Measuring the absorption of electromagnetic radiation
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Electron Paramagnetic Resonance (EPR) measures the absorption of electromagnetic radiation by a paramagnetic system placed in a static magnetic field. Standard applications of EPR include characterization of free radicals, studies of organic reactions involving radicals/paramagnetic metals, and investigations of the electronic and structural properties of paramagnetic centers.
The investigation of biological metal centers by EPR spectroscopy began about 50 years ago when low-temperature experiments revealed their presence in biological systems. The field of application of EPR spectroscopy has been considerably enlarged thanks to the development of new techniques such as pulsed EPR and, more recently, high-field EPR.
Nowadays in complex biological systems with stable or transient paramagnetic centers, which can be metal ions or clusters, spin labels, amino acid radicals, or organic cofactor radicals, EPR is used to study the arrangement of cofactors and subunits, the formation of secondary structure elements, or the interactions between biomolecules. The information obtained is used in the determination of molecular structure. The accuracy of this structural information often exceeds that of other methods.
In many cases, EPR spectroscopic data provide the only structural information, in particular when high-resolution crystals are not available and systems are too large for high-resolution NMR spectroscopy. EPR data can complement the information gained by other structural methodologies, and has proved to be an essential technique for interdisciplinary investigations of biological systems.