From Big Medical Encyclopedia

X-RAY CRYSTALLOGRAPHIC ANALYSIS — the method of definition of structure of objects based on the physical phenomenon of dispersion (diffraction) of electromagnetic radiation in the studied substance. By means of R. and. fundamental data on a structure of viruses, various proteins, including many enzymes, about molecular bases of diseases of blood, in particular a sickemia etc. of R. are obtained and. finds the increasing application in medicobiological researches.

The possibility of obtaining information on structure and degree of its detail (so-called permission) is defined by degree of structural orderliness of an object and wavelength of the used radiation (see. Radiation ).

If the disseminating centers are located in the planes divided by distance of d, and the wavelength of the falling x-ray emission of X, then the directions of preferential dispersion will be set by corners of v according to Bragg's condition — Woolf:

nX = 2d sin (v),

where n — an integer. Small wavelength of X-ray (about 0,1 nanometers) corresponds to scales of frequency in crystals of the majority of molecules (units or tens of nanometers). Therefore by means of R. and. are defined as the nature of frequency in crystals (see) and fibers, and structure molecules (see), located from each other with these frequency.

First stage P. and. consists in receiving a diffraction pattern: distributions of scattered beams in space and their intensity.

Fig. 1. Roentgenograms of fibrous protein of collagen: and — a diffraction picture of normal fiber; position of lines reflects geometry of a three-spiral molecule of collagen; — a diffraction picture of the fiber emitted from patholologically to the changed fabric; shooters specified the additional reflexes arising owing to biochemical changes of fabric.

The diffraction pattern is registered or on a film (the roentgenogram, a X-ray analysis), or the counter of ionizing radiation (difraktometriya). In case of fibrous structures (collagen, a muscle, molecule DNA) useful information is concluded also in geometry of the roentgenogram characterizing frequency in the most fibrillar molecule (fig. 1).

Experiments on X-ray diffraction on various biomembranes allowed to receive profiles of their electron density and the sizes of separate elements of membranes (thickness, width of a hydrophobic zone, asymmetry), in particular membranes of a myelin, photoreceptor membranes. X-ray diffraction researches of molecular mechanisms of reduction of muscle fiber are conducted, atomic structures of vitamins, many medicinal substances are received.

Outstanding success was establishment by means of R. and. molecular compositions of nucleic acids and many proteins. The main method of interpretation of structure of macromolecules — an isomorphous replacement method (see. Isomerism ), the essence to-rogo consists in receiving the identical crystals of the same protein differing with introduction to one of provisions of the heavy, i.e. strongly disseminating group. On change of intensivnost of dispersion at the expense of heavy group it is possible to recover an arrangement of atoms in relation to this group and to each other. Calculations require use electronic computer (see).

Fig. 2. Crystallogram of protein: the arrangement of diffraction spots reflects geometry of an arrangement of molecules of protein in a crystal.

The atomic structure more than 100 proteins is established. In fig. 2 the crystallogram of protein is shown. The frequency of small scale (within 0,2 nanometers) reflecting packaging of molecules of protein in supramolecular units are visible. Having model of atomic structure of protein, it is possible to localize an active center, to understand work of enzyme, to find ways of chemical influence for the purpose of change of its functional activity. Data on structure of groups of the proteins close on functions, allow to reveal a difference in a structure and to connect it with functional differences. So, the research of group a myoglobin-gemoglobinovykh of proteins allowed to understand a number of diseases of blood, in particular a disease of a sickemia at molecular level.

Exclusively difficult X-ray diffraction researches of crystals of viruses are conducted that allowed to reveal patterns of laying of polypeptide chains in subunits of a cover. The received results can, apparently, help to find ways of medicamentous impact on viruses, so and to promote an izlechivaniye of many viral diseases.

Bibliography: Blundell T. and Johnson L. A crystallography of protein, the lane with English, M., 1979; Weinstein B. K. Diffraction of rentgenovy beams on chain molecules, M., 1963; And about with t of X. Physiology of a cell, the lane with English, page 19, etc., M., 1975; M and r sh e l of l E. Biophysical chemistry, the lane with English, t. 2, page 749, M., 1981; The Modern crystallography, under the editorship of B. K. Weinstein, etc., t. 1 — 4, M., 1981.

N. G. Yesipova.