Electron paramagnetic resonance (EPR) spectroscopy is now part of the armory available to probe the structural aspects of proteins, nucleic acids and protein–nucleic acid complexes. Since the mobility of a spin label covalently attached to a macromolecule is influenced by its microenvironment, analysis of the EPR spectra of site-specifically incorporated spin labels (probes) provides a powerful tool for investigating structure–function correlates in biological macromolecules. This technique has become readily amenable to address various problems in biology in large measure due to the advent of techniques like site-directed mutagenesis, which enables site-specific substitution of cysteine residues in proteins, and the commercial availability of thiol-specific spin-labeling reagents (Figure 1)1. In addition to the underlying principle and the experimental strategy, several recent applications are discussed in this review.
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