Streptavidin is a 52,800 dalton tetrameric protein purified from the bacterium Streptomyces avidinii. It has an extraordinarily high affinity for biotin, the dissociation constant (kDa) of the biotin-streptavidin complex is on the order of ~10-14 mol/L, making it one of the strongest non-covalent interactions known in nature. It is used extensively in molecular biology and bionanotechnology as, in addition to the high affinity, biotin-binding is resistant to extremes of pH, temperature, organic solvents, denaturants (e.g. guanidinium chloride), detergents (e.g. SDS, Triton) and proteolytic enzymes. .