APPENDIX G - Chirality

Summary

Chirality is the property of those molecules that can exist into two symmetrical forms corresponding to mirror reflections, but cannot be superimposed on each other by a mere rotation in space. Left-hand and right-hand gloves are an example of chirality. Chiral objects must be three-dimensional, since two symmetrical plane objects can always be superimposed by a reversal in space.

Many of the molecules used by life are chiral. However, when they exist in non-living matter, most of the time one half is in the right-hand form and the other half is in the left-hand form. This is what is called a racemic mixture. In contrast, life nearly always chooses only one of these two forms. For instance, all proteins consist of left-hand amino acids, whereas RNA and DNA are always built up from right-handed sugars. When a living organism dies and decays, thermal fluctuations change molecular shapes at random, so that, in the long run, there is racemization. Since the opposite process does not exist, a mechanism was needed to trigger the emergence of life by selecting preferentially one of the two chiral forms. The continuity of life then becomes only a mere copying process.

Was the choice random? Two forms of life of different chirality could have emerged. Left-handed proteins could have eliminated righthanded proteins by a random evolutionary process. This matter does not seem fundamental for elucidating the origins of life, because all biochemical processes depend on chemistry; that is, on the electromagnetic interaction which is mirror-symmetric.