A quick way to weigh molecules

A mystery substance is analysed and found to contain, let’s say, five compounds. How is that analysis actually done? How do scientists know what substances they’re working with? There’s no magic box that identifies all chemicals: it takes a lot of time, and complicated machinery. But researchers from Griffith University have figured out one way to speed the process up.

“When we look at complex mixtures of molecules, we can do that by either one of two techniques,” explains Professor Anthony Carroll, from Griffith’s School of Environment and Science and the Griffith Institute for Drug Discovery.

The first technique is called liquid chromatography/mass spectrometry: separating the mixture into discrete substances (liquid chromatography), and then analysing the molecular weight of those substances (mass spectrometry). This tells us which atoms are present, but it doesn’t yield much information on the structure of the molecules.

The second technique is called nuclear magnetic resonance spectroscopy (NMR): it can give information on the structures of molecules, but it doesn’t provide details on mass. It yields data in the form of peaks on a “spectrum”, which chemists can use to learn more about how the atoms are linked – but a mixture with lots of different molecules generates very complicated readings.

Example of an nmr spectrum for a single molecule. It shows a line graph, with a line making 10 peaks and lots of smaller peaks. The peaks correspond to certain atoms (in this case, phosphorus atoms) in the molecule.
Example of an NMR spectrum for a single molecule. The peaks in this graph correspond to certain atoms (in this case, phosphorus atoms) in the molecule. If there are lots of different molecules in the mixture, many more peaks appear on the spectrum. Credit: Smokefoot / Wikimedia Commons, CC BY 4.0

“We’ve got these two techniques that give us really useful information, but they’re not tied to each other,” says Carroll.

“We’ve got this mass spectrometry technique which shows all these particular compounds with these particular masses, and we’ve got NMR that gives us a complex forest of peaks associated with all of those compounds, but we’ve got no way of actually knowing what is the molecular weight associated with each of the peaks that we see in the NMR spectrum.”

Carroll and colleagues have figured out how to simplify this, though, by using NMR to determine weight as well as structure.

“What we’ve developed is a technique where we can identify the molecular weights of every component by simply doing an NMR spectrum. And what that then means is that we’ve got not only the masses for the individual molecules, but we’ve got their unique NMR signatures directly tied to a specific mass.”

This is done by looking at the speed of the molecules as they move – or diffuse – through the liquid they’re dissolved in.

“Every molecule is actually moving constantly through a liquid, but depending on the size of the compound, they move at different rates,” explains Carroll.

“The method that we’ve applied in this process is a thing called diffusion NMR or DOSY … we’re not doing any separation in a physical sense, we’re separating all of the molecules using this particular technique in the NMR tube.”

This means that the time taken to do analyses is drastically shorter. Carroll has found it very handy for his research, which looks at finding potential medications from living sources.

“I work on the chemistry of living organisms. We go out and we collect sponges and we collect rainforest plants and we collect microorganisms, and we extract them so that all of the organic molecules that occur within the tissues of those organisms are in one sample.”

Previously, if something in that sample showed promise as a medical treatment, Carroll and colleagues would need to spend one or two weeks in the lab isolating the substance for further analysis.

But with this new technique, “it takes us somewhere between about 10 minutes and maybe an hour to acquire the data.”

The technique doesn’t require any new equipment: it can be done on existing NMR spectrometers, meaning that “this is something that’s transferable”.

A paper describing the technique was published last week in Chemical Science.

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