Crystal structure analysis
Crystal structure analysis is the determination of the atomic structure of a crystal by diffraction of suitable radiation on the crystal lattice . Monochromatic X-ray radiation is very often used for this , since this can be generated relatively easily as characteristic X-ray radiation from an X-ray tube. The term X-ray structure analysis ( X-ray crystallography ) has become established for this. Alternatively, neutron beams or synchrotron radiation can also be used. The crystal structure analysis with electron beams is particularly difficult due to the strong interaction between the irradiated electrons and the crystal and is not yet fully developed for routine investigations.
The crystal structure can then be calculated from the observed diffraction pattern . The geometry of the unit cell of the crystal lattice can be derived completely from the angles at which the diffraction maxima occur. From the strength of the diffraction maxima, the arrangement of the atoms within the unit cell can be calculated using various mathematical methods. However, the calculations required for this are already so complex for medium-sized molecules (from around 10 non-hydrogen atoms) that they can not be carried out without a computer . The first protein structure was analyzed in 1958 on the Cambridge high-performance computer EDSAC from 1949, while researchers already found out in 1934 that the enzyme pepsin forms regular crystals.
In the case of crystal structure analysis using X-ray, electron or synchrotron radiation, strictly speaking, it is not the positions of the atoms that are determined, but the distribution of the electrons in the unit cell , as these interact with the radiation. So what you actually get is an electron density map, and very precise crystal structure analyzes of molecules with light atoms do indeed reveal binding electrons. Neutrons, on the other hand, interact with the atomic nucleus. However, the difference in position is negligible in most cases. A detailed description of the diffraction effects on crystals and their interpretation can be found in the article X-ray diffraction .
Ideally, the diffraction is carried out on a single crystal . However, this is often not possible because there are not always enough large single crystals of a substance available. Nowadays it is also possible to evaluate the diffraction pattern of crystal powders as part of a crystal structure analysis ( Rietveld method ). However, information is lost due to the superimposition of diffraction maxima that occurs here, so that the results are generally of lower quality.
In addition to the actual crystallographic application of the method, in which the crystal itself is of interest, crystal structure analysis is also used to elucidate molecular structures . Today this is a standard method in chemistry and biochemistry and thus a sub-area of structural biology . However, this requires the molecules to crystallize , which can be very difficult , especially with protein crystals . Crystallization in the presence of substrates can be used to attempt to record various metabolic states of the protein.
A competing method for determining the structure of proteins by means of X-ray crystal structure analysis is NMR spectroscopy , which, however, can currently only be used for proteins of small or medium size.
There have been several Nobel Prizes in the field of crystal structure analysis, starting with Max von Laue and Wilhelm Conrad Röntgen , who laid the foundations, through, for example, Dorothy Crowfoot Hodgkin , who was the first to structurally determine many biologically relevant molecules, to Robert Huber , Johann Deisenhofer and Hartmut Michel , who studied proteins (including the chlorophyll- containing photoreaction center) as protein crystals . One of the best-known examples of structure elucidation using X-ray diffraction is the deciphering of the DNA structure by James Watson and Francis Crick , whose model was largely based on X-ray diffraction data from Maurice Wilkins and Rosalind Franklin . In 1985 Jerome Karle and Herbert A. Hauptman were awarded the Nobel Prize in Chemistry for their contributions to the development of “direct methods” for crystal structure analysis.
Since the expression x-ray analysis is semantically misleading insist on X-ray, some authors beam structure analysis.
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