ELECTRONIC PARAMAGNETIC RESONANCE (EPR) — resonant absorption of electromagnetic waves the substances containing paramagnetic particles. The methods based on EPR found broad application in a lab. to practice. With their help study kinetics of chemical and biochemical changes (see Kinetics of biological processes. The kinetics chemical), a role of free radicals in processes of life activity of an organism is normal also at pathology (see Radicals free), origins and courses of photobiological processes (see Photobiology) etc.
The phenomenon of EPR was opened by the Soviet scientific B. K. Zavoysky in 1944. The electronic paramagnetic resonance is characteristic only of the paramagnetic particles i.e. particles capable to be magnetized at the annex to them of magnetic field) with uncompensated electronic magnetic moment, to-ry, in turn, is caused by own mechanical moment of an electron — spin. Special type of internal movement is inherent in electrons, a cut it is possible to compare to rotation of a top round its pivot-center. The related moment of number of the movement is called spin. Thanks to spin the electron has the constant magnetic moment directed opposite to spin. In the majority of molecules electrons are located on orbitals thus, chgo their spins are directed opposite, magnetic moments are compensated, and the signal of EPR from them does not manage to be observed. If magnetic field of an electron is not compensated by spin of other electron (i.e. the molecule contains not coupled electrons), then the signal of EPR is registered. Particles with not coupled electrons are free radicals, ions of many metals (iron, copper, manganese, cobalt, nickel, etc.), a number of free atoms (hydrogen, nitrogen, alkali metals, etc.).
For lack of external magnetic field the direction (orientation) of magnetic moment of an electron in space can be any; energy of such electron does not depend on orientation of its magnetic moment. According to laws of quantum mechanics in external magnetic field orientation of magnetic moment of an electron cannot be any — it can be directed or in the direction of magnetic field, or is opposite to it.
According to orientation of magnetic moment of an electron its energy in magnetic field can also accept only two values: the minimum E1 — at orientation of magnetic moment «across the field» and the maximum E2 — at its orientation «against the field» and a difference in energy of these states (the delta E) is calculated on a formula: ΔЕ = gβH, where β — Bohr's magneton (unit of magnetic moment of an electron), H — a magnetic field strength, g — the constant depending on electronic structure of a paramagnetic particle. If on system of not coupled electrons in external magnetic field to work electromagnetic radiation, energy of quantum to-rogo is equal ΔE, then under the influence of radiation electrons will begin to pass from a state with smaller energy into a state with greater energy that will be followed by absorption of radiation substance.
EPR carry to methods of radiospectroscopy as radiations in the radio-frequency range of electromagnetic waves are applied to observation of an electronic paramagnetic resonance.
EPR is registered by means of special devices — radio spectrometers. Are their part: the electromagnet, a source of radio-frequency radiation, a transmission line of radiation from a source to a sample (wave guide), the resonator, in Krom is the studied sample, systems of detecting, strengthening and registration of a signal. Radio spectrometers are most widespread, in to-rykh electromagnetic radiations with lengths of waves of 3,2 cm or 8 mm are used.
Registration of a signal of EPR is made as follows. The magnetic field strength, created by an electromagnet, linearly changes in the la defined to a preda. At the values of tension answering to a condition of a resonance, the sample absorbs energy of electromagnetic radiation. The line of absorption (signal of EPR) represents dependence of power of the radiation absorbed by a sample on a magnetic field strength. In the existing radio spectrometers the signal of EPR is registered in the form of the first derivative line of absorption.
For the description and the analysis of ranges of EPR a number of the parameters characterizing intensity of lines, their width, a form, and also situation in magnetic field is used. Intensity of the EPR lines is with other things being equal proportional to concentration of paramagnetic particles that allows to carry out quantification.
By consideration of the phenomenon of EPR it is necessary to consider that magnetic moment of not coupled electron interacts not only with magnetic field of an electromagnet, but also with the magnetic fields created by an environment of an electron: other not coupled electrons, magnetic kernels (see Nuclear magnetic resonance). Interaction of not coupled electrons with kernels often leads to splitting of a range of EPR to a number of lines. The analysis of such ranges allows to identify the nature of paramagnetic particles, to estimate character and extent of their interaction with each other.
Participation of paramagnetic particles in chemical reactions, the molecular movement and other kinetic effects also influence a form of a range of EPR. Therefore EPR use for detection, assessment of quantity and identification of paramagnetic particles, a research of kinetics of chemical and biochemical changes and molecular dynamics.
