Especially in proteins, electron transfer often involves hopping of an electron from one redox-active center to another. The hopping pathway, which is viewed as a vector, guides and facilitates ET within an insulating matrix. Typical redox centers are iron-sulfur clusters, e.g. the 4Fe-4S ferredoxins. These site are often separated by 7-10 Å, a distance compatible with fast outer-sphere ET.