Thread: Chirality in biologically active molecules

This is an interesting topic. Chirality, of handedness of molecules, refers to the connections between atoms in a molecule. Apparently a lot of compounds that are used in biological settings are only active in one form. It is difficult to make stereoselective compounds in the laboratory, while organisms can effortlessly produce single enantiomers/diastereomers/etc. for us, something we can't do.

You can see an example of chirality by sticking both hands out in front of you, with the palms facing each other. If you look at your left hand and imagine placing a mirror in between both hands, you would see the right hand image. The opposite would hold true. Now both hands are "the same" -- they're just copies of each other. Yet if you try to superimpose them on one another, they do clearly are not the same.

So it is true with molecules; despite having the same composition and damn near identical structures, one form -- right or left handed, as it were -- works while the other doesn't. In fact the R,S system of naming for chiral compounds refers to rectus and sinister, Latin for right and left. In terms of organic chemistry, you can find this if you take saturated carbon atoms with different substituents on each of the four available valences and do the hand-mirror exercise.


Here, these two molecules are mirror images of each other but cannot be superimposed on another. They are different, although composed of the same atoms.

I've heard that this is a consequence of all enzymes using steric arrangement in the lock and key model, but nonetheless it is still cool and that's just a theory. If you were to think of the Tree of Life and the ultimate progenitor of all life, the first living organism and how its enzymes and proteins were organized this way, and were passed on through billions of years then this is terrifically cool. That every living thing carries within it the same chirality of the original organism. If you don't go with the idea of homochirality originating in the original species, then it'd be interesting to evaluate how convergent evolution managed to pull this off.