The enantiomers of life… explained at last?

A close look at amino acids in a meteorite that has lain buried under Antarctic ice has found that they show a similar ‘handedness’ to the molecules in living organisms, as – crucially – do the precursor molecules (Science Daily).

Any organic molecule comes in two forms – mirror images – that are chemically identical. Run a reaction in a test tube, and you’ll get an even mix of the two. Life, however, is different. Because the reactive site of enzymes is 3-dimensional, typically only one of the mirror images (enantiomers) will fit. And so the molecules of life show a characteristic signature, with a large preponderance of one or other enantiomer for any given molecule. Life is chiral.

The question is, how did this imbalance arise in the earliest life forms. It’s a question that has puzzled researchers for some time, with several possible solutions proposed. Of course, anything that’s not yet been explained is used by creationists as evidence for the existence of god – and the chirality of life is no exception.

One possible solution is provided by the fact that the amino acids in meteorites show a preponderance of left-handed enantiomers (they are chiral). But that still leaves open the question of how that happened.

What the new research adds is that probable precursor molecules, aldehydes, found in the meteorite are also chiral. From the paper:

Analyses of the meteorite diastereomeric amino acids alloisoleucine and isoleucine allowed us to show that their likely precursor molecules, the aldehydes, also carried a sizable molecular asymmetry of up to 14% in the asteroidal parent body. Aldehydes are widespread and abundant interstellar molecules; that they came to be present, survived, and evolved in the solar system carrying ee gives support to the idea that biomolecular traits such as chiral asymmetry could have been seeded in abiotic chemistry ahead of life.

Ref:

Pizzarello et al. Molecular asymmetry in extraterrestrial chemistry: Insights from a pristine meteorite PNAS published February 29, 2008, 10.1073/pnas.0709909105