4-Amino-TEMPO, free radical

4-Amino-TEMPO, free radical, or 4-amino-2,2,6,6-tetramethyl-piperidinyloxyl, is a radical species used as a spin label probe for cellular study. 4-amino-TEMPO is derivitized onto more complex molecular structures, particularly selected to provide target specificity to the probe. Once the 4-amino-TEMPO-containing molecule has reached its target, electron paramagnetic resonance spectroscopy can be used to gather information about the environment in which the radical electron has been approximated. An ATP analog derivitized with 4-Amino-TEMPO is described to bind to actin filaments, generating rotational information of the binding cleft, and providing distinct spectral signatures for different conformations of the filament structure encountered by the bound probe.