Mirror, Mirror

Chirality is a word that rarely passes the lips of inorganic chemists. For those that don’t recognize the term, chiral molecules are molecules which are not superimposable on their mirror images. Think about left and right handed gloves; they are mirror images of each other, but they are obviously different. A much better explanation of chirality is available here. Chiral molecules and their mirror images are also referred to as optical isomers, due to their ability to rotate polarized light in opposite directions. While chiral compounds are commonplace in organic chemistry and biochemistry, relatively few inorganic molecules are chiral, many of them belonging to a class of compounds known as multidentate transition metal complexes. Not surprisingly, chiral molecules and their mirror images have essentially identical chemical properties -- except when interacting with other chiral molecules. You can see why chiral chemistry plays a relatively small role in the world of inorganic chemistry.

To the biochemist, however, chirality is everything.

I am just guessing here, but it wouldn’t surprise me to find that 80-90% of the molecules present in a living organism are chiral. After all, amino acids, the building blocks of life, are all chiral (except for glycine). And since they all react with each other, chiral chemistry is very important. Now if all the chiral molecules in our bodies were evenly divided between both mirror images (known as a racemic mixture), this might be a moot point -- but it’s not. For some reason, all life on this planet chose long ago to selectively use only one form of most chiral molecules. For example, we all use the “left-handed” version of amino acids. Their mirror images, known surprisingly as the right-handed versions, are mostly unusable by our bodies.

The big question has always been, “Why (and when and how) did this occur in the first place?” A part of the puzzle has now been answered by researchers at Arizona State University. According to the article, extraterrestrial amino acids found on a meteorite demonstrate the same preponderance of left-handed molecules that we see in biological systems on Earth. Now this doesn’t explain whether the same process which operated in space operated on Earth or if the Earth was merely seeded with organic materials from space, but at least we are one step closer to knowing.