4. When performing X-ray crystallography to determine protein structure, the
scattering of x-rays from the protein crystal is primarily due to:
a. amino acids
b. Carbon atoms
c. electrons
d. protons
X-Ray Crystallography Protein Structure: MCQ Answer and Explanation
The correct answer is c. electrons. In X-ray crystallography for protein structure determination, X-rays scatter primarily from electrons in the atoms of the protein crystal, producing diffraction patterns that reveal atomic positions.
Option Analysis
X-rays interact with matter through elastic scattering, where photons are deflected by electron clouds around atoms, as proteins consist mostly of light elements like C, N, O, and H.
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a. Amino acids: These are protein building blocks, but scattering occurs at the atomic level, not the residue level; amino acids contribute indirectly via their atoms.
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b. Carbon atoms: Carbon is abundant in proteins, yet scattering depends on electron density per atom, not specific elements; all atoms scatter proportionally to their electrons.
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d. Protons: Located in atomic nuclei, protons contribute negligibly since X-ray wavelengths (~1 Å) interact with electrons, not massive nuclei.
X-ray scattering protein structure electrons form the core principle in determining high-resolution protein models. This technique revolutionized structural biology by enabling atomic-level insights into protein folding and function.
Principles of X-Ray Crystallography
Protein crystals diffract X-rays due to periodic electron density arrangements matching X-ray wavelengths (0.5-2 Å), following Bragg’s law: nλ=2dsinθ. The resulting pattern maps electron density, from which atomic models are built. Light atoms dominate protein scattering, making electron interactions key.
Why Electrons, Not Other Options?
Electrons surround nuclei and coherently scatter X-rays elastically, preserving phase for interference patterns. Amino acids (option a) and carbon atoms (option b) provide the scaffold but do not directly scatter; protons (option d) are irrelevant due to their charge and mass. This electron-centric mechanism ensures precise structure elucidation for CSIR NET Life Sciences topics like protein dynamics.
