Many drugs have mirror image chemical structures – while one may be helpful, the other may be harmful

The effects a drug or chemical compound have on the body depend on how its atoms are arranged in space. Some compounds have a dark twin with the same molecular formula but different 3D structure – and this can have consequences for what they do or don’t do in the body.

Consider the tragic story of thalidomide, a morning sickness drug that caused thousands of birth defects and miscarriages. While one form, or isomer, of thalidomide has a sedative effect, the other is thought to cause abnormal physiological development. Because the two versions can convert back and forth in the body, it’s dangerous to take either form of thalidomide while pregnant.

My research has focused on one such compound found in red grapes and peanuts, resveratrol. It has been a scientific mystery why clinical trials on using resveratrol to treat Alzheimer’s disease have had inconsistent results. Turns out, it may be because two different forms were used – while one may help with cognition and memory, the other may be toxic to the nervous system.

Thalidomide has since been repurposed to treat other conditions, including cancer.

Isomers and amino acids

Many drugs have the same atoms and bonds but are arranged differently in space. These drugs are called chiral compounds – meaning they exist as two nonsuperimposable mirror images. For example, your hands are also nonsuperimposable mirror images of each other. Although they look the same, they don’t overlap when you put one on top of the other.

Usually these mirror-image versions have very similar properties because they share the same elements and bonds. But the way they are arranged in space can drastically change the effects they have in the body. Just as you wouldn’t be able to fit a left-handed glove on your right hand, a left-handed version of a drug wouldn’t be able to fit into a target in the body shaped to fit a right-handed molecule.

Chiral molecules come in two versions, or isomers, defined by their optical activity. This means that if you shine polarized light on a chiral molecule, one will rotate the light to the left (indicated by the prefix L-, or levorotatory) while the other will rotate it to the right (indicated by the prefix D, or dextrorotatory).

Amino acids, the building blocks of proteins, are chiral molecules. Living organisms primarily make proteins from amino acids with L configurations. The D configuration, however, has many other functions in nature. Bacteria, for example, use D configuration amino acids to make their cell walls. Mammals use D configuration amino acids as messengers in their nervous and endocrine systems.

The structure of a compound matters just as much as its individual atoms and bonds.

The amino acid tyrosine is one important exception to the L configuration rule. Unlike other amino acids, both the L and D…

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