The Era of Crystallography ends…
For over 100 years, crystallography has been used to determine the atom arrangements of molecules; specifically, it has become the workhorse of routine macromolecular structure solution, being responsible for over 90% of the atomic structures in the PDB. Whilst this achievement is impressive, in some ways it has come around despite the crystallographic method, rather than because of it…
The problem, generally, is this: to perform crystallography, you need crystals. Crystals require the spontaneous assembly of billions of molecules into a regular repeated arrangement. For proteins — large, complex, irregularly shaped molecules — this is not generally a natural state for them to exist in, and getting a protein to crystallise can be a difficult process (the notable exception is Lysozyme, which it is difficult NOT to crystallise, and there are subsequently currently ~1700 crystal structures of it in the PDB). Determining the conditions under which proteins will crystallise requires extensive screening: placing the protein into a variety of difference solutions, in the hope that in one of these, the protein will spontaneously self-assemble into (robust, homogeneous) crystals. As for membrane proteins, which… exist in membranes, crystallisation solutions are sort of ridiculous (clever, but ridiculous).
But even once a crystal is obtained (and assuming it is a “good” well-diffracting crystal), diffraction experiments alone are generally not enough to determine the atomic structure of the crystal. In a crystallographic experiment, only half of the data required to solve the structure of the crystal is measured — the amplitudes. The other half of the data — the phases — are not measured. This constitutes the “phase problem” of crystallography, and “causes some problems”: developing methods to solve the phase problem is essentially a field of its own.
…and the Era of Cryo-Electron Microscopy begins
Cryo-electron microscopy (cryo-EM; primers here and here), circumnavigates both of the problems with crystallography described above (although of course it has some of its own). Single-particles of the protein (or protein complex) are deposited onto grids and immobilised, removing the need for crystals altogether. Furthermore, the phases can be measured directly, removing the need to overcome the phase problem.
Cryo-EM is also really good for determining the structures of large complexes, which are normally out of the reach of crystallography, and although cryo-EM structures used to only be determined at low resolution, this is changing quickly with improved experimental hardware.
Cryo-Electron Microscopy is getting better and better every day. For structural biologists, it seems like it’s going to be difficult to avoid it. However, for crystallographers, don’t worry, there is hope.