ABSTRACT

Electron spin resonance (ESR) spectroscopy has been widely used as a sensitive tool for the detection and characterization of paramagnetic species generated during many chemical and biochemical reactions. With the exception of transition metals, such species are rarely observed to occur naturally. Chemical, electrochemical, and enzymatic reduction and/or oxidation of compounds of interest have been extensively utilized in the production of paramagnetic species from their nonparamagnetic precursors in biological systems. Additionally, two other techniques have been developed for the study of many biological phenomenon and reactions by ESR spectroscopy. These techniques involve the attachment of a stable free radical, usually called spin labels or spin probes (for example, the nitroxide radical), to a normally nonparamagnetic species; or the trapping of an unstable (short-lived) free radical using a spin trapping reagent. The spin labeling technique facilitates the study of molecular motion and local fluidity and local dielectric and electromagnetic environments. In spin trapping, a nonparamagnetic molecule (spin trap) reacts rapidly with a labile, free radical intermediate to form a stable paramagnetic adduct. The ESR spectrum of the resulting paramagnetic species (adduct) is then characterized by measurement of the hyperfine splitting constants and g factor. Excellent reviews on the theory and application of ESR spectroscopy in the biochemical field are Spin Labeling by L. J. Berliner, Biological Applications of Electron Spin Resonance Spectroscopy by H. Swartz, R. J. Bolton, and D. Borg, and Free Radicals by W. Pryor.