Chirality of Amino Acids

by Dr. Tahir Yaqoob on February 16, 2012

This post highlights how science reporting, including reporting by scientists themselves, often presents something as an explanation of a problem, whereas in reality the proposed explanation does not in fact solve the problem but simply defers the question.

The issue in this post concerns the problem of chirality (or handedness) of amino acids in living organisms. It turns out that the geometrical arrangement of the component atoms of an amino acid molecule in three-dimensional space is critical for an amino acid to function correctly. In particular, there are two arrangements of an amino-acid molecule that are mirror images of each other but otherwise their chemical properties are identical. However, it turns out that in a biological system, the behavior and functionality of the two forms of amino acid are different. The two forms are referred to as left-handed and right-handed molecules respectively, because they cannot be mapped onto each other by a rotation or any other spatial manipulation. Which one is labeled left and which one is labeled right, is simply a matter of convention. The property of “handedness” is generally known as chirality. In nature the two mirror images of a molecule occur in equal proportions. However, it is a remarkable fact that the molecules in living organisms are always left-handed. The right-handed counterpart cannot function with the left-handed apparatus. In a test tube, or in an organism that has died, left-handed (or right-handed) molecules revert to a mixture containing equal proportions of both types. The reason for the chirality of life is a complete mystery.

No satisfactory explanation has been found, but plenty of “non-explanations” abound. ou may have heard that an excess of left-handed amino acids has been found in meteorites. One of the articles about this appeared in the Astrobiology Magazine in 2011 and was entitled, “Meteorites May Answer Life’s Chirality Question” (other articles appeared in different places as usual, based on the same information, and the story in such cases is usually simply replicated many times over). The article is very strange and is a fine example of extreme handwaving. Even before reading the article, just from the title, a red flag should go up in your head. You should realize that the meteorites alone cannot answer the question because the question is simply deferred. How was the bias introduced in the meteorites?

Still, let’s continue with the article, maybe it does offer an explanation. But soon, the article admits that the mechanism is not known, and postulates that primordial water could be responsible for the asymmetry. However, no actual mechanism is proposed. Then it gets worse. The article admits that even if water could do it, it can only amplify an already existing asymmetry, and could not create one. Having admitted that, the article then postulates that radiation could have produced the asymmetry, but no mechanism is proposed. At this point the article has really gone way off course. There is no reason
why radiation should introduce an asymmetry. If there is, the article gives no hint about it. It has never been demonstrated in the laboratory, or theoretically (the article does not cite any study). The article then ends with a pretty graphic of an artist’s impression of a neutron star, which has nothing to do with amino acids, let alone left-handed ones. The only connection is that the neutron star produces radiation, but a large variety of other objects in the Universe produce radiation too.

So, by the time the article ends, we are no wiser about the origin of the chirality of amino acids. It seems to me that whoever wrote the article really needed to go home (maybe it was a Friday afternoon), and was stuck for a cool graphic to put in the article (after all how do you illustrate the chirality problem with a graphic?). Putting a picture of a meteorite in the article would have looked boring. Let’s see what I’ve got in my image archive. Aha, here’s an exciting graphic of an artists impression of a neutron star (neutron stars cannot be directly imaged because they are too small). Hmm, how can I work that in to the article? Radiation. Maybe radiation causes chirality? Okay, I’ll put in something about radiation so I can use this cool graphic and then I can go home.

So we were right in the original interpretation of the title of the article. How can meteorites possibly explain the origin of chirality without a proposed mechanism?

The Wikipedia article on amino-acid chirality is even stranger. It states that “Most scientists believe that Earth life’s ‘choice’ of chirality was purely random.” This is nonsense because random processes produce equal numbers of left-handed and right-handed amino acids. That is the point! In order to introduce a bias, a nonrandom process is explicitly required, and that process has not been identified. So, scientists who believe in the “random choice” (and such scientists do exist) actually believe in nothing, no explanation at all, because the mechanism producing a choice in the first place is not known.

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