POLARIMETRY

From Big Medical Encyclopedia

POLARIMETRY (late lat. polaris polar + grech, metreo to measure, measure) — set of the physicochemical methods of a research based on definition of an angle of rotation of the plane of polarization of the polarized light passing through the optically active environment.

The item finds broad application in a dignity. - a gigabyte., clinical and fiziol, researches. Determine by methods P. existence and concentration of carbohydrates in vegetable raw materials, proteins and amino acids in solutions; by these methods investigate activity of the enzymes splitting carbohydrates, etc. The Item

is the cornerstone passing of the polarized ray of light through the optically active environment (e.g., through any biol, test, the studied solution). At the same time the plane of polarization of a beam (see. Polarization of light ), passed through the studied substance, it is turned on some corner (an angle of rotation of the plane of polarization). In size of this corner, and also in the direction of shift (sign of rotation) of the plane of polarization identify optically active substance and define its concentration.

For quantitative assessment of an angle of rotation of the plane of polarization are used the specific rotation [α] which is expressed a formula:

[α] = α/lc,

where α — an angle of rotation of the plane of polarization (in degrees); l — length ditches in dm; with — concentration of optically active substance in g/ml, and also molecular rotation:

[M] = αM/100,

where M — the molecular weight (weight) of optically active substance. For the characteristic of optical properties of high-molecular substances (e.g., for proteins or nucleinic to - the t) uses also definition of «rotation on a monomer unit or the rest» — m or R (t. e. per the molecular weight of a monomer unit or the rest). Specific and molecular rotation — are specific to each optically active connection (depend on structure of substance, temperature, type of solvent, etc.).

The angle of rotation of the plane of polarization depends on the wavelength of a transmitted light; such dependence carries the name of optical rotatory dispersion. It is shown that each optically active chromophore of a molecule in the field of the strip of absorption is characterized by specific, so-called abnormal optical rotatory dispersion. Out of area of a strip of absorption optical rotatory dispersion carries the name of normal. In the presence of several optically active transitions normal dispersion is defined by the sum of deposits from all chromophores. Optical rotatory dispersion is studied by means of devices — spektropolyarimetr; the method of studying is called a spektropolyarimetriya.

Along with rotation of the plane of polarization of incident light any optically active substance differently absorbs the components of light polarized around to the left and to the right i.e. possesses a circular dichroism. Its measure is size ΔЕ, calculated on a formula:

ΔЕ = E L — E R ,

where E L and E R — molecular absorbtion coefficients for two a component of the polarized light. Size ΔЕ is connected with a difference of optical density for two a component of light ΔD = D L — D R ratio:

ΔЕ = ΔD/cl

where with — concentration of substance in mol/l, l — length ditches in see Values ΔE change with the wavelength of light and can be both positive, and negative. This size has great values only in narrow area of frequencies near a maximum of absorption therefore the range of a circular dichroism of the molecules having several hromoformny groups has bigger resolving power, than ranges of optical rotatory dispersion.

Ranges of a circular dichroism study by means of the devices called by dikhrograf.

Methods of measurement of optical rotatory dispersion and a circular dichroism are widely applied to studying of structure of many biologically important connections, in particular secondary and tertiary molecular compositions of proteins, conformational changes of proteins in solutions at change of conditions of the environment or at their interaction with other molecules. These methods study structure of enzymes, peptide hormones, membrane proteins, various proteinaceous complexes (e.g., antigen — an antibody), difficult complexes (e.g., chromatin, ribosomes, viruses and phages), processes of nucleinic and proteinaceous recognition etc.

Fig. 1. Scheme of the simple polarimeter: 1 — a source of optical radiation 2 — the light filter, or the monochromator, 3 — a lens, 4 — a polarizer, 5, 7, 11 — diaphragms 6 — the inherited solution; 8 — the analyzer, 9 — the goniometric device, 10 — a lens 12 — an eyepiece, 13 — outlet opening; I \the polyariziruyushchy device, II - the analyzing device, III — the telescope.

