Chirality
of that handedness. One suggestion is a meteor, similar to the Murchison meteorite, carried an enantiomeric excess of chiral biological molecules as it landed on Earth.
In terms of amplification, Charles Frank, a British physicist, made a hypothesis in 1953. He proposed a theoretical model of an autocatalytic reaction where the product is the catalyst for forming more of itself. Additionally, it also acts as an anti-catalyst to inhibit the production of molecules of the other handedness. Given that there is a slight imbalance of the enantiomeric ratio at the start, the homochirality of the product will be amplified during this catalysis. Fortuitously, this concept was successfully confirmed in the Soai reaction by the alkylation of pyrimidine-5-carbaldehyde with diisopropylzinc. 5 This reaction leads to the rapid enantioenrichment of the product catalyst from a low enantiomeric excess of reactant. Unfortunately, this is the only example up to now that demonstrates Frank’s model but is able to demonstrate a simple and sufficient life model for the evolution of homochirality. In the primordial soup, sugars and amino acids have represented the basic building blocks for biological homochirality. Glyceraldehyde was the simplest chiral sugar, and aminonitriles were the first chiral molecules able to be converted into amino acids. Recent research has proven that there is some sort of synergistic connection between the two types of biomolecules. This relationship was shown in an experiment carried out by John Sutherland and Matthew Powner in 2009. Using small amounts of L- peptide to act as a mediator, they reacted a racemic mixture of prebiotic molecules that included glyceraldehyde with inorganic phosphate to form pyrimidine ribonucleotides. 6 This result was a breakthrough in prebiotic chemistry, as it created an RNA precursor and provided evidence for the RNA world hypothesis. Furthermore, the enantioenrichment of specifically the D-glyceraldehyde was observed. Similarly, when a racemic mixture of aminonitriles is catalysed and mediated by a D-ribose sugar, an amino-sugar intermediate is formed, which later produces an L-amino acid. To sum up, both reactions reveal that D-sugar and L-amino acid can enrich each other, which creates a chicken or egg paradox: did the sugar or the amino acid first became homochiral and mediate the other? The fact that homochirality induces a chiral environment in life allows for discrimination of chiral molecules. Precisely because of this, enantiomers of chiral drugs display variations of taste, odour, safety, and the effectiveness of the medication in the body. Despite the different optical properties between enantiomers, they have the same chemical and physical properties. Based on the ideas of the Easson-Stedman hypothesis, the biological activities of chiral molecules can be demonstrated by a three-point interaction model. Shown in their model, a pair of chiral twins interact with three distinct regions of a binding site. The active enantiomer will fit in perfectly (like a key to lock), whereas the other will not. Nonetheless, the distomer at that receptor can still be the eutomer of a different receptor or enzyme, resulting in unwanted effects.
In particular, Ibuprofen is sold as a racemic mixture deemed safe. The S-isomer is anti-inflammatory, but the inactive R-enantiomer luckily has no effect. On the contrary, this anomaly does not apply to the distomer of other chiral drugs. Notably, levomethamphetamine is sold over the counter as a nasal
5 Soai Reaction 2021. 6 Brazil 2015.
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