Critical thinking club talk

Critical thinking club talk: one of Chemistry’s unsolved questions

On Tuesday 14 May, Comet P in Year 12 gave an incredible talk during lunchtime to their fellow peers and teachers about one of chemistry’s most intriguing unsolved questions: what is the origin of homochirality in biomolecules.

Please read the text below from Comet to learn more about this fascinating topic and the ideas explored during the session.

My talk as part of critical thinking club was on one of Chemistry’s unsolved questions: What is the origin of homochirality in biomolecules. To break this question down, a biomolecule is an organic compound produced by a living thing and a molecule is chiral if its mirror image is not identical to it. For example, hands are chiral. Your left and right hands are mirror images of each other, however, if you lay your hands over each other palm up, you can see that they are not identical and therefore are chiral. The chirality of molecules can also be described as left- and right-handed and if a sample of molecules are homochiral, then that sample only contains molecules of one chiral, e.g. all left-handed or all right-handed. Each handedness of a molecule is called an enantiomer.

When you produce a chiral chemical in a lab, you would expect to see a roughly 50/50 split of left-handed and right-handed molecules. An infamous example of this is thalidomide, which was marketed as a morning sickness drug in the 1950s. Unfortunately, the women who took thalidomide ended up giving birth to babies with birth defects. This is due to how one handedness of the molecule helped with morning sickness, but the other caused birth defects.

However, if you were to buy sugar at the shops, all the sugar in that bag will be right-handed sugar. This is because we get sugar from plants and plants always produce right-handed sugar. But wait… Why do plants always produce right-handed sugar? Well, plants create sugar using enzymes during photosynthesis and all of the enzymes used to make sugar are right-handed.

But that doesn’t really answer the question, does it? Because then, why are all the enzymes that make sugar right-handed? That, like many things in life, is all down to efficiency and evolution. If you have two different enantiomers of enzymes and both enantiomers of the molecules you’re trying to react then the chance that the wrong enzyme collides with the wrong molecule is relatively high.

So, why is it right-handed sugar, and not left? This is the crux of the question which chemistry cannot definitively answer, but we do have some theories. One of these theories is due to how if you use mechanical stirring on a solution with both enantiomers this will create friction on the molecules, causing them to clump together and break apart, forming clumps of homochiral molecules. This could have been achieved in nature from waves in the ocean stirring and thrusting matter into the shore, leading to areas dominant in a single enantiomer.

A second theory is due to how one enantiomer is usually more soluble than the other. Experimentally, this can be shown by using a minimum volume of solvent to dissolve each enantiomer in two separate beakers and seeing which one requires less. Again, this could have led to areas in nature with a dominant enantiomer, encouraging evolution to choose the dominant one.

There are many more theories on the origin of homochirality in biomolecules, if you’re interested in reading more then here’s the link to the paper I used to source them: https://chemrxiv.org/doi/pdf/10.26434/chemrxiv-2025-h5kck?download=true&redirectToLatest=false

Thank you for reading!

-Comet P, year 12