Tricyclic aromatic compounds (TCA) are promising candidates for treatment of transmissible spongiform encephalopathies. Direct binding to the cellular prion protein (PrP^C) has been proposed as anti-prion active mechanism. We here show by means of NMR-spectroscopy that binding of TCA occurs with millimolar affinity to motifs consisting of two neighboring aromatic residues (Ar-Ar motif). It is independent of the secondary structure of this motif and of the side chain attached to the TCA and it is not specific to PrP^C. Because biologically inactive 9-aminoacridine (9-aa) binds with similar K_D as anti-prion active quinacrine, direct interaction with PrP^C as mechanism of action appears highly unlikely. However, binding of 9-aa to Ar-Ar-motifs in proteins can be used as reporter for biological macromolecule interactions, by measuring changes in T₁-NMR relaxation times of 9-aa.