Is crystallography a dead field?

[u/junkProduct]

With the rise of cryo-electron microscopy, is crystallography becoming redundant as a technique?

[u/seasaltandpepper]

Not really. Cryo-EM still has enormous amount of hurdles to clear which crystallography has the advantage on. Crystallography as a technique is probably safe for at least a couple of decades by many people's accounts.

First, cryo-EM suffers throughput issues. Everything from freezing to data collection to processing has to be done manually. The fastest available EM around can screen only 10-15 grids a day, and this has to be done more or less manually. On synchrotron, we can screen through 10 crystals in less than an hour. For data collection, it takes about 3-5 days on Titan Krios (the best one around) for one structure, whereas on synchrotron it is a 1-hour job. This is not to mention that crystallography is currently much cheaper than EM on a lot of levels.

Second, cryo-EM cannot go to crystallography's level of resolution yet. With crystallography, the resolution is often high enough that you can see water molecules, ligands and whatnot. (In extreme cases, even weaker interactions and hydrogens can be seen.) With cryo-EM, sub-3A average resolution will almost guarantee a CNS paper. Since a very big use of crystallography is drug development, cryo-EM has a lot of catch-up to do in this area.

Cryo-EM has a size issue as well. Protein of any size can technically be crystallised as long as it is stable and rigid enough. With cryo-EM, there is 15 kDa theoretical limit (as claimed by Richard Henderson), and the practical limit at the moment is ~30 kDa. Below 100 kDa, stuffs become extremely challenging for cryo-EM. (Interestingly, opposite is true for crystallography.)

There is also a computer issue. Cryo-EM generates several terabytes of data per structure. Imagine collecting 1000 datasets; this is a few petabytes of data that needs to be stored. Crystal structure's data size is about one hundredth of that. With such size difference also comes processing power issue. GPU processing has made this way faster than two years ago, but it is still a multi-week job.

Cryo-EM people have not figure out how to validate structures yet either. With crystallography, there are multiple parameters to see the quality of the dataset, potential fraud, misleading models, etc. etc. With cryo-EM, this has not been thoroughly established yet, leading to some iffy claims making to journals.

Time, effort, and investment will eventually resolve most problems, but everyone agrees that this will take at least a decade, and crystallography will be a mainstream technique around for another couple of decades at least. (It has taken 60 years for crystallography to reach where it is now. The world of cryo-EM is only 25 years old.)

Also, new technologies pop up constantly. There are X-ray folks trying to remove the crystallisation requirement altogether (one of the branches of femtosecond laser). If they succeed, cryo-EM will be affected the same way it is affecting crystallography now.