Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Methods for Experimental Investigation of Protein Structure
X-ray Diffraction Analysis
Physical Principles of XRD

X-rays were discovered by Röntgen in 1895. They are a stream of high-energy photons.

X-ray crystallography was first applied to Proteins in the early 1930s by William Astbury (England), who obtained the initial X-Ray Diffraction patterns for A number of Fibrous proteins. In selecting these specific proteins, Astbury proceeded from the assumption that their peptide chains were ordered and oriented in a definite, uniform direction.

X-rays are generated by the bombardment of matter with high-energy (accelerated) electrons. Copper and molybdenum are most commonly used as target Materials.

X-rays are produced in X-ray tubes, which are evacuated two-electrode devices. Electron emission occurs at the cathode. Driven by a potential difference of approximately 20 - 90 kV between the cathode and the anode, the electrons are accelerated and bombard the anode material. Upon colliding with the target (the anode mirror), the electrons are decelerated, meaning they lose energy. This energy is partially or entirely converted into electromagnetic radiation in accordance with the law:

Class="center">hv = ∆E

where h is Planck's constant, v is the frequency of the emitted X-ray radiation, and ∆E is the magnitude of the energy lost by the electron during the collision (rapid deceleration).

The collision of electrons with the anode produces a continuous bremsstrahlung (braking radiation) spectrum across a certain frequency interval ∆v. This is the so-called white X-ray radiation, which exhibits a sharp cutoff at the short-wavelength end of the spectrum. THE POSITION OF this boundary is determined by the potential difference between the cathode and the anode in the X-ray tube, i.e., the maximum energy of the accelerated electron:

h v_max = ∆E_max = eU

where e is the elementary charge, and U is the voltage.