From a Scattering Pattern to an Atomic Map
An X-ray crystallography experiment does not photograph atoms. X-rays scatter when they encounter electrons in a molecule, and the scattered waves overlap. In a crystal, many copies of the molecule sit in a regular repeating arrangement, so their scattering reinforces in some directions and cancels in others. The resulting spots form a diffraction pattern that records clues about the molecule’s internal structure.
Researchers measure the spots’ positions and intensities, then use mathematical methods to work backward from that pattern to a three-dimensional map of electron density—the regions where electrons are most likely to be. They fit an atomic model into the map, adjusting it until the model accounts for the observed data. The shape is therefore inferred, not directly seen.
Why the Crystal Matters
A single molecule scatters too weakly to produce a useful pattern in many experiments. A crystal solves that problem by lining up huge numbers of identical molecules, making their signals add together. Growing a suitable crystal can be the difficult part: the molecules must pack into an orderly lattice without losing the structure researchers want to study.
That packing also imposes a constraint. A crystal shows a molecule held in a particular, ordered environment, which may differ from how it moves or interacts in a cell. Even so, the method can reveal where atoms sit closely enough to explain how a molecule is built—and, in the case of DNA, helped establish its iconic structure.
