The Mirror Image Of Life

by Lorien E. Menhennett

I was talking to a dear friend on the phone this morning (you know who you are!). She is a very intelligent, wonderful, beautiful, and insightful woman, and we got talking about the basis and origins of life. Yes, the basis and origins of life … at 6:30 a.m. This is what the two of us do. We are a bit nuts. We come at these conversations (which happen with great frequency) from very different perspectives. She is a Muslim, I am an atheist. She is a future chemist, I am a future geneticist/physician. But the fact that we have different backgrounds enriches our dialogues, and brings us both to a greater understanding of the world around us, each other, and ourselves. It is an amazing tango of science and philosophy.

Back to the origins of life. In biochemistry, which I took last semester, we learned various theories about how life *may* have begun. Theories about RNA, DNA, and various other macromolecules coming into existence due to environmental and atmospheric changes. My friend and I were talking about those theories this morning. She, who also took biochemistry (although in a different section), then said something very interesting. She said that science cannot explain life. It just can’t. It’s not about whether God does or does not exist. (She knows my views on that subject, and respects them.) Science simply cannot explain how molecules went from nonliving to living. That transformation eludes explanation. In one of her most brilliant future-chemist moments, she then went on to say that one difference between life and non-life, one way to understand this difference, is related to chirality.

To quote my dear friend: There is an inherent relationship between chirality and life.

Two examples of chirality: a hand, and the amino acid alanine.

Two examples of chirality: a hand, and the amino acid alanine.

For those of you who have never suffered through organic chemistry, allow me to explain the concept of chirality. An object that is chiral is non-superimposable on its mirror image. An object that is achiral is superimposable on its mirror image. Here is an example. Take your hand. Hold it up to a mirror. The mirror image of your hand cannot be placed exactly over your actual hand, with the physical features of your hands aligning properly. Therefore, your hand is chiral. (According to Wikipedia, the word “chiral” is actually derived from the Greek word for hand, kheir.)

In chemistry, as well as in the pharmaceutical industry, chirality has profound implications. For example, one version (called an “enantiomer”) of a chiral molecule may be an active pain-relieving drug, while the other “enantiomer” may be very toxic to the body. The simple spatial rearrangement of the atoms can cause this dramatic shift in effect.

But what my dear friend was talking about is much more profound. I will explain. Take amino acids, the building blocks of proteins. In living organisms, amino acids are found in only one of their two enantiomeric forms. In the picture above, there are images of both L-alanine and D-alanine. These are the two enantiomeric versions of alanine, the simplest amino acid that exists. They contain the exact same atoms, but arranged differently in space around the central carbon atom. As we learned in biochemistry, “L” means “life.” In other words, living organisms contain the “L” enantiomer of amino acids. Not the “D.” This seemingly simple spatial rearrangement is literally the difference between a molecule that supports life and one that does not. If that isn’t profound, I don’t know what is.