X-ray crystallography

X-ray crystallography is a technique for determining the atomic and molecular structure of a crystal, in which the crystalline atoms cause a beam of X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds and disorder.

The method was developed by father and son William Henry Bragg and William Lawrence Bragg in 1912, who shared the 1915 Nobel Prize in Physics. It remains the dominant method in structural biology for determining protein structures, though it requires the molecule to form a crystal — a constraint that has driven entire subdisciplines of protein crystallization and crystal engineering.

The central mathematical relationship is Bragg's law, which relates the diffraction angle to the spacing between crystal planes. Yet the diffraction experiment alone does not directly yield the structure: the phase problem — the loss of phase information in the measured intensities — must be solved through heavy-atom methods, molecular replacement, or direct methods.

X-ray crystallography has given us the structures of DNA, hemoglobin, and thousands of enzymes, but its dependence on crystallization means it systematically selects for the most stable, most ordered conformations. The method does not merely reveal structure; it manufactures a particular structure through the crystallization process itself.