Which technique is used to determine the three-dimensional structure of a protein at high resolution?

• A. Immunoprecipitation
• B. Flow cytometry
• C. Polyacrylamide gel electrophoresis
• D. X-ray crystallography

Answer: (D)

Understanding how to reveal the exact arrangement of atoms in a protein at high resolution relies on methods that map electron density and atomic positions. X-ray crystallography does this by diffracting X-rays through a protein crystal and analyzing the diffraction pattern to generate an electron-density map, from which an atomic model is built. That ability to resolve individual atoms and their positions is what gives high-resolution 3D structures.

In contrast, immunoprecipitation uses antibodies to pull out a protein from a mixture to study interactions or abundance, not its atomic arrangement. Flow cytometry analyzes physical and chemical properties of cells or particles, not the detailed structure of a protein. Polyacrylamide gel electrophoresis separates proteins primarily by size or charge, providing information about purity or molecular weight rather than three-dimensional structure.