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Chemical Reaction: The Chirality of Life

By: Steve Herman
Posted: October 14, 2008, from the May 2006 issue of GCI Magazine.

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Figure 1 has a simple organic molecule, with all the covalent bonds pointed to the corners of a tetrahedron. If all the colored balls were hydrogen it would be methane. If all the balls are different colors, the mirror image becomes a different molecule, and the two images cannot be superimposed. If you don’t have a molecular model kit handy to test this idea, try putting a left-handed glove on your right hand.

Figure 2 illustrates the “handedness” of an amino acid. These amino acids are the building blocks of protein. Thus skin and hair, at the molecular level, are stereospecific and left-handed. By contrast, the DNA double helix has right-handed chemistry in its turns.

Louis Pasteur discovered chemical asymmetry in 1840. He noticed that a sample of sodium ammonium tartrate, a salt of tartaric acid used in wine making, contained two crystalline forms. Pasteur separated them using a pair of tweezers. One form was the mirror image of the other, and each was optically active in an equal but opposite way. Mixtures were optically inactive. This was the birth of stereochemistry. Lord Kelvin is credited with introducing the word “chirality” (from the Latin chiro, meaning hand) into science.

How important is chirality? Thalidamide is the classic, and tragic, example. In the early 1960s when the synthetic tranquilizer thalidomide was widely prescribed as a sedative, some pregnant women used it for morning sickness. They later gave birth to deformed children. The drug had been administered as a racemic mixture. Later, it was found that one isomer was teratogenic and interfered with DNA metabolism, while the other isomer was safe: the l-isomer produced the severe fetal abnormalities Life does not have to be left-handed. Some factor operating when life started here preferred l-enantiomers, or it just happened by chance. Another significant natural event that was not symmetrical was the Big Bang. Matter and antimatter should have been created in equal amounts, and it would have self-annihilated. Thus life on earth, and the entire universe, would not exist except for some imbalance at the moment of creation.

The proteins produced by nature are made from polymeric chains of 20 amino acids. What if there are more amino acids available? Then new proteins can be constructed with novel properties. Non-natural amino acids already are being made, and the first microorganism that both produces a non-natural amino acid and incorporates it into proteins has been created.

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