Site-directed nitroxide spin labeling of biopolymers

The application of electron paramagnetic resonance (EPR) spectroscopy to the study of biopolymer structure and dynamics has seen rapid growth in the last decade. In addition to advances in instrumentation – in particular, the development of high-field spectrometers and pulsed-EPR methods – spin-labeling techniques have evolved. Nitroxide spin labels can now routinely be incorporated at selected sites to interrogate how structure and dynamics at specific locations relate to biopolymer function. Furthermore, spin labels with improved properties have emerged, in particular, rigid labels that yield more accurate distance measurements, give information about orientation, and faithfully report site-specific dynamics. This review recounts how the three main approaches for site-directed spin labeling of biopolymers, namely, postsynthetic labeling, labeling during biopolymer synthesis, and noncovalent labeling, have been used to label proteins as well as nucleic acids.