Chirality and the origin of life

Life uses left-handed amino acids almost exclusively. Some meteorites contain a small excess of left-handed forms, a clue to how this preference may have begun.

Optical isomers

A carbon atom bonded to four different groups is a chiral centre. The molecule can exist as two enantiomers, non-superimposable mirror images, which rotate plane-polarised light in opposite directions. Amino acids other than glycine have a chiral centre: in alanine, the central carbon is bonded to H, CH₃, NH₂ and COOH.

A racemic mixture contains equal amounts of the two enantiomers and has no overall optical activity. Chemical reactions without a chiral influence produce racemic mixtures. Yet the proteins of life on Earth are built almost entirely from L-amino acids.

Excesses in meteorites

Most amino acids in carbonaceous chondrites are close to racemic, as expected for non-biological synthesis. But some, notably isovaline, show an excess of the L-form. In the Murchison meteorite, isovaline L-excesses up to about 18% have been reported in some samples. Isovaline is rare in Earth's biosphere, which argues against contamination, and the excesses tend to be largest in samples that were most altered by water on the parent asteroid.

Hypotheses

Proposed causes include circularly polarised ultraviolet light in the star-forming region, which would destroy one enantiomer slightly faster than the other, followed by amplification of a small excess during aqueous alteration. If meteorites delivered amino acids with a slight L-excess to the early Earth, that could have biased the chemistry from which life emerged. This remains a hypothesis: there is evidence for each step, but not a complete chain.

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