Is Cocaine Protonated When it Binds to the Dopamine Transporter?

There has been much controversy about whether the well-known alkaloid and tertiary amine base cocaine (pK_a = 8.5) binds to the human dopamine transporter (DAT) in its protonated form. Most potent DAT inhibitors are also strong amines─yet there are some noteworthy examples where neutral cocaine analogues have high affinity, while the quaternary ammonium analog of cocaine, cocaine methiodide, is a comparatively poor inhibitor. In this paper, we show that a fluorescent cocaine analog, with a lower pK_a than cocaine, becomes protonated in the DAT binding site and conclude that similar behavior must be expected from cocaine. By determining the pK_a of the aspartate residue in DAT believed to interact with the amine of cocaine, we are able to explain the apparently contradictory structure–activity data of cocaine analogues.