An alternative approach is to slow down reactions by studying samples held at cryogenic temperatures, such as 77 K (liquid nitrogen). This can be a particularly severe problem in studying reactions in liquids.
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Spin-labeled fatty acids have been extensively used to study dynamic organisation of lipids in biological membranes, lipid-protein interactions and temperature of transition of gel to liquid crystalline phases.ĮPR/ESR spectroscopy can be applied only to systems in which the balance between radical decay and radical formation keeps the free radicals concentration above the detection limit of the spectrometer used. Specially-designed nonreactive radical molecules can attach to specific sites in a biological cell, and EPR spectra can then give information on the environment of these so-called spin labels or spin probes. These reagents are particularly useful in biological systems. Although radicals are very reactive, and so do not normally occur in high concentrations in biology, special reagents have been developed to spin-label molecules of interest. Medical and biological applications of EPR also exist. It can be applied to a wide range of materials such as carbonates, sulfates, phosphates, silica or other silicates. EPR/ESR spectroscopy is also used in geology and archaeology as a dating tool. In many cases, the reactions to make the radicals and the subsequent reactions of the radicals are of interest, while in other cases EPR is used to provide information on a radical's geometry and the orbital of the unpaired electron. Organic and inorganic radicals can be detected in electrochemical systems and in materials exposed to UV light.
Such radicals can be identified and studied by EPR. For example, when ice (solid H2O) is decomposed by exposure to high-energy radiation, radicals such as H, OH, and HO2 are produced. EPR is a sensitive, specific method for studying both radicals formed in chemical reactions and the reactions themselves. EPR/ESR spectroscopy is used in various branches of science, such as biology, chemistry and physics, for the detection and identification of free radicals and paramagnetic centers such as F-centers.
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