Abstract
Electron spin resonance (ESR) refers to spectroscopy of unpaired electrons, and is sometimes called electron paramagnetic resonance (EPR) or electron magnetic resonance (EMR). The theoretical bases of ESR spectroscopy are similar to those of nuclear magnetic resonance (NMR), except that an electron spin, rather than a nuclear spin, is the focus. Unpaired electrons in biological systems are in much lower abundance than nuclei, so ESR is a technique that focuses on local sites while NMR is more global. Two electrons are paired, with antiparallel spins, in a single chemical bond. The ESR requirement of an unpaired electron spin is met in biology when (1) a chemical bond is broken homolytically, as in formation of a free-radical enzyme intermediate; (2) there are unfilled valence orbitals, as there are in oxygen, nitric oxide, or many metal ions; and (3) one-electron oxidation or reduction of a nonparamagnetic biomolecule has occurred. Biological subjects for ESR include free-radical enzyme intermediates, metal ions, nitric oxide and some of its complexes, and redox-active cofactors such as quinones and flavins. The range of applications of ESR spectroscopy is not limited to natural sources of unpaired electrons. An ESR probe technique, site-directed spin labeling (SDSL), is widely applied to examine dynamics and folding of biomolecules. Oter paramagnetic probes can be used to image oxygen or nitric oxide in biological tissues in vitro and in vivo. Keywords: cwESR; EPR, PMR, EMR; EPRI; ESR; ENDOR; ESEEM; g-factor; Hyperfine Splitting; Isotropic or Anisotropic; Pulsed ESR; Spin Label; Spin Trap
Published Version
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