One of the most powerful analytical techniques that is available to organic chemists is called Nuclear Magnetic Resonance imaging, or NMR. It’s essentially the same technique that’s used in hospital MRI machines, although the medical field chose to drop the word “nuclear” from the phrase as patients were reluctant to expose themselves to a “nuclear” machine, for fear of radiation. As an organic chemist of fifteen years experience, I just have to laugh. An NMR doesn’t have any harmful radiation. It simply uses an immensely strong magnetic field to surround a sample (whether a glass tube or a human being) and then bombards the sample with radio waves. The resulting nuclear excitation / relaxation can be displayed on a computer screen, and if you’re trained, you can get a huge amount of information from the spectrum. Whenever I have an unknown sample that needs characterizing, the very first thing I do is dissolve it in some chloroform, place it in an NMR sample tube, and head to the NMR instrument. It’s something that organic chemists use every single day.
The instrumentation can be “tuned” to excite one particular atom over another. For MRIs, the atom is hydrogen. The protons (hydrogens) in water molecules inside a human body show up on the display screen and give doctors good information. Proton NMR (H-NMR) is also used a lot by organic chemists. It’s a quick analysis, taking about 5 minutes from start to finish, and protons heavily decorate a carbon skeleton. However, because organic chemists are primarily interested in carbon, a much more valuable analysis is carbon NMR, or C-NMR. Naturally occurring carbon is invisible to the NMR scan, but an isotope of carbon (called carbon-13) makes up about 1% of carbon samples, and it can be seen via NMR. Because the scan only picks up carbon-13 (which is scarce) the scan takes longer – overnight, in some cases. However, the resulting spectrum gives vast amounts of information that organic chemists can use to decipher the molecular structure of an unknown sample. I’ve gathered several thousand C-NMR spectra over my career, and it was always exciting to be able to use the information to make statements about the molecule and it’s electronic environment.
One technique that chemists can use to get more information from their NMR scans is to enrich a sample with carbon-13. Although carbon-13 usually only makes up about 1% of a molecule, it’s possible to make (and even purchase) chemical starting materials that are >99% carbon-13. They’re quite expensive, but they are readily available. Imagine you have a reaction that produces an organic material. If you obtain the carbon-NMR of the unknown mixture formed by the reaction, you’ll see small peaks for the naturally-occuring carbon-12’s but if any of the carbon-13 starting material had been incorporated, you’d see a huge peak (100 times the size of the others). It would be proof that the reaction had occurred, as that large signal could only have come from the enriched starting material. It’s a way of tracing the progress of a reaction and it doesn’t involve anything more complicated than a regular carbon NMR analysis, which we organic chemists do multiple times a day in any case. While it’s not a common technique (due to the cost of the carbon-13 starting materials), I have used it myself in the past, and I’m always on the lookout for interesting science which takes advantage of this powerful method.
That’s why I was excited to read a recent article published by some Australian researchers in the Journal of Biological Chemistry. The scientists were studying a tropical parasite called Leishmania, which is widespread and which a parasite / protozoa which infects hundreds of thousands of people a year. The researchers were interested in the metabolic pathway of the parasite. If they could discover how a nutrient was used as it traveled through the parasites digestion, they could possibly make statements about how new medications could attack those pathways. Hopefully, the new medications would be more selective and therefore less toxic to the human hosts. The chemists purchased carbon-13 enriched glucose and fed it to the parasites. They then used carbon NMR to study the samples, and found that the carbon-13 glucose atoms were broken down and used in a biological cycle called TCA. This was surprising, as other studies on parasites haven’t found much activity in the TCA cycle. With this particular parasite (which is spread by sandflies), the TCA had crucial anabolic functions. With this result, scientists can begin to design drugs which disrupt the TCA cycle inside the parasite, which will hopefully lead to new medications with a larger safety profile. As an organic chemist, I’m very pleased that a technique I use everyday has played a role in studying a troublesome parasite. It reinforces the notion that organic chemistry is relevant, even central, to everyday life.
The source of this article can be found at: Saunders, E., et al. “Isotopomer profiling of Leishmania mexicana promastigotes reverals important roles for succinate fermentation and aspartate uptake in tricarboxylic acid cycle (TCA) anaplerosis, glutamate synthesis, and growth”. Journal of Biological Chemistry 2011, 286, 27706.