Questions: X-ray Crystallography

Question 1

A protein crystal diffracts X-rays to 2.0 Angstrom resolution. What level of structural detail can be reliably determined?

• A. Only the overall protein shape with no atomic detail
• B. Individual atoms can be resolved in well-ordered regions, water molecules are visible, and side chain conformations can be assigned with confidence
• C. Only the positions of alpha-carbon backbone atoms
• D. Hydrogen atom positions are precisely determined

Answer: (B)

At 2.0 Angstrom resolution, the electron density map shows distinct features for individual atoms (though not fully resolved as separate peaks), backbone and side chain conformations are clear, ordered water molecules appear as discrete density peaks, and ligand binding modes can be determined. This is considered good resolution for drug design and mechanistic analysis. At lower resolution (3.0+ A), only the backbone trace and large side chains are reliable. At very high resolution (<1.2 A), individual hydrogen atoms become visible. The 2.0 A resolution represents the 'sweet spot' where most biologically important structural features are well defined.

Question 2

X-ray crystallography captures the dynamic motion of a protein in solution.

• T. True
• F. False

Answer: False

Crystallography produces a static, time-averaged and space-averaged structure. The crystal lattice constrains molecular motion, and the diffraction experiment averages over billions of molecules and the duration of data collection. Dynamic information is partially encoded in B-factors (temperature factors), which reflect the degree of atomic disorder — higher B-factors indicate more mobile regions. But B-factors conflate true dynamics with crystal packing disorder and data quality issues. For genuine dynamic information, NMR spectroscopy, molecular dynamics simulations, or time-resolved crystallography (using X-ray free-electron lasers) are needed.

Question 3

Why is the 'phase problem' the central computational challenge in X-ray crystallography?

Model answer: The diffraction experiment measures the intensities (squared amplitudes) of the diffracted X-rays, but not their phases. Both amplitude and phase are needed to reconstruct the electron density map via Fourier transform. The phase information is lost during measurement because X-ray detectors record only intensity (number of photons), not the relative timing of the waves. Without phases, the electron density cannot be calculated — the intensities alone are compatible with an astronomically large number of possible electron density maps. Phase determination methods (molecular replacement, isomorphous replacement, anomalous dispersion) are therefore essential and often the rate-limiting step in structure determination.

The phase problem has driven much of the methodological innovation in crystallography. Molecular replacement (using a known homologous structure's phases as a starting estimate) works when a similar structure exists. For novel folds, experimental phasing using heavy atoms (isomorphous replacement) or anomalous scattering (selenomethionine labeling) provides initial phase estimates that are refined iteratively.