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What is cryo-electron microscopy, the Nobel prize-winning technique?
The 2017 chemistry laureates were recognised for developing cryo-electron microscopy. But what is it, why is it exciting and where will it take us next?

A trio of scientists share this year’s Nobel prize for chemistry: Jacques Dubochet, Joachim Frank and Richard Henderson.

Their win is for work on a technique known as cryo-electron microscopy that has allowed scientists to study biological molecules in unprecedented sharpness, not least the Zika virus and proteins thought to be involved in Alzheimer’s disease.

Being able to capture images of these biological molecules at atomic resolution not only helps scientists to understand their structures, but has opened up the possibility of exploring biological processes by stitching together images taken at different points in time. 

Cryo-electron microscopy has proved valuable in helping scientists to develop drugs.It has been used in visualising the way in which antibodies can work to stop viruses being dangerous, leading to new ideas for medicines as just one example.


Why do we need cryo-electron microscopy?

Microscopes allow scientists to look at structures that cannot be seen with the naked eye – but when these structures are very tiny, it is no longer possible to use rays of light to do the job because their wavelengths are not short enough. Instead, beams of electrons can be used – with a technique known as transmission electron microscopy (TEM) – or scientists can employ a method known as x-ray crystallography in which x-rays are scattered as they pass through samples, creating patterns that can be analysed to reveal the structure of molecules.

The trouble is, x-ray crystallography relies on biological molecules forming ordered structures, which many fail to do, and the technique does not allow researchers to probe how molecules move.

Historically, TEM also presented difficulties. The beam itself fried the biological molecules being studied, while the technique involved the use of a vacuum which resulted in biological molecules drying out and collapsing, throwing a spanner in the works when it came to probing their structure.

This year’s chemistry laureates tackled these conundrums, enabling scientists to use TEM to image biological molecules in incredible resolution.

What did they do?
Henderson and his team, using a glucose solution to prevent molecules drying out, combined a weaker beam of electrons with images taken from many angles and mathematical approaches to build up a 3D image of a protein neatly organised within a biological membrane. It was a breakthrough moment. Henderson later succeeded in unveiling its 3D structure at atomic resolution – a first for a protein.

Meanwhile Frank developed ingenious image processing techniques to unpick TEM data and build up images of biological molecules as they are in solution, where they point in many different directions.

Dubochet came up with a sophisticated approach to prevent molecules from drying out. Henderson’s technique did not work for water-soluble biological molecules, while freezing samples resulted in the formation of ice crystals which caused damage and made the resulting images challenging to interpret.

Dubochet’s solution was to rapidly cool samples at such speed that the water molecules did not have time to adopt a regular structure. Rather, they were left pointing every which way, resulting in a glass within which biological molecules were frozen in time – in their natural shape.

What’s next?

The trio’s work, and subsequent efforts to perfect these approaches, has already led to astonishing developments.The technique of cryo-TEM has really opened up the molecular world of the cell to direct observation.

Among the processes it has made clearer is the mechanism by which DNA is copied into the single-stranded molecule RNA. 

But the future is also exciting, with scientists using the technique to probe the structure of drug targets, as well as components within cells involved in sensing pain, temperature and pressure. Further improvements in resolution are also afoot.

Cryo-electron microscopy is one of those techniques so basic and important that its use spans all of biology – including understanding the human body and human disease and in designing new medicines.
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Lavkesh Sharma

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