Chirality

Molecules like amino acids are chiral, meaning they exist as optical isomers of each other. A "chiral" molecule (from the, , a hand) is one that can not be superimposed on its mirror image. Just as left and right hands are mirror images and not the same, chiral molecules have the same things attached in the same order, but opposite of each other.

The two isomeric forms (enantiomers) of amino acids are known as the D and L forms. Enantiomeric molecules are physically and chemically indistinguishable by most of the techniques available and only when probed asymmetrically, for instance, by plane-polarized light can they be distinguished. Although most amino acids (except for glycine, which is non-chiral) can exist in both L and D forms, life on Earth is made of only L-form amino acids. The L form is found in proteins. The D form is found in only some proteins that are formed by exotic sea dwelling organisms. No one knows why this is the case, but it offers strong evidence that life was designed rather than the result of random chemical evolution.

Like amino acids, sugars ribose and deoxyribose come in two chiralities but living things include only 'right-handed' sugars in its DNA or RNA.

Discovery
In 1848, Louis Pasteur working as a chemist with a solution of synthetic ammonium tartrate tetrahydrate contaminated it with a mold and the solution became more optically active as the time passed. For the first time anyone had demonstrated chiral molecules.

Related references

 * Origin of life: the chirality problem by Jonathan Sarfati. TJ 12(3):263–266. December 1998
 * Evolution Hopes You Don't Know Chemistry: The Problem with Chirality by Charles McCombs, Ph.D.

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