How’s it put together? Blow it up to find out!

Ok, this is just way cool. Researcher Martin Pitzer and a German-Swiss-Canadian team want to know how molecules are put together spatially, so in true mad scientist fashion, they’re blowing ’em up (Chemistry World writeup here). More specifically, they track bits of exploding molecule to figure out the stereochemical arrangement of the molecule’s atomic components – that is, where all the molecule’s pieces sit relative to each other pre-explosion.

Brace yourself, because their method also has a requisite badass name: Coulomb Explosion Imaging.

If you’ve studied organic chemistry, stereochemistry is probably familiar (if perhaps also the subject of recurring nightmares). It’s the study of how the various atoms of a molecule are arranged around each other. A non symmetric molecule can have identical atomic structure as its neighbor, even identical connections between atoms, but those atoms may be sticking out in front, while its neighbor’s are sticking out back. The arrangement is not simply rotational. Instead, the atoms are bound together differently. Images of two molecules cannot be superimposed over each other, no matter how you arrange them in space.

Blasting these with lasers is not recommended. (From unina.it, which coincidentally has a very accessible overview of stereochemistry.)

Far and away, everyone’s favorite metaphor for stereochemistry is your hands. Both the left and the right have five fingers: a thumb, a pinky, a few in the middle. Their constituent parts are identical. But there’s no way to superimpose one over the other in the same orientation. They’re mirror images of each other. In chemistry terms, they have opposite stereochemistry.

So who cares? Your body does actually. You may have heard of thalidomide. The story is quite famous and rather horrifying. Thalidomide was a drug prescribed to pregnant women to help fight morning sickness. Until it started causing genetic defects. Whoops. The idea is that one stereochemical arrangement of thalidomide was safe, but the other messed you up badly. Same molecule, if you count the atoms. But the relative position of those atoms made a drastic difference (the mechanism actually is assumed to be more complicated than that, where your body can turn one arrangement into the other, so really neither is “safe”).

So what’s new now? Well, not surprisingly, studying stereochemistry is a big deal, particularly for the medicinal community. The problem is that actually figuring out how a molecule is arranged ain’t easy. The most common method is to X-ray a very pure, pristine crystalline sample. But large complicated molecules and proteins are fiendishly difficult to make into perfect crystals. Here’s where the German-Swiss-Canadian team comes in: instead of trying to make the cleanest most perfect crystalline samples, they blow them up. By blasting samples with a laser, the molecules get stripped of their electrons. And in a strong electric field – BOOM! They blow apart, and the pieces hit a detector. Depending where each fragment lands, Pitzer and his colleagues can do some CSI work and trace the atoms back to their original molecular arrangement. Voila! X-ray-free stereochemistry!

Little known fact: Darth Vader was actually an unsung hero of stereochemistry.

A caveat here is how hard it is to track all those busted molecular fragments.  The trigger-happy researchers were only working with a small molecule made of five atoms.  Tracking all the possible fragments from just those five seems to be pushing the bounds of what their detector can do.  So this doesn’t immediately solve the problem of stereochemistry in large complex molecules, but it’s immensely cool and impressive.

Full, pay-walled Science article here: Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging

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