Distance information for disordered proteins from NMR and ESR measurements using paramagnetic spin labels.

The growing recognition of the many roles that disordered protein states play in biology places an increasing importance on developing approaches to characterize the structural properties of this class of proteins and to clarify the links between these properties and the associated biological functions. Disordered proteins, when isolated in solution, do not adopt a fixed structure, but can and often do contain detectable and significant residual or transient structure, including both secondary and long-range structure. Such residual structure can play a role in nucleating local structural transitions as well as modulating intramolecular or intermolecular tertiary interactions, including those involved in ordered protein aggregation. An increasing array of tools has been recruited to help characterize the structural properties of disordered proteins. While a number of methods can report on residual secondary structure, detecting and quantifying transient long-range structure has proven to be more difficult. This chapter describes the use of paramagnetic spin labeling in combination with paramagnetic relaxation enhancement (PRE) in NMR spectroscopy and pulsed dipolar ESR spectroscopy (PDS) for this purpose.