x-ray crystallography

Examples of x-ray crystallography in the following topics:

• Determining Atomic Structures by X-Ray Crystallography

  • X-ray crystallography is a method of determining the arrangement of atoms within molecules.
  • X-ray crystallography is a method for determining the arrangement of atoms within a crystal structure.
  • X-ray crystallography is a powerful tool that has broad applications in the determination of the structures of both organic and inorganic compounds.
  • Throughout the history of chemistry and biochemistry, x-ray crystallography has been one of the most important methods in helping scientists understand the atomic structure and bonding.
  • Describe the method of x-ray crystallography as it is used for determining the structure of molecules.

• Crystallographic Analysis

  • The field has greatly advanced with the development of x-ray diffraction methods, where the matter analyzed is usually in its crystal form.
  • Nuclear magnetic resonance spectroscopy and x-ray crystallography have become the methods of choice for understanding three-dimensional protein structures.
  • X-ray crystallography is the primary method for determining the molecular conformation of biological macromolecules, particularly proteins and nucleic acids such as DNA and RNA.
  • Neutron crystallography is often used to help refine structures obtained by x-ray methods or to solve a specific bond; the methods are often viewed as complementary, as x-rays are sensitive to electron positions and scatter most strongly off heavy atoms, while neutrons are sensitive to nucleus positions and scatter strongly off many light isotopes, including hydrogen and deuterium.
  • Distinguish between the three methods of crystallography: X-ray, neturon and electron crystallography

• X-Ray Diffraction

  • The principle of diffraction is applied to record interference on a subatomic level in the study of x-ray crystallography.
  • X-ray diffraction was discovered by Max von Laue, who won the Nobel Prize in physics in 1914 for his mathematical evaluation of observed x-ray diffraction patterns.
  • In x-ray crystallography, the term for diffraction is Bragg diffraction, which is the scattering of waves from a crystalline structure.
  • Similarly, the x-ray beam that is diffracted off a crystal will have some parts that have stronger energy, and others that lose energy.
  • The XRD machine uses copper metal as the element for the x-ray source.

• Basic Techniques in Protein Analysis

  • The basic techniques used to analyze proteins are mass spectrometry, x-ray crystallography, NMR, and protein microarrays.
  • X-ray crystallography enables scientists to determine the three-dimensional structure of a protein crystal at atomic resolution.
  • Crystallographers aim high-powered X-rays at a tiny crystal containing trillions of identical molecules.
  • After each blast of X-rays, lasting from a few seconds to several hours, the researchers precisely rotate the crystal by entering its desired orientation into the computer that controls the X-ray apparatus.
  • This enables the scientists to capture in three dimensions how the crystal scatters, or diffracts, X-rays.

• X-Ray Spectra: Origins, Diffraction by Crystals, and Importance

  • In a previous Atom on X-rays, we have seen that there are two processes by which x-rays are produced in the anode of an x-ray tube.
  • In one process, the deceleration of electrons produces x-rays, and these x-rays are called Bremsstrahlung, or braking radiation.
  • The x-ray spectrum in is typical of what is produced by an x-ray tube, showing a broad curve of Bremsstrahlung radiation with characteristic x-ray peaks on it.
  • This process is known as x-ray crystallography because of the information it can yield about crystal structure.
  • These can be studied using x-ray crystallography.

• X-Ray Diffraction Analysis

  • Since many materials can form crystals—such as salts, metals, minerals, semiconductors, as well as various inorganic, organic, and biological molecules—X-ray crystallography has been fundamental in the development of many scientific fields.
  • X-ray diffraction is caused by constructive interference of x-ray waves that reflect off internal crystal planes.
  • A thin film or layer of powder is fixed in the path of monochromatic x-rays.
  • A detector measures x-rays from the sample over a range of angles.
  • X-ray diffraction analysis workflow.

• X-Rays

  • X-rays with photon energies above 5 to 10 keV (below 0.2-0.1 nm wavelength), are called hard X-rays, while those with lower energy are called soft X-rays.
  • Since the wavelength of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by X-ray crystallography.
  • By contrast, soft X-rays are easily absorbed in air and the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer.
  • The distinction between X-rays and gamma rays is somewhat arbitrary.
  • Different applications use different parts of the X-ray spectrum.

• Diffraction Gratings: X-Ray, Grating, Reflection

  • X-ray crystallography is a method of determining the atomic and molecular structure of a crystal, in which the crystalline atoms cause a beam of X-rays to diffract into many specific directions.
  • In an X-ray diffraction measurement, a crystal is mounted on a goniometer and gradually rotated while being bombarded with X-rays, producing a diffraction pattern of regularly spaced spots known as reflections (see ).
  • Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal.

• Discovery of DNA

  • Pauling had discovered the secondary structure of proteins using X-ray crystallography.
  • In Wilkins’ lab, researcher Rosalind Franklin used X-ray diffraction methods to understand the structure of DNA.
  • Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin's data, because Crick had also studied X-ray diffraction.
  • Scientist Rosalind Franklin discovered the X-ray diffraction pattern of DNA (pictured at right), which helped to elucidate its double helix structure.

• X-Rays

  • X-radiation (composed of x-rays) is a form of electromagnetic radiation.
  • X-rays can be generated by an x-ray tube, a vacuum tube that uses high voltage to accelerate the electrons released by a hot cathode to a high velocity.
  • These x-rays have a continuous spectrum.
  • The intensity of the x-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the x-ray tube.
  • Its unique features are x-ray outputs many orders of magnitude greater than those of x-ray tubes, wide x-ray spectra, excellent collimation, and linear polarization.