Thursday, September 01, 2005

Left-handedness in amino acids

Amino acids exhibit chirality, which means they can come in a left-handed or right-handed version. Geological processes tend to make in exactly equal yields but life on Earth overwhelmingly uses only the left-hand versions. This strong bias is now necessary for proteins to fold together properly, but when did it begin? The New Scientist describes some speculation of how polarized light generates a bias in the chirality of amino acids prior to the emergence of life. Circularly polarized light, of the sort generated in vast amounts by white dwarfs can generate an excess of the left-handed racemer of the amino acid leucine. Possibly some such process at work in the early solar system meant that our common ancestor had an excess of left-handed amino acids to work with. The current polarized light experiments can generate excesses on the order of a few percent, but perhaps that's enough. It could be chemically amplified , or, once the bias was established, living things may have standardized and codified it. Some earlier work suggests that strong chiral bias in only one of the amino acids- serine- could force the other ones to come along.

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