SEDIMENTATION

From Big Medical Encyclopedia

SEDIMENTATION (Latin sedimentum sedimentation) — sedimentation of the dispersed particles or the dissolved molecules under the influence of forces of gravitational or centrifugal field. Qualitative or quantification of S. can have diagnostic value, napr, determination of speed of S. of erythrocytes — a so-called erythrocyte dimentation test (see. Sedimentation of erythrocytes ), sedimentation division of serum proteins at a makroglobulinovy reticulosis (see. Valdenstrema disease ), etc. S.'s studying lipoproteids of blood serum is very effective for establishment of quantity of separate classes of these connections and definition thus of lipoproteinemiya (see. Lipoproteids ). Nevertheless S.'s Studying natural compounds is carried out by hl. obr. in the research purposes. It is explained by complexity of the equipment that limits possibilities of wide use of a method C. in a wedge, practice.

In case of dispersions (see. Colloids , Suspensions ) speed of Page (v) is described by Stokes's equation:

v = [2r 2 g (ρ '-ρ)]/9η,

where ρ' both r — density and radius of the dispersed particles respectively, ρ and η — density and viscosity of the environment respectively, g — acceleration of gravity. This equation is fair for the spherical particles with a smooth surface settling with a small speed. Practical use of the equation of Stokes demands introduction of a number of amendments. S.'s speed is estimated on accumulation of a deposit, change of density of suspensions, on change of concentration of particles at the certain level, applying for this purpose methods of weighing, measurement of hydrostatic pressure or density of suspension.

The important characteristic of disperse systems is distribution of the dispersed particles by the sizes that can be established by S.'s studying in time. Such sedimetric analysis is of interest at the description of properties of the dispersions formed by polymers, medicinal substances, etc.

In case of solutions of macromolecules C. it is reached by use ultracentrifuging (see), at Krom acceleration of centrifugal force can exceed acceleration of the gravity (g) in hundreds of thousands of times. At the same time S.'s speed is characterized by coefficient of sedimentation (s):

s = (dx/dt) * (1/ω 2 x) = M (1 - vρ) / f

where x — distance of border of S. from an axis of rotation of a separating bowl, t — S.'s time, ω — the angular speed of rotation of a separating bowl, M — the molecular weight (weight), V — the partial specific volume, f — the frictional coefficient testimonial of degree of asymmetry of the sedimentating particles or macromolecules, ρ — density of solvent, d — the discrete increase in an indicator registered during observation, in this case — the distance passed by the sedimentating particle (dx) and time, for a cut this distance is passed (dt). Determination of size s these coefficients by means of tabular allowances for carry out by means of ultracentrifuging, then density, viscosity, lead temperature to so-called reference conditions, at to-rykh S. would occur in water at 20 ° and at infinite dilution of the sedimentating substance, i.e. at its zero concentration. Thus receive the size of a constant C. (s 20 °, w ). Coefficients and constants C. express in svedberga; one svedberg (1 S) is equal to 1*10 - 13 sec. Nek-ry substances, napr, liioproteida, density to-rykh less than density of solvent, during the ultracentrifuging do not sedimentate, and float, i.e. emerge. In this case the coefficient of flotation is designated by s f  ; it is measured at strictly certain values of density of solvent, napr, for a number of lipoproteids of blood serum at 1,063 g/ml, and to reference conditions (water at 20 °) not led. Constants C. and coefficients of flotation are important physical. - chemical parameters proteins (see), nucleic acids (see), polysaccharides (see) and others biol. polymers; they are used sometimes for designation of such polymers, napr, spoken about 7S-and 19S-immunoglobulins, about fraction of lipoproteids with s f 20 — 400 S.

S.'s studying in ultracentrifuges gives the chance, in addition to determination of sizes s, s 20 °, w and s f , to determine the molecular weight of substance. For this purpose earlier used Svedberg's equation:

M = sRT / [D (1 - Vρ)],

where R — a gas constant, T — Kelvin temperature, D — a diffusion coefficient, V — partial specific volume, ρ — density of solvent. However because of complexity of determination of sizes D Svedberg's equation for establishment of molecular weights in practice is almost not used. Molecular weights usually establish by methods of sedimentation equilibrium, a cut arises at rather small speeds of rotation of a separating bowl when transfer of substance at the expense of S. is counterbalanced with its return transfer due to diffusion, i.e. through any section of a nappe in a cell of the ultracentrifuge of effective transfer of substance does not occur. In that case molecular weight is determined by the equation:

M = RT / [D (1 - Vρ)ω 2 ] * (dc/dx)/cx,

where with — concentration of the studied substance, the same other designations, as in the equations which are stated above.

For preparative works with proteins, nucleinic to-tami, viruses, subcellular components (ribosomes, kernels, mitochondrions), etc. widely use S. in a gradient of concentration of sucrose, and also in a gradient of concentration of D 2 About (oxydeuterium) or glycerin, to-ry it is created for prevention of convection washing out of zones of the sedimentating substances. The gradient of concentration of sucrose (e.g., from 5 to 20%) is created in a test tube of the preparative ultracentrifuge in advance, then solution of the studied substances accumulates on solution of sucrose. After ultracentrifuging the studied substances are located in the form of zones depending on their coefficients of Page. These zones one way or another select and analyze the substances entering them.

For a research of hl. obr. nucleinic to - t and nucleoproteids use S. in the gradient of density created by salts of heavy metals, most often chloride or sulfate caesium and also metrizamidy — the organic matter containing iodine. E.g., during the ultracentrifuging nucleinic to - the t in solution of chloride caesium (concentration to 2 M) occurs S. of chloride caesium and creation of a gradient of its density. In a nek-swarm to a zone of a test tube the work V*ρ = 1 (V — partial specific volume nucleinic to - you, ρ — density of solution of chloride caesium), are equal in this zone of density of the sedimentating particles and solution of salt, t. e. s = 0, therefore, the studied substances will sedimentate from a meniscus and to float from a bottom of a test tube of the centrifuge until get to a zone with V*ρ = 1 where they also remain. The method allows to carry out division of substances according to their partial specific volumes (or to density) therefore he received the name of equilibrium centrifuging in a gradient of density. The size of floating density of macromolecules determined with its help is expressed in g/ml and is the important characteristic nucleinic to - t, proteins and virus particles.



Bibliography: Bowen T. Introduction to ultracentrifuging. the lane with English, M., 1973; Osterman L. A. Methods of a research of proteins and nucleic acids, Electrophoresis and ultracentrifuging, M., 1981; Figurovsky N. A. Sedimetric analysis, M. — JI., 1948; Frayfelder D. Physical biochemistry, the lane with English, page 275, M., 1980; S h e e 1 e of P. Centrifugation in biology and medical science, N. Y., 1981.


V. O. Shpikiter.

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