LETTER

Distance measurement between two flexible sites in proteins in high viscosity medium at physiological temperature using continuous wave EPR Dear Editor, Accurate measurement of distances within and between biological macromolecules is important for structure-function studies, for investigating conformational changes and dynamics, and for improving restraints for molecular dynamics and modeling programs (Hellmich and Glaubitz, 2009). Several methods have been developed to measure the distances between atoms, residues, and domains in single proteins and larger complexes. Fluorescence reso-nance energy transfer (FRET) is currently the most fre-quently used method to derive distances in cells or protei