Life is remarkably selective. Life relies on many molecules with what is termed “handedness”—they’re chemically identical but are mirror images of each other. Most chemical reactions will make a 50-50 mix of the left- and right-handed forms of a chemical, but life uses only one of them. In fact, because of the differences between left- and right-handed chemicals, most of the key enzymes used by organisms will fail to work if they’re presented with chemicals that have the wrong handedness.
That poses a bit of a challenge for origin-of-life research, as we’re forced to explain how a world that may have started with an even mix of left- and right-handed chemicals produced organisms that only used one of them.
Today’s Nobel Prize in Chemistry goes to two individuals, Henri Kagan and Kenso Soai, who discovered chemical reactions could be biased, producing large excesses of one of the two forms of a chemical.
Chirality and life
The technical term for molecular handedness is “chirality,” and scientists replace left and right with dextro (D) and levo (L). But the ideas are largely the same. Your hands have all the same components—fingers and thumbs—organized in the same way. Yet if you point your thumb upward, the fingers curl in opposite directions, making one the mirror image of the other. Depending on the arrangements of the chemical bonds, many molecules can form similar mirror-image forms, with all the same parts oriented slightly differently in space.
(For the geekier among the readership: carbon atoms have four potential sites that can form bonds, spread evenly across the surface of the atom’s sphere. If each of those sites is linked to a different chemical, then swapping the chemicals located in any two of them can potentially change the way it’s arranged in 3D space.)