Chapter 2 Electron spin resonance

Abstract

This chapter provides an overview of electron spin resonance (ESR). From the point of view of the biochemist, there are three major classes of samples that might be investigated with ESR spectroscopy: (1) samples containing transition or lanthanide elements; (2) samples containing free radicals; (3) samples that have been extrinsically labeled by the introduction of nitroxide radical spin labels or spin probes. The chapter also reviews selective development of some of the key aspects of ESR instrumentation. Emphasis is placed on the microwave bridge and microwave resonant structure. The two double resonance techniques in ESR spectroscopy are electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR). In ENDOR, two radio frequencies are incident on the sample, one the ESR frequency, which is at a saturating level, the other at the frequency of nuclei, which are coupled to the electron. An ENDOR display is of ESR signal height versus nuclear radio frequency. When nuclear resonance is induced, the ESR signals height changes. Thus, ENDOR uses ESR to detect nuclear magnetic resonance (NMR). Its primary utility is in improving resolution.