The dipolar coupling between two paramagnetic centres depends upon their spin-spin distance and relative orientation with respect to the spectrometer field vector B0.1 Generally most double electron-electron resonance (DEER) spectroscopy experiments (or synonymously pulsed electron double resonance (PELDOR)) are carried out under conditions where the signal only reports on the spin-spin distance and for this type of dipolar data sophisticated analysis method for obtaining the distance distribution have been developed. Recently there has been an increasing number of studies on systems where the DEER response depends upon both distance and spin-spin orientation. In our case we have been interested in employing DEER spectroscopy to gain structural information with biological system containing iron-sulfur clusters, a spin which provides good orientation selectively in DEER experiments.
This contribution will present the salient points to consider when analysing data from orientation selective DEER experiments, and discuss two applications: a) assignment of the redox potentials and corresponding paramagnetic iron-sulfur clusters comprising the molecular wire of mitochondrial complex I to a structural model using DEER spectroscopy,2 and b) determining the 3D structure of an electron-transfer protein-protein complex in a P450 system using DEER measurements between paramagnetic iron-sulfur clusters and MTLS spin labels.3