X-ray crystallography and single-particle cryo-electron microscopy (cryo-EM) both produce molecular structures, but they start with different kinds of samples and collect different kinds of data. Crystallography measures how X-rays diffract from an ordered crystal; cryo-EM combines images of many frozen individual particles into a 3D reconstruction. Neither method is best for every target: the right choice depends on the molecule, sample quality and biological question.
How X-ray crystallography turns a crystal into a molecular model
Researchers first purify the molecule and encourage it to form an ordered three-dimensional crystal. This is often a major practical hurdle: a sample may be difficult to crystallize, or the resulting crystals may not diffract well.
An X-ray beam produces a diffraction pattern. The measured spot intensities provide information about diffraction amplitudes, but a structure also requires phase information. Researchers obtain those phases using experimental or computational approaches, then combine phases and amplitudes to calculate an electron-density map. They interpret and refine that map into an atomic model. The method and its underlying data are described in the International Union of Crystallography review.
Because the molecule is arranged in a crystal, crystal packing can favor or constrain particular conformations. The resulting structure is therefore a view of the molecule in its crystallization conditions, not necessarily the only biologically relevant state.
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How single-particle cryo-EM builds a 3D reconstruction
For single-particle cryo-EM, a purified sample is placed on an electron-microscopy grid and rapidly frozen so that its water forms vitreous ice. A transmission electron microscope records images containing many individual particles, often in different orientations. Computational software estimates particle positions and orientations, sorts particles into classes and combines the images to reconstruct a three-dimensional map.
Because the method uses individual particles rather than a crystal, crystal growth is not required. The approach can also help researchers examine conformational or compositional variation. Its success still depends on a high-quality biochemical specimen: particles need to be sufficiently intact and well behaved, and image signal, particle orientations, beam effects and reconstruction choices can all affect the result. Flexibility may leave parts of a map blurred or structurally distinct from other regions. See the method overview and discussion of cryo-EM capabilities and limitations in the review.
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Key differences at a glance
| Question | X-ray crystallography | Single-particle cryo-EM |
|---|---|---|
| What sample is needed? | An ordered crystal; growing and optimizing one may be difficult. | A purified sample frozen in vitreous ice on a grid; no crystal is needed. |
| What is measured? | X-ray diffraction intensities from the crystal; phase information must also be obtained. | Images of many individual frozen particles, computationally combined into a 3D map. |
| Where can it be especially useful? | Detailed atomic coordinates and ligand interactions when suitable crystals are available; crystallographic screening can also be useful when crystals are in hand. | Large macromolecular assemblies and targets with conformational or compositional variability. |
| Common bottlenecks | Obtaining high-quality crystals and accounting for conformations favored by crystal packing. | Specimen quality and homogeneity, image signal, particle orientations, beam effects and computational classification or reconstruction. |
| Interpretive caution | A crystal structure captures a state under crystallization conditions, which may not represent every biologically relevant state. | Map resolution and interpretability can vary across a molecule; flexibility can blur or separate features. |
These are tendencies rather than mutually exclusive rules. The comparative review describes how the methods can address different parts of a structural question.
Which method is a better fit for a particular question?
Choose crystallography when crystals are achievable and detail is the priority
If a target forms suitable crystals and the aim is a detailed atomic model of a ligand-binding site, crystallography can be a strong option. It can also support structural screening when researchers already have crystals. The practical advantage depends on having crystals that diffract well; it is not a guarantee that any target will yield a high-detail structure.
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Consider cryo-EM for large assemblies or multiple states
Cryo-EM is particularly useful for large protein complexes and systems with multiple conformational or compositional states. As structural biology researcher Catherine Vénien-Bryan and coauthors put it, “Indeed, cryo-EM is particularly well suited for obtaining structural information on large protein complexes and for systems that exhibit multiple conformational or compositional states.” The specimen still needs to be suitable for imaging and computational reconstruction.
Let the biological question and sample guide the choice
Ask what molecular state needs to be understood, whether the sample can form an ordered crystal, whether it is a large assembly, and whether multiple states matter. A target that is hard to crystallize may favor trying cryo-EM; a well-crystallizing target studied for a ligand interaction may favor crystallography. These considerations guide method selection, but do not make the techniques exclusive.
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Why resolution statistics need a date and context
A 2023 International Union of Crystallography review compared structures released in 2021. In that deposited set, 92% of protein crystal structures had resolution better than 3 Å, compared with 22% of cryo-EM structures; 47% of crystal structures were below 2 Å, compared with 0.4% of cryo-EM structures. These are shares of structures released in 2021, not current limits on either method or predictions for a specific sample. The figures are reported in the review’s comparison of structures better than 3 Å and comparison of structures below 2 Å.
Resolution is not a complete measure of usefulness. The biological question, the quality of the model in the region of interest and how well the data support interpretation all matter. A single overall resolution value also does not describe whether every part of a flexible molecule is equally clear.
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How the methods can complement each other
Researchers can combine results rather than choosing a single winner. For example, a cryo-EM map can show the overall shape of a large complex while crystallographic structures of its subunits are fitted into that map. A cryo-EM reconstruction can also help with crystallographic phasing. Used together, the methods can connect a broad view of an assembly with detailed structural information about its components.
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