Biomolecular Structure Determination

How the atomic-resolution shapes of proteins and nucleic acids are obtained, by diffracting, scattering, or imaging the molecules and reconstructing a model from the signal.

Definition

Biomolecular structure determination is the set of experimental methods that yield the three-dimensional atomic coordinates of biological macromolecules from diffraction, resonance, or imaging data.

Scope

This topic surveys the physical basis of the principal structure-determination methods—X-ray crystallography, nuclear magnetic resonance, and cryo-electron microscopy—at the conceptual level: what physical quantity each measures, what sample and what limitation each entails, and how a three-dimensional model is built from the data. Detailed instrumentation belongs to the biophysical-techniques area; here the focus is the logic of going from experiment to structure.

Core questions

• What physical signal does each major method measure, and how does it encode structure?
• Why do crystallography, NMR, and cryo-EM suit different molecules and conditions?
• What sets the achievable resolution of a structure?
• How is an atomic model fitted to and validated against the experimental data?

Key theories

Diffraction and the phase problem

A crystal's diffraction pattern gives the amplitudes of the scattered waves but not their phases; recovering the phases is the central obstacle, and once solved it yields an electron-density map into which a model is built.

Single-particle reconstruction

Cryo-EM records many noisy two-dimensional projections of identical particles in random orientations and combines them computationally into a three-dimensional density, an approach whose resolution improved dramatically with direct detectors.

Mechanisms

In crystallography, X-rays scatter from the ordered electrons of a crystal, and the measured intensities—after the phases are recovered—are Fourier-transformed into an electron-density map. In NMR, the resonance frequencies and through-space couplings of nuclei report interatomic distances that constrain the structure in solution. In cryo-EM, electrons scatter from flash-frozen single particles whose many projection images are aligned and averaged into a density. In every case an atomic model is refined to fit the data and assessed by agreement statistics and stereochemical validation.

Clinical relevance

Determined structures of drug targets and disease-associated macromolecules underpin structure-based drug design and the interpretation of mutations; the methods here provide the educational background for that work without offering clinical recommendations.

History

X-ray analysis gave the first protein structures, myoglobin and haemoglobin, in the late 1950s; solution NMR extended structure determination to molecules in their native state from the 1980s; and the cryo-EM resolution revolution of the 2010s, enabled by direct electron detectors, made near-atomic structures of large complexes routine.

Key figures

• John Kendrew
• Max Perutz
• Kurt Wüthrich
• Richard Henderson

Seminal works

• kendrew1958
• kuhlbrandt2014

Frequently asked questions

Why is the phase problem important in crystallography?

A diffraction experiment records intensities, which give wave amplitudes but lose the phases; without the phases the electron-density map cannot be computed, so recovering them is essential to solving a structure.

Does a single structure capture how a molecule moves?

Not fully; most methods yield a representative structure or ensemble, and capturing motion requires additional dynamics measurements, which is why structural and dynamic studies are complementary.