Distance Measurements between Paramagnetic Centers by High-Field EPR

The resonance frequency of an electron is highly sensitive to the local magnetic field at the electron and can therefore be used as a probe of the electron's direct surroundings. Electron Paramagnetic Resonance (EPR or ESR) is a method by which this canbe studied. The presence of a nearby electron B may cause faster relaxation of electron A, which depends on the distance between the two electrons. Performing EPR relaxation measurements at highmagnetic fields (6.4 Tesla) provides the spectral resolution from which one can extract information about the distance between and orientation of electrons A and B with respect to each other. High-field EPR relaxation measurements have been performed on the electron-transfer proteins cytochrome c and cytochrome c oxidase. These proteins form a short-lived complex in which an electron is transferred from cytochrome c to its partner. The goal was todetermine the structure of this transient complex. Based onextensive analysis and simulation of the data from conventional as well as high-field EPR it is proposed in this work that the two proteins do not build a single, well-defined complex, but rather form unspecific, short-lived complexes.