by Andre on 9 July 2005
If you’ve found your way to this blog, you may be familiar with the layout of some simple biological molecules like amino acids and sugars, and if you know about chirality you probably also know that molecules like this can’t be superimposed on their mirror images. But if you’ve never taken a course in organic chemistry or biochemistry, this simple fact of geometry may sound totally uninteresting, but that’s wrong. In fact, it’s one of life’s many great mysteries.
Let me start the story by pointing out that most chemical reactions are not enantioselective: they produce equal amounts of left- and right-handed products (that is, of course, assuming that the products have some handedness at all). This fact is especially important to chemists trying to synthesize drugs because geometry is critical to function. To drive this point home, I was going to retell the oft-quoted story about one of the enantiomers of thalidomide causing birth defects while the other one was safe, but this turns out to be a myth.* Anyway, it is true that we interact differently with different enantiomers. Spearmint just wouldn’t be spearmint without chirality.**
Back to the mystery. All of the amino acids in your body have the L-configuration and the sugars in your RNA have the D-configuration. The differences in the two configurations are described in the Wikipedia article cited above.
So how did this situation develop? There is still no definitive answer, but there have been some interesting suggestions. Here are some of the ideas:
1) One of the physical properties of chiral molecules that is different for the different enantiomers is their interaction with polarized light (this is why they are also referred to as optical isomers), so this is a natural place to look first. But where is this polarized light coming from? One possibility is from sunlight scattered in the atmosphere, but this would result in a very small effect at best. A more interesting possibility was recently suggested by an Australian group looking at polarized light in space (they were also nice enough to include free preprints of their articles!). Although they only detected infrared light from this region, which doesn’t have the energy to destroy amino acids, they calculate that 17% of the UV light from the region should also be circularly polarized. Cicularly polarized UV radiation could preferentially destroy the D-amino acids and lead to a different concentration on the early Earth. This possibility is particularly interesting since similar conditions could have been found at the birth of our solar system and because meteorites have been found that contain a 3-9% excess of L-amino acids.
Of course, there are some problems with this scenario. The L-amino acids would have to be created in large quantity in space and would have to survive their trip to Earth. Perhaps a bigger problem is that many amino acids interconvert between the L- and R-configurations in water, a process called racemization. (apparently this can be used for dating...I learn a lot writing for this blog!)
2) Another source of chirality in nature is found in beta-decay. There is a natural asymmetry in our universe that shows up in interactions mediated by the weak force called parity violation. This effect can lead to slight energy differences between the L- and D-amino acids and sugars. But the difference really is slight. Never the less, some people suggest that it could be the sought-after source of homochirality in biology.
3) It turns out that stirring can act as a chiral force (subscription required) that determines how porphyrins aggregate. The experiment didn’t affect chirality at the level of single molecules: instead it affected the aggregation of molecules, but the authors still suggest that something similar may have played a role in biology:
From the perspective of the above results, one could look toward designing experiments aimed at preparing chiral soft materials that would be further used for symmetry amplification at lower scale levels, i.e., as chiral catalysts for chemical synthesis and chiral selective membranes. On the other hand, colloids, micelles, and lyotropic liquid crystals are presently well recognized for their preponderant role at many levels of cellular structures. One is thus tempted to speculate about the role of the different hemispherical vorticity in relation to the origin of biological chirality. In this regard, symmetry breaking of mesophases and chiral microobjects, whose chirality could have been induced by the permanent sign of vortices in primeval mixtures, could have acted as initial asymmetric factors at prebiotic stages.
4) Another possible explanation is that simply by chance the first successful self-replicating systems on earth had parts with a given chirality and that as these systems evolved they ultimately produced L-amino acids and D-sugars. I don’t find this possibility very convincing in the absence of some further physical idea since there would be no selective pressure to amplify this putative fluctuation in the proportion of L- and D-amino acids. At least, I can’t think of one. Let me know if you can!
Yet another open question for you to answer! Are you biocurious yet or what?
*Of course, that didn’t stop the release of this public information page after the announcement of the 2001 Nobel Prize in chemistry.
**It was actually not straightforward to find a decent page that mentioned the effect of chirality on spearmint flavour that didn’t also get the thalidomide story wrong, but through the wonder of Wikipedia, I just changed the original incorrect entry to reflect this new-found knowledge. I hope the editors don’t change it back!