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One specific interest in long-range electron transfer (ET) is attributed to their importance in biological reactions involving metalloproteins, such as the electron flow through proteins between metals or other redox cofactors. Proteins can facilitate the process by isolating the metals participating in the electron transfer in hydrophobic spaces within the protein. This shields them from the polar solvent in order to reduce the reorganization energy required. However, when these redox centers are buried within the protein, they no longer establish close-contact complexes, and as a result electrons must tunnel across larger distances within the polypeptide matrix. In addition to their ability to reduce reorganization barriers, proteins can facilitate electron coupling via superexchange coupling. This refers to the joining of the donor to the acceptor via electronic interactions, thus strengthening the donor/acceptor electronic interaction. Therefore, if long-range ET in a protein is facilitated by strong coupling pathways, single-site mutations would predictably generate significant consequences on the enzyme function. This is where the work of Harry B. Gray comes in. Gray looks at ruthenium-modified pseudomonas aeruginosa azurin with six specifically located Ru-modified sites on the β-strands of the protein. As a result, long-range ET between different Ru-modified sites on the protein and azurin (which is a blue copper ET protein) were investigated. The kinetics of long-range ET demonstrates an exponential dependence on the distance between the reactants, where the distance decay constant, β, is the time with which it takes for an ET to occur at a set radius. There comes a point where the rate of transfer no longer depends on coupling strength, but instead sees greater influence by solvent reorientation dynamics. For example, in a vacuum the rate of ET could predictably take 10^17 years, because there is no matter through which the ET can occur. Additionally, in water the electron may interact with this polar solvent too much by spending copious time associating with the water molecules, thus the rate of ET still takes approximately 5x10^4 years. However, ET within a hydrophobic polypeptide is considerably smaller (compared to a vacuum or water medium) with a rate in the millisecond to microsecond range within the hydrophobic interior of a protein.