The Handedness of Carvone
I love the smell of spearmint, growing wild in fields in the hills above my home. I first became aware of its fragrance long ago. Early one summer evening, coming down a mountain road at too high a speed, my motorcycle and I missed a curve. I found myself lying rather suddenly face down in large patch of a previously unnoticed and unknown but highly aromatic plant. As I lay there, assessing which parts of me were broken and which still seemed to work properly, I could not ignore the overwhelming odor enveloping me and that I found so oddly soothing. Even after washing the blood and the dirt from my clothes the smell filled my room. A few weeks after my accident a friend drove me to the site and I cut myself a bouquet of the plant that had so gallantly tried to cushion and comfort me. I have loved it ever since.
When we smell something we are usually detecting a multitude of different molecules. The scent of most things is rarely pure but a mixture of this, that, and the other. In the case of spearmint the compound most prominent in its aromatic oil is carvone. We also find carvone in abundance in caraway and dill. And yet these plants smell and taste nothing like spearmint. How can that be?
Place the palm of your right hand against the palm of your left hand. The effect is similar to that of holding the palm of your right hand flush against a mirror. Your hands are mirror images of each other. And yet, were you able, just for a moment, to detach your left hand you would find that there is no possible way to bend, twist, or rotate it such that you might make it an exact copy of your right. Your hands are mirror images of each other and yet, paradoxically, they are unique.
There are molecules in the world around us that, like our bodies, have this attribute of handedness. For simplicity’s sake we can think of carvone as existing in left-handed and right-handed forms. The latter is found in caraway and dill, the former in spearmint. It is amazing to realize that our bodies are so exquisitely sensitive that they respond to and correctly interpret this handedness in something as small as molecules, but this response is not unique to carvone. Many of the molecules essential to life exist in left and right-handed forms and our bodies are very specific in their requirements for one, or the other, but not both. The amino acids and sugar molecules on which our very lives depend have this property of handedness. For the most part we can only use left-handed amino acids and right-handed sugars. The majority of the most widely-sold pharmaceutical compounds in the United States of necessity also have this attribute of handedness.
How can the handedness of molecules be important? Brew yourself a cup of spearmint tea and take a sip. If it doesn’t taste like pickles you have the beginning of an answer.