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The rate of scientific progress — often driven by the development of new methods and new technologies, is truly staggering — even for us scientists. I do a lot of staggering. Every day around 4000 papers are published and it just gets harder and harder to keep up. Today is no exception.
I have written before on this blog about how the scattering of a fine beam of X-rays from crystals can reveal to us the atomic architecture of the minerals, chemicals & biological molecules of which the world is made. The technique was one of the signature scientific breakthroughs of the twentieth century, as I have recently been telling my students (and anyone else who cares to listen).
The trick to X-ray crystallography is to grow crystals from the molecules you want to study. Within a crystal the molecules — typically billions and billions of them — are lined up in orderly rows like ranks of disciplined infantry, all pointing in the same direction. This ordering and the sheer numbers of molecules in crystalline sample provides crucial amplification of the intensity of the scattered X-rays, turning a theoretical possibility into an experimental reality by yielding diffraction patterns that are strong enough to be measured.
The structure of any molecule that can be crystallised can be determined by this technique but here's the rub: not every molecule can be crystallised. What can be even more frustrating is that some molecules only produce microcrystals that are too small to used for X-ray analysis.
This is a well-known problem that has been tackled over the years by developing more intense source of X-rays, the gold standard now being third-generation synchrotrons that produce beams millions of times brighter than a standard X-ray generator. As a result, the size of useable crystals has dropped dramatically. It used to be that crystals of up to a millimetre in size were needed to obtained good quality data but these days the micro-focus beam lines available at synchrotrons allow data to be collected from samples that are only around 10 microns thick. That's one hundredth of a millimetre.
But a study published today in the open access journal eLife by scientists at Janelia Farm in the USA looks set to lower the size of useable crystals by another factor of ten. By using beams of electrons rather than X-rays Tamir Gonen and colleagues are now putting previously discarded protein microcrystals to work. The technique relies on the fact that electron beams behave as waves (a property first observed at the dawn of quantum mechanics in the early 20th century), and so interact with crystals in much the same way as X-rays, but also, critically, on their much stronger interactions with matter. This means that you get more intense scattering than with X-rays and can use much smaller crystals.
Remarkably, the Janelia Farm scientists have shown that useable data can be obtained from crystals as small as 1 micron (one thousandth of a millimetre) and have solved the structure of the protein lysozyme to demonstrate the power of the technique. Electron diffraction is not new but has been problematic to press into service because of the severe chemical damage that electrons cause when they bounce of proteins in crystals. Gonen's team have shown that by greatly reducing the intensity of the beam their protein crystals were able to survive long enough to allow collection of a complete data set, which requires repeated exposures of the crystal as it is slowly rotated through a large angle.
The diffraction patterns that they obtained by this new protocol are of high quality and allowed the structure of lysozyme to be determined at high resolution using essentially the same analytical methods that are applied to X-ray data. It would be nice if they had solved a new protein structure, rather than once that whose structure was first worked our by standard methods about 50 years ago, but let's not be ungrateful.
The technique — micro electron diffraction — is an important step forward for crystallographic methods and our ability to determine the structures of biological molecules that are difficult to crystallise. It opens a new window, albeit one that is for the time being a rather narrow aperture because although crystals of only a micron in size can now be analysed, crystals that are much bigger aren't useable because of complexities in the scattering process that cannot yet be corrected in the data analysis. Crystals that are much smaller take us back to the problem of scattering that is too weak to measure. MicroED might equally have been called the Goldilocks Protocol.
So the frustrations of crystallographers aren't quite at an end, but if reliable methods can be developed to grow micron-scale crystals or to cut larger crystals to the right size, the technique may well be adopted widely.
Perhaps that development will turn up among the 4000 papers to be published tomorrow.
@Stephen_Curry is a professor of structural biology at Imperial College, vice-chair of Science is Vital and a director of CaSE.
This blog post was modified on Tue 19th Nov at 17:29 to correct the spelling of Janelia Farm (full name: the Howard Hughes Medical Institute's Janelia Farm Research Campus).
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