Mirror Images and Molecules

In my last entry on turkey, I mentioned that the proper name for the amino acid tryptophan is really L-tryptophan. Though we usually leave the L out, it is actually very important, at least as far as the chemistry is concerned. The L designates a characteristic of the molecule that is called chirality (pronounced k-EYE-ral-it-ee).

Chirality comes from the Greek word meaning “hand,” and is a property of asymmetry. To understand chirality, take a look at your hands. To a certain extent, your hands are identical – 4 fingers and 1 thumb, all of comparable sizes and arranged the same way around palms of equal size. However, your hands are different in one important respect – you cannot overlay them. If you put your right and left hands in the same orientation (say, palm down), and then put your right hand on top of your left, they don’t match. Your thumbs are on opposite sides. That’s because your hands are really mirror images of each other. Hold them palm-to-palm, and they match up.

An object is chiral if it cannot be superimposed on its mirror image. I’ve given you an example in the diagram below. There are a few important notes that you need to know to understand a picture like this. First, it is a 2-dimensional image of a 3-dimensional object, so you’ll have to use your imagination a little. Second, each letter represents a different atom. Third, the lines connecting the atoms are bonds – chemical links that hold the 2 atoms together. You can’t break the bonds, or shuffle the orientation of the atoms relative to each other. A bond designated by a solid triangle points out of the picture at you, while an empty triangle points into the picture away from you. A single line represents a bond flat in the plane of the picture.

In this diagram, the central atom (N) is bonded to 3 other atoms (X, Y, and Z). In the diagram on the left, N and X are in the plane of the picture (flat), Y points out of the picture at you, and Z is behind the picture away from you. In the center panel is the mirror image. Its mirror image is the same (N and X in the plane, Y pointing out, and Z pointing back). However, if I now flip the image on the left to try and superimpose back on the original (that's on the right), all of a sudden it doesn’t work. Though N and X are still in the plane, now Z points out at you and Y points back. These molecules have the same chemical composition, the same type and number of atoms, and the bonds connect the atoms in the same way in both. But the 2 molecules are mirror images of each other, and they cannot be overlaid. That makes them chiral.

But who cares if you can superimpose them or not? They’re still the same molecule, right? Actually, no, they’re not. So let’s go back to L-tryptophan. Tryptophan is a chiral molecule, which means that it is not the same as its mirror image. The 2 mirror image versions of tryptophan (called isomers) are L-tryptophan and D-tryptophan. Your body cannot use D-tryptophan at all. It can only use L-tryptophan - as a building block for proteins, as a precursor for serotonin, or for any other metabolic process. In fact, almost all of the amino acids in your body are L. (The exception is glycine, which is so simple a molecule that it doesn’t have chirality.) The only organisms to use D amino acids are some bacteria and a few exotic sea creatures. Even if you were to eat D-amino acids, you can’t use them. Your body would just get rid of them.

I find it fascinating that a molecule - like an amino acid – can be chemically identical to another (as in its mirror image), and yet the 2 have totally different properties in your body. Our body chemistries are really exquisitely specific!