What was the mechanism, at the origin of life, by which D nucleotide polymers were selected and amplified to homochirality, while the L species became extinct?
Most of the other main building blocks of life – the amino acids – are chiral too, and in this case life uses the L enantiomers exclusively. Last year, it was proposed that a special type of L-glycine (normal glycine is the simplest amino acid, and is not chiral), may have helped produce the other L amino acids. But this mechanism cannot have directed the D-nucleotides of life, such as DNA and RNA.
Enter Hydrogen Peroxide
Late last year, my colleague and I proposed that hydrogen peroxide was the agent that mediated amplification of an initial small excess of D polynucleotides to homochirality.
We know that hydrogen peroxide is present on Mars, Enceladus and Europa, and it was produced on the ancient Earth, more than 3.8 billion years ago, which is around the time that life emerged. As mentioned above, it has also been detected outside the solar system.
In our most recent study, we focused on another remarkable property of hydrogen peroxide: it is the smallest and simplest chiral molecule itself, occurring as a pair of enantiomers called M and P.
Now, chirality begets chirality. This is a consequence of Curie’s principle, which states that “the symmetry of a cause is always preserved in its effects”. In other words, to achieve a chirally selective synthesis, separation or amplification, a chiral agent or force is needed.
In fact, chiral organic peroxides have been used in the lab to mediate the production of homochiral molecules. This tells us that, in principle, hydrogen peroxide can act similarly.