighten or loosen the screw. On the other….um…. hand, if the mirror image of screws were the preferred type that was manufactured, then the phrase we would use to teach carpentry would become the less alliterate “lefty tighty, righty loosey.”
And here are just some quick examples of chirality that we could encounter in day to day life:
In the case of this building, which is like a screw, I will leave it up to the reader to use a mirror:
This may help:
Like the examples above, chemicals can also exhibit chirality. In chemistry, these are known as “stereoisomers”. What they share with hands is that they are made of the same atoms, are connected the exact same way, but can not be overlayed with their mirror image.
As a result, their handedness at the invisible molecular level can sometimes be observed at the macroscale in their crystal forms.
The most famous examples, and where the study of chiral compounds began, were crystals of tartaric acid, which were isolated in their pure forms using a microscope and tweezers by the famous scientist Louis Pasteur!
In many ways, Pasteur got lucky because most chiral compounds will not readily crystallize into separate, distinct forms such as this. Chemists make use of a technique called polarimetry instead, which involves polarized light being passed through a solution of preferably PURE samples of chiral compounds.
Without needing to go into too much detail, a “left-handed” molecule will rotate the polarized light in one direction, while its “right-handed” counterpart will rotate it in the exact opposite direction.
As a small note for examples to follow, you may notice that the chiral molecules have a (+)- or (-)- prefix before its name, or a D- or L- prefix. This is related to polarimetry, which remains to this day one of the only ways to distinguish between two stereoisomers. I will not explain these prefixes for the sake of brevity.
Chirality in Nature
Chirality is found throughout nature as exhibited by the one directional turn of the shells of the snails above, and this can also be found in sea shells. Why these snails have different handedness is a topic of debate, and may be due to mutation at the genetic level. In fact, there is a prevalence of one spiral direction over the other.
Since we are on the topic of genetics…most are familiar with the helical, “screw-like”, structure of DNA and RNA, however, most might be surprised to learn that they all show an essentially absolute preference for one turn versus the other possibility, especially across one type of species, but also across all species, perhaps due to our common origins and reinforcing the theory of evolution.
But these are not the only examples of the preference of one type of handedness in bulk molecules found in biology. This is also seen in enzymes, and proteins:
And just as it would be akward to shake the left hand of someone else with your right hand as Trudeau tried to do once:
These proteins and enzymes, due to their shape will often only recognize and react with molecules of a specific handedness:
What was left until now to describe is that the shape of the protein above is partly due to the preference for one type of handedness of amino acids in nature. This same preference is also found in sugars as well, and in the building blocks of DNA (Note: the overwhelmingly predominant chirality found in nature is circled.):
As a small aside, some bacteria, known as gram-negative bacteria construct their cell walls from the less prevalent D- amino acids and L- sugars, which makes them highly resistant to antibiotics. This is in part due to the fact that most antibiotics are also handed, with their mirror image being mostly innefective.
To finish the biochemistry section, we return to the what was stated above about polarimetry, which was indicated as one of the only, and most general, ways to distinguish between stereoisomers. However, in a limited sense, the nose also offers a means, in a few rare cases, of distinguishing between handed molecules.
If you are familiar with cooking, have ever chewed gum, and regularily brush your teeth, then you are well aware of the refreshing scent of mint, or spearmint. Most, though are not quite as familiar with the smell of caraway seeds, which has a smell similar to fennel, commonly associated with odor of black licorice.
Well, it may surprise you to know that the compound that create these two smells are due to the stereoisomers of the same compound – carvone.
Another example, though there may be some controversy, are the stereisomers of limonene, which offer lemons and oranges their distinct, though similar, odors:
Other examples include nookatone:
(You will need a mirror to see the other. Try all angles.)
The isomer on the left smells of grapefruit, while the odor of the isomer on the right is characterized as woody and spicy.
Linalool, with one isomer described as sweet lavender, with the other as being a woody lavender:
An explanation offered as to how the nose can distinguish between these stereoisomers is that the receptors of the nose are themselves chiral. Many may not be able to clearly distinguish between the latter compounds, but the carvone examples are quite clear. (I, for one, have smelled both carvones in pure form.)
Chirality in Medecine
When chemists isolate or synthesize compounds for medical use, acquiring a pure form of the drug is usually essential for two reasons: 1) one steroisomer is inactive, and in some cases 2) the undesired “hand” could be dangerous. A disturbing example of this was the drug thalidomide.
While there is some controversy as to why, it is suspected that the “R” form was safe, but the “S” form had very adverse affects, including deformation of fetuses in the womb, or death soon after birth. I will not show the effects, since they may be hard for some to see, and painful for myself as well.
One issue that is suspected was that the human body is able to convert one form to the other. The drug was originally sold as a 50/50 mixture of the two isomers, but even in pure form of the “R” variant, these terrible effects occur.
Though the drug has a terrible history, there is somewhat of a silver lining. Thalidomide has been found to be effective against some forms of cancer, and against viruses such as HIV. Cancer consists of rapidly developing cells, just as in a fetus, so it readily absorbs the drug. The teratogenic (mutating) ability of the drug causes the cancer to grow improperly, allowing the immune system more time to attack the cancer, destroying it in some cases.
The part of chirality that should make your head spin is not that these isomers exist, or their properties. The intriguing part is that there is an overwhelming preference for one “hand” over the other in the natural world.
There are many theories proposed to explain this selection and my own, if offered, may result in accusations of “pseudoscience”, so I will not comment on them.
Nevertheless, chirality is intriguing in just about every way, and it is part of the natural world that is known to mostly those involved in the sciences, especially mathematicians, physicists, architects, but especially, chemists, biochemists, and biologists.