röntgenkristallografiaa

Röntgenkristallografi, or X-ray crystallography, is a set of experimental methods used to determine the three-dimensional arrangement of atoms within crystalline materials by analyzing the diffraction of X-rays. When X-rays pass through a crystal, the orderly array of atoms causes scattered waves that interfere to produce a diffraction pattern. By applying Bragg's law, nλ = 2d sin θ, researchers relate diffraction data to crystal planes and derive an electron density map, from which atomic coordinates are built.

Historically, the technique emerged in the early 1910s. Max von Laue demonstrated diffraction of X-rays by crystals in 1912, and William Henry Bragg and William Lawrence Bragg formulated the law that bears their name in 1913, enabling quantitative interpretation of diffraction data. The method matured with advances in radiation sources, detectors, and computational algorithms.

In practice, a small, well-ordered crystal is exposed to X-rays, and the resulting diffraction pattern is recorded. Phase information, not directly present in the measured amplitudes, is recovered by methods such as multiple isomorphous replacement, anomalous dispersion, or molecular replacement. Iterative refinement adjusts a structural model to best fit the observed data, yielding a model with atomic coordinates and a map of electron density. Modern XRD often employs synchrotron radiation and cryogenic cooling, and may involve single-crystal or powder diffraction, with time-resolved or microcrystal variants.

Applications span chemistry, materials science, and especially structural biology, where protein and nucleic acid structures are determined and deposited in public databases. Limitations include the need for high-quality crystals, potential radiation damage, and unresolved hydrogen positions at limited resolution.