Science and Reason: Amino acid chirality

We've discussed the mystery of amino acid chirality – the fact that all biological amino acids on Earth seem to be left-handed. Previous discussion and background are here.

In that earlier discussion we considered a hypothesis that was described last year. It is known that amino acids can be formed in interstellar space. The hypothesis suggests that the first amino acids on Earth were carried here by meteorites. Further, there was some excess of left-handed over right-handed amino acids (but not necessarily entirely the left-handed kind) that arrived this way.

The last detail is to explain how the imbalance of one handedness over the other occurred in space. That could be explained by the fact that light that is strongly polarized could selectively destroy one enantiomer (alternative chiral form of a molecule) of an amino acid. Such light is emitted by rapidly spinning neutron stars. But, as we'll see, there are other possibilities.

This hypothesis was expounded by Ronald Breslow of Colombia University. He has conducted experiments that showed a small initial imbalance could be amplified by successive evaporations of solutions containing phenylalanine, a process that could easily have occurred on Earth. [2] [3]

Although experiments were done to verify the plausibility of the various mechanisms required, there's no direct evidence that the supposed sequence of events is actually responsible for the observed chirality. However, ten years ago analyses of the Murchison meteorite by Sandra Pizzarello and John Cronin did find a preponderance of left-handed amino acids. There was a possibility that this finding could have resulted from some process occurring after the meteorite hit the ground. But last year Pizzarello and others analyzed another meteorite that fell in Antarctica and seemed especially pristine. The researchers reported that there was a similar chiral imbalance in that sample as well, and not only in amino acids but also in precursor aldehydes. [1]

Now there is still more evidence for chiral imbalances – in the Orgueil meteorite, which fell almost 150 years ago:

Clues To A Secret Of Life Found In Meteorite Dust (3/17/09)

Over the last four years, the team carefully analyzed samples of meteorites with an abundance of carbon, called carbonaceous chondrites. The researchers looked for the amino acid isovaline and discovered that three types of carbonaceous meteorites had more of the left-handed version than the right-handed variety – as much as a record 18 percent more in the often-studied Murchison meteorite. "Finding more left-handed isovaline in a variety of meteorites supports the theory that amino acids brought to the early Earth by asteroids and comets contributed to the origin of only left-handed based protein life on Earth," said [Dr. Daniel] Glavin.

There's also evidence about how the imbalance may have occurred:

The team also found a pattern to the excess. Different types of meteorites had different amounts of water, as determined by the clays and water-bearing minerals found in the meteorites. The team discovered meteorites with more water also had greater amounts of left-handed isovaline. "This gives us a hint that the creation of extra left-handed amino acids had something to do with alteration by water," said [Dr. Jason] Dworkin.

The researchers focused on the amino acid isovaline, because it can "preserve its handedness for billions of years, and it is extremely rarely used by life, so its presence in meteorites is unlikely to be from contamination by terrestrial life."

Here's the abstract of the research paper:

Enrichment of the amino acid l-isovaline by aqueous alteration on CI and CM meteorite parent bodies

A large l-enantiomeric excess (ee) of the α-methyl amino acid isovaline was found in the CM meteorite Murchison (lee = 18.5 ± 2.6%) and the CI meteorite Orgueil (lee = 15.2 ± 4.0%). The measured value for Murchison is the largest enantiomeric excess in any meteorite reported to date, and the Orgueil measurement of an isovaline excess has not been reported previously for this or any CI meteorite. The l-isovaline enrichments in these two carbonaceous meteorites cannot be the result of interference from other C5 amino acid isomers present in the samples, analytical biases, or terrestrial amino acid contamination. We observed no l-isovaline enrichment for the most primitive unaltered Antarctic CR meteorites EET 92042 and QUE 99177. These results are inconsistent with UV circularly polarized light as the primary mechanism for l-isovaline enrichment and indicate that amplification of a small initial isovaline asymmetry in Murchison and Orgueil occurred during an extended aqueous alteration phase on the meteorite parent bodies. The large asymmetry in isovaline and other α-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life.

Note that evidence in the samples analyzed was not consistent with circularly polarized light as a cause of the imbalance, and a process involving water seems more likely.

Other accounts of this research: [6], [7]

There's other relatively recent research that demonstrates another way enantiomeric excesses of one form of amino acid could be produced. When inorganic materials are irradiated with high-energy photons (e. g. X-rays), they may emit electrons whose spins are polarized in a specific direction by a magnetic field. Experiments showed that this could affect the chirality of organic molecules adsorbed on the surface of a magnetic material:

Electrons put a new spin on chirality (11/5/08)

Researchers in the US have shown that the presence of spin-polarized electrons can make a chemical reaction involving “right-handed” molecules occur faster than the same reaction involving “left-handed” molecules. The discovery could help scientists understand why nature favours a certain handedness in many biological molecules.

Although the organic molecule used in this experiment was butanol (not an amino acid), the researchers plan to perform a similar experiment with the amino acid alanine. While this research doesn't show this is the mechanism actually responsible for left-handedness of amino acids found on meteorites, it's a least a possibility.

Press release on this research: [4]

Glavin, D., & Dworkin, J. (2009). Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811618106