How life lost its symmetry

Extra-terrestrial amino acids answer chiral question.

Amino acids in the meteorites that pummelled the early Earth could be responsible for the chirality of the sugars subsequently used to support life, report US chemists. ¹ Their research ties in with earlier work showing that the amino acid serine can act as a chiral template for the formation of clusters of amino acids.

Although amino acids and sugars can form as one of two enantiomers - left-handed (l) or right-handed (d) - all life on Earth preferentially uses left-handed amino acids and right-handed sugars. The reasons for this remain unknown.

Now, research at Arizona State University and the NASA Ames Research Center, California, might go some way to providing an explanation. The researchers noted that carbonaceous meteorites which land on Earth often contain an overabundance of left-handed amino acids. They hypothesised that this enantiomeric excess could have been passed onto other prebiotic compounds, such as sugars, on the early Earth.

To test this, they examined the catalytic influence of the amino acids alanine and isovaline on the synthesis of the sugars threose and eruthrose, via aldol condensation of glycolaldehyde and formaldehyde. Alanine and isovaline are often found in meteorites, while both aldehydes are commonly present in models of the early Earth’s chemical environment.

The researchers discovered that the chirality of both alanine and isovaline could affect the chirality of the resultant sugars. l-alanine and l-isovaline produced an excess of d-threose, and a smaller excess of l-erythrose, while d-alanine and d-isovaline produced an excess of l-threose, and a smaller excess of d-erythrose. The effect was most pronounced when enantiomerically pure forms of the amino acids were used as the catalyst, but was still seen when the ratio of l- and d-enantiomers matched that found in meteorites.

Amino acids might also have acted as chiral templates for other amino acids, according to chemists at Purdue University, Indiana. ² They discovered that the amino acid serine would form into stable clusters of eight molecules, but only when all the molecules had the same chirality. These clusters could then combine with other amino acids, but again only if they had the same chirality.

If an excess of left-handed amino acids was delivered by meteorites, then these two processes could have produced an abundance of certain left-handed amino acids and right-handed sugars, all ready to be used to create life.

Jon Evans