AbstractThe structural determinants of the interaction of the G‐quadruplex (G4) motif found in precursor miRNA 149 (rG4) with the acridine orange derivative C₈, a G4 ligand stabilizer possessing anticancer activity, and the protein nucleolin (overexpressed in cancer cells) were investigated by Nuclear Magnetic Resonance (NMR) spectroscopy. For the rG4/C₈ complex, the results revealed a strong stabilizing interaction between the aromatic core and the iodinated ring of the C₈ ligand with the rG4 structure. The NMR study revealed also different interaction patterns between nucleolin and rG4 and nucleolin and rG4/C₈ complex. In the absence of the ligand, rG4 establishes interactions with polar residues of the protein while for the rG4/C₈ complex, these contacts are mainly established with amino acids that have hydrophobic side chains. However, nucleolin chemical shift perturbation studies in the presence of rG4 or rG4/C₈ reveal the same location between domains 1 and 2 of the protein, which suggests that the rG4 and rG4/C₈ complex bind in this region. This puzzling structural study opens a new framework to study rG4/ligand/nucleolin complexes that might impact the biogenesis of miRNA 149.