Thanks to the universality EPR finds broad application in various fields of science. Use of EPR in biology and medicine is caused by existence in cells, fabrics and biol. liquids of various paramagnetic centers by nature. By means of EPR presence of free radicals practically in all animals and vegetable fabrics was revealed. Such connections as the flavins, coenzyme Q and other substances which are carrying out a role of carriers of electrons In reactions of energy balance in vegetable and a zooblast are a source of free radicals; the paramagnetic centers found in the isolated fabrics belong generally an electron - to transport chains of mitochondrions, microsomes, chlorolayers (see Breath). It is revealed that keeping of free radicals in fabrics correlates with their metabolic activity. In numerous works change of quantity of free radicals at various patol is shown. states, e.g. at a carcinogenesis (see), development of beam damages (see), toxicosis (see Intoxication) that is explained by disturbance of power metabolism at pathology (see Bio-energetics).
In tissues of animals and plants define paramagnetic ions by EPR (iron, copper, manganese, cobalt etc.), to-rye are a part of the metalloproteins participating in reactions of electron transfer on elektrontransportny chains and an enzyme catalysis and also in kislorodperenosyashchy pigments (hemoglobin). By means of EPR it is possible to investigate oxidation-reduction transformations of ions of metals and the nature of interaction of ions with their environment that allows to establish fine structure of metallic complexes.
Patol. changes of fabrics lead to changes of signals of EPR of metalloproteins that connect with disintegration of paramagnetic complexes of metals, change of an environment of paramagnetic ions, transition of ions to other complexes. However the research of the nature of the paramagnetic centers of fabrics, especially free radicals, is connected with certain difficulties because of complexity of interpretation of ranges of EPR.
By means of EPR it was possible to investigate mechanisms of enzymatic reactions (see Enzymes). In particular, it is possible to study at the same time as kinetics of education and an expenditure of free radicals during enzymatic reactions, and kinetics of oxidation-reduction transformations of the metals which are a part of enzymes that allows to establish the sequence of stages of enzymatic reaction.
Use of EPR at a research of radiation injury in biol. objects allows to obtain information on the nature of the radicals which are formed in biopolymers, on mechanisms and kinetics of the radical reactions developing in the irradiated objects and bringing to biol. to effect. The EPR method can be applied in emergency dosimetry, e.g. at accidental radiation of people to assessment of an exposure dose, using for this purpose objects from the radiation zone.
The important place is taken by EPR in a research of the photobiological processes proceeding with participation of free radicals (see the Molecule, Radicals free, Photobiology, Fotosensibilization). By means of EPR in detail study processes of formation of free radicals in proteins, nucleic acids and their components at action of ultraviolet radiation, a role of these radicals in photodestruction of biopolymers (see Light). Use of EPR gave important information on primary mechanisms of photosynthesis (see). It is shown that priming reaction of photosynthesis is transfer of an electron from the molecule of a chlorophyll excited by light and education a radical cation of a chlorophyll. Also the nature of the molecules accepting the electron given by the excited molecule of a chlorophyll is identified.
EPR is applied as well to a research of structure of biologically important macromolecules and biomembranes. E.g., the ions of iron which are a part gem in gemsoderzhashchy proteins can be in a high-spin state (electrons in external orbits are not coupled, total spin is maximum) and low-spin (external electrons are in whole or in part coupled, spin is minimum). Researches of features of signals of EPR of high-spin and low-spin conditions of ions of iron in hemoglobin and its derivatives promoted understanding of spatial structure of a molecule of hemoglobin.
Considerable progress in studying of structure of biomembranes and biopolymers was reached after emergence of methods of spin probes and tags (see Membranes biological). As spin tags and probes stable nitroksilny radicals are generally used (see Radicals free). The Nitroksilny radical can be covalently connected with molecules (a spin tag) or to keep in the studied system due to physical interactions (spin the probe). The essence is that the form of a range of EPR of nitroksilny radicals depends on properties of a microenvironment: viscosity, character and molecular movement, local magnetic fields, etc. Spin tag, covalently connected with various groups of biopolymers, are the indicator of a condition of structure of biopolymer. By means of spin tags spatial structure of biopolymers, structural changes of proteins at a denaturation, formation of complexes are investigated enzyme — substrate, antigen — an antibody, etc.
By means of a method of spin probes ways of packaging and mobility of lipids in biomembranes, a lipid - the proteinaceous interactions, structural transitions in membranes caused by effect of various substances, etc. are studied. On the basis of a research of spin tags and probes methods of definition of pharmaceuticals in biol are offered. liquids, and also questions of the directed transport of pharmaceuticals etc.
Thus are investigated, by means of EPR the wide spread occurance of electronic processes in an organism normal is shown and during the developing of any pathology. Creation of the theory and improvement of the equipment of the EPR method formed the basis of quantum electronics as division of science, led to creation of molecular generators and amplifiers of radio waves (masers) and light — the lasers (see) which found broad application in many fields of the national economy.
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K. N. Timofeev.