CRYSTALLIZATION

From Big Medical Encyclopedia

CRYSTALLIZATION — process of education and crystal growth upon transition of substance from liquid or gaseous state in strong crystalline state. With the phenomenon To. many are connected biol, processes — patol, adjournment of salts, education uric and gallstones (see. Gallstones , Urinary stones ), processes of ossification (see. Bone ), etc. To. is one of methods of purification of biopolymers (proteins, nucleinic to - t, etc.). Crystalline state of enzyme testifies to high extent of purification of fermental drug (fig. 1 and 2). The phenomena To. play a large role in the equipment, chemical - pharm., chemical and food industry.

Fig. 1. Crystals of ribonuclease in alcohol (X 190)
Fig. 2. Crystals of gamma chymotrypsin (X 15,5)

To. substances comes from fusions at their cooling. To. substances from solutions (see) occurs in that case when solution becomes peresyshchenny in relation to this substance. Supersaturation can be caused cooling of solution, evaporation of solvent, addition in solution of the substances reducing solubility of the crystallizing substance. Sometimes molecules of solvent are a part of the crystals which are allocated from solution, forming kristallosolvata. If at the same time substance crystallizes from water solution, then are formed crystalline hydrates (see). Patterns To. speak a basis of the thermodynamic theory of phase transformations (see. Thermodynamics ). Saturated solutions are in a stable equilibrium state. Unlike them, peresyshchenny solutions represent unstable, nonequilibrium systems. Transition from a nonequilibrium state to equilibrium is followed To. Conditions of formation of peresyshchenny solutions and their property in many respects define K. Stepen's course, supersaturations can be characterized by any of three sizes: absolute supersaturation (α) — α = With — With 0 , relative supersaturation (β) — β = (With — With 0 )/С 0 or coefficient of supersaturation (γ) — γ = With/with 0 , where With — concentration of solute at present, and With 0 — its solubility, i.e. concentration in saturated solution.

Process To. — it is process of formation of a new phase. The smallest parts of a new phase are called germs, or the centers K. As such centers also motes, parts of glass, etc., capable to occlude the crystallizing substance on the surface can serve.

To. it is widely used as a method of purification of solid matters of impurity (so-called recrystallization). This method is based on different solubility of solid matters in certain solvents. For purification of substance with way To. it is dissolved in suitable solvent (often with heating), sometimes add active coal for adsorption of impurity, filter hot solution, the filtrate is cooled and separate the dropped-out crystals of the necessary substance from solution, in Krom there are impurity. Solid hydrocarbons and other unpolar connections usually crystallize from their solutions in petroleum ether, benzene, toluene; polar compounds — from water or other polar solvents, napr, ethanol, ice acetic to - you, etc. If solubility of substances in one solvents is very small, and in others is big, then apply mix of several solvents which can mix up in any relations. Often apply the following mixes: alcohol — ice acetic to - that is water; ether — acetone — benzene — petroleum ether; pyridine — water — alcohol.

In 50 — the 70th of 20 century great success in was achieved To. biopolymers — proteins and nucleinic to - since also recrystallization of proteins were considered as the most effective methods of their cleaning. Development of highly selective methods of preparative chemistry of proteins reduced further value K. as method of purification of proteins and enzymes, however method K. biopolymers kept the value during the receiving rather large crystals suitable for X-ray diffraction researches (see. X-ray crystallographic analysis ). Such crystals shall have the sizes not less than 0,3 mm in all measurements. Receiving crystals of proteins of such size often is a complex challenge. Crystals of protein differ in the high content of water (to 50% on volume) and big fragility. Most often apply the following methods K. proteins: a) To. in test tubes when concentration of solution of protein slowly increases by evaporation of solvent through a capillary or as a result of slow diffusion of solvent in the organic liquid layered on solution of protein; b) To. by means of equilibrium dialysis (see) against solutions of the substances reducing solubility of protein (To. carry usually out in micropermeability cells which one end is closed by a semipermeable membrane, or in capillaries for X-ray diffraction researches); c) To. by diffusion of solvent through a gas phase, for this purpose drops of proteinaceous solution place on the glass which is in the closed camera containing organic solvents or salt solutions.

See also Crystals .



Bibliography: V. D smiths. Crystals and crystallization, M., 1954; Melik-Adamyan V. R. and Zhilyaeva T. I. Crystallization of proteins and transport RNA, in book: Molek. biol., under the editorship of M. V. Volken-stein, t. 2, page 7, M., 1973, bibliogr.; Boorish E. V. Crystallization from solutions, L., 1967.


O. V. Kazakova.

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