Pulsed electron–electron double resonance: beyond nanometre distance measurements on biomacromolecules

Pulsed Electron-Electron Double Resonance (PELDOR or DEER) is an Electron Paramagnetic Resonance (EPR) method that measures via the dipolar electron-electron coupling distances in the nanometer range, currently 1.5 to 8 nm, with high precision and reliability. Depending on the quality of the data, the error can be as small as 1 Å. Beyond mere mean distances, PELDOR yields distance distributions which provide access to conformational distributions and dynamics. The PELDOR modulation depth can be used to count the number of monomers in a complex and high-field/high-frequency PELDOR or the use of rigid labels allows determining the orientations of these labels with respect to each other. If in addition to the dipolar through space coupling a through bond mechanism contributes to the overall coupling both can be easily separated and quantified. This method has been applied to a large variety of nucleic acids as well as proteins and protein complexes either soluble or within membranes. As spin centres small nitroxide labels, paramagnetic metal ions and clusters or intrinsic amino acid or cofactor radicals have been used.