Effect of NH₂ rotation on the fluorescence of 2-aminopurine in solution

Since the introduction of 2-aminopurine (2AP) in 1969 as a fluorescent analogue of adenine, its intense fluorescence in aqueous solution and the subsequent reduction of this intensity in DNA has been a powerful tool for studies of structural changes in DNA. Herein, we show that the unusual intense fluorescence of 2AP in water is attributed to the formation of a closed complex between one water molecule and 2AP in the excited state. This configuration restricts the rotation of the 2-NH₂ group which subsequently lowers the nonradiative decay rate. We supported this finding by attaching heavy masses to the amino group, dimethyl (2-(N(Me)₂)) and diethyl (2-(N(Et) ₂)). By examining the fluorescence behavior in dioxane (an apolar, aprotic solvent), the lighter NH₂ group can rotate in the excited state more freely which enhances the nonradiative loss of fluorescence. On the other hand, this rotation slows down sharply in the two heavy-group derivatives, leading to a restoration of the fluorescence intensity and lifetime very close to that of 2AP in water. Depletion of fluorescence was observed in the 2AP derivatives in water and is attributed to the population of a twisted intramolecular charge transfer (TICT) state due to the strong electron donating power of the NR₂ groups, an effect that is absent for the parent 2AP.