Polarimeters — devices for measurement of angles of rotation of the plane of polarization. Any device for P. contains a source of the polarized radiation (a light source and a polarizer) and the device for the analysis of this radiation which passed through the studied device (analyzer). In the elementary polarimeters at measurement by turn of the analyzer try to obtain full blackout of a field of vision. In such polarimeters (fig. 1) the flow of radiation leaving a source passes through the light filter, or the monochromator, via the polyariziruyushchy device, the studied sample, the analyzer which is rigidly connected with the goniometric device and gets to the telescope. The limb of the goniometric device is located perpendicularly to an axis of rotation of the analyzer. For the measurement of an angle of rotation performed by optically active substance, the analyzer is twice installed on the identical brightness of a field of vision and carry out counting on the goniometric device twice — without the studied substance between a polarizer and the N0 analyzer and with the studied substance (N1). The required angle of rotation φ is equal to a difference of results of two counting φ = to N1 — N0.

Fig. 2. Scheme of a saccharimeter: 1 — a source of optical radiation, 2 — a lens, 3 - a half-shade polarizer, 4 — solution of sugar, 5 — a plate, 6 — a motionless wedge, 7 — a mobile wedge, 8 — the otschetny device, 9 — the analyzer, 10 — the telescope.

For increase in accuracy of measurements polarimeters supply with half-shade devices — the polarizers (analyzers) of a special design providing small shift of the planes of polarization under review and dividing it into two or three fields of comparison. At the same time sensitivity of the device increases. The polarimeters intended for definition of a sugar content in solutions are called saccharimeters, and process of definition of concentration of sugar in solutions — a sakharometriya. Radiation from a light source (fig. 2) passes through the focusing lens and a half-shade polarizer. The analyzer is rigidly installed on half-shade equality in zero situation. For compensation of the rotation caused by solution of sugar the compensator in the form of the plane-parallel plate of crystalline quartz of the variable thickness established in front of the analyzer and consisting of a plane-parallel plate dextrorotatory and wedges of left-handed quartz serves. The angle of rotation is counted by means of a linear scale and the nonius, otgraduirovanny in terms of the International sugar scale. Half-shade equality is established at observation in the telescope.

Fig. 3. Scheme of the photo-electric polarimeter: 1 — a light source, 2 — a polarizer, 3 — the modulator, 4 — the studied solution, 5 — the analyzer, 6 — the goniometric device, 7 — a photodetector, 8 — the amplifier, 9 — a phase-sensitive null indicator.

Use of photo-electric receivers of radiation in polarimeters allows to increase considerably their sensitivity, to refuse the darkened rooms, to increase the speed of performance of measurements, to take measurements in ultra-violet and infrared spectral ranges, to investigate substances with the big optical plane. In a habit view the scheme of one of photo-electric polarimeters is provided on fig. 3. The flow of radiation from a light source passes a polarizer, the modulator, the studied sample, the analyzer with the goniometric device in it and gets on the photo-electric receiver, and then in the resonant amplifier who is adjusted on the frequency of modulation, and a phase-sensitive null indicator. At corners of a skreshcheniye, excellent from 90 °, the light flux will have f (frequency of the alternating current feeding the modulator) making modulations at a frequency, the phase a cut depends on the direction of turn of the plane of polarization by the studied substance. Errors of measurements by photo-electric polarimeters lie within 0,02 ° for devices with measuring ranges about several degrees.



Bibliography: Vellyuz L., Legrand M. and Groshan M. An optical circular dichroism, the lane with English, M., 1967; Volkova E. A. Polarizing measurements, M., 1974; Wolkenstein M. W. Molecular biophysics, page 315, M., 1975; Optical rotatory dispersion and a circular dichroism in organic chemistry, under the editorship of G. Snatts-ke, the lane with English, M., 1970; Kudryavtsev V. I. Automatic saccharimeter, Sugar prom-st, No. 11, page 14, 1953; The cell nucleus, ed. by H. Busch, v. 5’ p. 55, 99, N. Y., 1978; J i r g e n s o n s B.’ Optical activity of proteins and other mac-r o m about 1 ec u 1 e s, B. — N. Y., 1973.


I. A. Bolotina; M. Ya. Kaabak (tekhn.).

Яндекс.Метрика