The amino acid chirality mystery

If the analysis here is correct, it solves one of the more puzzling mysteries of life on Earth – namely, the fact that all 20 amino acids found in biological proteins are "left-handed".
Meteorites Delivered The 'Seeds' Of Earth's Left-hand Life, Experts Argue

                                                                   In a report at the 235th national meeting of the American Chemical Society, Ronald Breslow, Ph.D., University Professor, Columbia University, and former ACS President, described how our amino acid signature came from outer space.

Chains of amino acids make up the protein found in people, plants, and all other forms of life on Earth. There are two orientations of amino acids, left and right, which mirror each other in the same way your hands do. This is known as "chirality." In order for life to arise, proteins must contain only one chiral form of amino acids, left or right, Breslow noted.

"If you mix up chirality, a protein's properties change enormously. Life couldn't operate with just random mixtures of stuff," he said.

Recall that a carbon atom can form up to four bonds with other atoms. (Sometimes there are 2 or more bonds with the same atom, such as a double bond to another carbon atom.)

You can imagine these bonds arranged in a tetrahedral shape, that is, from a central carbon atom in the direction of the 4 vertices of a tetrahedron. In an amino acid, three of the bonds are occupied by a hydrogen atom, an amino group (NH2), and a carboxyl group (COOH). The remaining bond is occupied by a fourth group, which is variable (but there are only 20 possibilities that normally occur in Earthly biology) and determines the specific amino acid. The simplest amino acid is glycine, in which the fourth bond is occupied by a single hydrogen atom.

If you think of the amino acid as a tetrahedron, with the carboxyl group at the top, the other three components are arranged around the three bottom vertices. In glycine, two of those positions will be hydrogen atoms. But in all other amino acids, there are two different orders in which the distinct components can be arranged. Think of the tetrahedron's axis running from the central carbon atom to the top. If you look down that axis towards the base of the tetrahedron, then the hydrogen atom will be in either the clockwise or counterclockwise direction from the amino group. The latter is (by convention) called the left-handed (L) version, and the former is called the right-handed (R) version.

A protein is a series of amino acids tied together by peptide bonds, which form between the amino group of one amino acid and the carboxyl group of the other (with an H2O molecule removed, since an H pairs with an OH). In biological proteins such bonds form only between amino acids of the same chirality (both R or both L). So all proteins must consist only of L or R amino acids. As it happens, only the L type of protein occurs in nature on Earth. Presumably that is because at some time back when life was first developing, L amino acids significantly outnumbered R amino acids, and hence L proteins predominated over R proteins.

So the mystery is reduced to that of why at some point in time there were many more L amino acids than the R form. It has been shown that amino acids can form spontaneously from inorganic materias under some conditions (the Miller-Urey experiments demonstrated this.) However, one would expect equal amounts of R and L amino acids under such circumstances.

But there's another way out, because we know that in fact amino acids can form in interstellar space, since they were found in parts of the Murchison meteorite (and later others) that were uncontaminated with Earthly material. Furthermore, there's one definite way that amino acids which existed originally in an equal mixture of L and R forms on a chunk of rock hurtling through space could have their proportion tilted in one direction or the other:

These amino acids "seeds" formed in interstellar space, possibly on asteroids as they careened through space. At the outset, they have equal amounts of left and right-handed amino acids. But as these rocks soar past neutron stars, their light rays trigger the selective destruction of one form of amino acid. The stars emit circularly polarized light--in one direction, its rays are polarized to the right. 180 degrees in the other direction, the star emits left-polarized light.

All earthbound meteors catch an excess of one of the two polarized rays. Breslow said that previous experiments confirmed that circularly polarized light selectively destroys one chiral form of amino acids over the other. The end result is a five to ten percent excess of one form, in this case, L-amino acids. Evidence of this left-handed excess was found on the surfaces of these meteorites, which have crashed into Earth even within the last hundred years, landing in Australia and Tennessee.

So, one asks, is it possible that this imbalance of R and L amino acids was transferred from a meteorite to prebiotic Earth? In a series of experiments Breslow confirmed that this could happen:

Breslow simulated what occurred after the dust settled following a meteor bombardment, when the amino acids on the meteor mixed with the primordial soup. Under "credible prebiotic conditions"-- desert-like temperatures and a little bit of water -- he exposed amino acid chemical precursors to those amino acids found on meteorites.

Breslow and Columbia chemistry grad student Mindy Levine found that these cosmic amino acids could directly transfer their chirality to simple amino acids found in living things. Thus far, Breslow's team is the first to demonstrate that this kind of handedness transfer is possible under these conditions.

On the prebiotic Earth, this transfer left a slight excess of left-handed amino acids, Breslow said. His next experiment replicated the chemistry that led to the amplification and eventual dominance of left-handed amino acids.

That's where things stand now. We have as yet no way of knowing whether this is the scenario that actually occurred. But it is the most credible scenario yet devised to explain the otherwise astonishing fact that essentially all life on Earth uses only left-handed amino acids.