What Is an Electron Spin Resonance?
Electron paramagnetic resonance (EPR) is a magnetic resonance technology derived from the magnetic moment of unpaired electrons. It can be used to detect qualitatively and quantitatively the unpaired electrons contained in the atoms or molecules of a substance and to explore their Structural characteristics of the surrounding environment. For free radicals, the orbital magnetic moment has almost no effect, and the vast majority (more than 99%) of the total magnetic moment's contribution comes from the electron spin, so the electron paramagnetic resonance is also called "electron spin resonance" (ESR).
- Electron paramagnetic resonance was first developed by former Soviet physicist EK Zavois from MnCl2, CuCl2, etc. in 1944.
- An electron is a basic particle with a certain mass and a negative charge. It can perform two types of motion; one is in an orbit around an atomic nucleus, and the other is a spin on an axis passing through its center. Due to the movement of electrons, moments are generated, and currents and magnetic moments are generated during the movement. In the applied constant magnetic field H, the magnetic moment of the electron acts like a small magnetic rod or needle. Since the spin quantum number of the electron is 1/2, the electron has only two orientations in the external magnetic field: one parallel to H, corresponding to low energy Level, the energy is -1 / 2gH; one is antiparallel to H, corresponding to a high energy level, the energy is + 1 / 2gH, and the energy difference between the two levels is gH. If in the direction perpendicular to H, an electromagnetic wave with a frequency of v is added to satisfy the condition of hv = gH, low-level electrons absorb the energy of the electromagnetic wave and
- There are three types of microwave frequencies commonly used in electronic paramagnetic resonance spectrometers (see table).
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- Among them, the X-wave band is most commonly used. [2]
- The electronic paramagnetic resonance spectrometer is composed of 4 parts: microwave generation and conduction system; resonant cavity system; electromagnet system; modulation and detection system. [1]
- Because the high-frequency field adjustment is usually used to improve the sensitivity of the instrument, what is recorded on the recorder is not the microwave absorption curve (the absorption coefficient X '' has a strong effect on the magnetic field).
- Founded by HM McConnell in the United States in 1965, it refers to a specific combination of a stable free radical (the most commonly used is nitroxide) to a single molecule or a molecule in a more complex system Information about the environment of the markers obtained from the electron paramagnetic resonance spectroscopy. When performing spin labeling, care should be taken to maintain specificity and minimize disturbances to the biological and molecular properties of natural systems. [2]
- Spin markers have 4 advantages: sensitive to the polarity of the solvent, so the hydrophobicity or hydrophilicity of the environment around the marker can be explored; extremely sensitive to the molecular rotation rate, so it can measure the internal environment of the marker Permissible degree of activity, especially measuring the conformational changes of biomolecules caused by a certain biochemical process; EPR spectrum is simple and easy to analyze, and the trimodal spectrum caused by 14N can provide a lot of valuable information; does not exist from the diamagnetic environment Interference signal. [2]
- Spin labels can be attached to the target by covalent bonds or by non-covalent gravitational forces involved in interactions such as enzymes and coenzymes, enzymes and substrates, antibodies and haptens, and membranes and steroids. The spin labeling method has been widely used to study the conformation of biopolymers, the structure of the active site of enzymes, the structure of liposomes and biofilms, and its application in immunoassay. [2]