Chemistry's Evil Twin

In the American-English spoken language, there are many metaphors for that member of the family that seems just a little bit more different from the others–the black sheep, the odd duck, the bad apple, for example. One often heard specifically with twins is that of an "evil twin." The idea of an evil twin is ancient and based in some of the oldest mythologies that folklore still knows of, including Native American creation myths and those from the Mandika people of southern Mali, but for the most part it has always been more of a fictional tool than a reflection of reality. The one major exception (because there's always one) is in chemistry.

An approximate visualization of what the glucose molecule looks like.
PC: Jessica Noviello

Every chemical has a chemical formula that shows how much of every element is in it. For example, the formula for glucose, a basic sugar created by cells for energy, is C6H12O6. This formula tells me, a definitive non-chemist, that there are 6 atoms of carbon (C), 12 atoms of hydrogen (H), and 6 atoms of oxygen (O) in every molecule of glucose. Unfortunately, this is only half of the story when talking about chemistry.

As small as atoms are, they are still three dimensional, which means the molecules they create are also three dimensional. Every molecule has a specific structure of how the atoms are arranged that give it a particular shape. It's hard to see in the two dimensional approximation of the glucose molecule that I made (above), but glucose has a slight twist in the molecule, and the loosely connected carbon to the top-right is more underneath the rest of the molecule than I've shown here

There are many different molecular structures. Some shapes look like a triangular pyramid (tetrahedral), a starfish (seesaw), and even two x's merged together (octahedral), but these are only some of the many shapes that molecules can have. Sometimes the only thing that's different between two molecules is the shape. The same chemical formula but a different molecular shape means that the two molecules are totally different. In special cases, the molecules are the same in both formula and are almost identical in structure, but they are mirror images of each other. These molecules are called enantiomers.

To visualize how enantiomers look, just look down at your hands. They each have five digits, palms, and fingernails. They both do the same thing (more or less) and allow you pick things up, move things around, write, type, hold, and touch. Even though they look very similar, you have a distinct left and a distinct right hand, and they are not the same. They are mirror images of each other, and though you may try to put one on top of the other, it will never be a perfect fit.

These "two" pictures are actually only a single picture! I took a
picture of my left hand, copied it, and flipped the image. It looks
pretty much like my actual right hand! PC: Jessica Noviello.

Chemical enantiomers are the same way, and chemists even refer to them as "right" or "left" handed molecules. Another word for this is chirality. There are many known chiral pairs in chemistry, and the differences in chemical effects between the two members of the pairs range from relatively harmless, such as things smelling different, to downright dangerous.

Oranges smell like oranges because of + limonene, but
its cousin would make them smell like lemons!
PC: Wikipedia, used under the Creative Commons license.

On the harmless side, there are plenty of good examples in the food world. One chemical, limonene, smells different depending on the handedness of the molecules being smelled. The right-handed (+) limonene molecule smells like orange, but its left-handed (-) counterpart smells like, as you may have guessed from the name, lemon. Another example, and one that we had at our November Science on Main table just a couple of weeks ago, is that of another chemical called carvone. L-carvone (-) smells like spearmint and is often used in essential oils products, and its counterpart D-carvone (+) smells like caraway seeds or rye.

A pack of thalidomide capsules. PC: Steven C. Dickson.
Used under the Creative Commons license.

On the more dangerous side, the best example is also a tragic one, a true chemical evil twin. Before chirality and enantiomers were understood, pharmaceutical companies and scientists didn't know that the almost-identical chemicals could produce very different outcomes. In the late 1950s, many doctors in Europe prescribed pregnant women a drug called thalidomide to ease morning sickness, anxiety, and trouble sleeping. It really did a great job doing all of that, but it also caused serious birth defects in babies whose mothers used the drug. 40% of these babies died at birth.

Doctors made the connection between thalidomide and the birth defects in 1961, and the drug was immediately pulled from the market. It turns out that one of the enantiomers caused the beneficial effects of a sedative that relieved the symptoms of pregnancy, but the other caused the birth defects, and both were present in the medicine prescribed. Once chemists figured this out, they also figured out how to isolate the helpful molecule from the dangerous one. Today, thalidomide is still used to manage and treat serious illnesses such as cancer, tuberculosis, leprosy, and HIV/AIDS. It is a drug with a terrible history and still is risky to take, but it is significantly safer than what it was in 1961.

This Thanksgiving, be thankful that your family isn't as bad as chemistry's evil twin!