Abstract
The interaction of the fluorinated antimalarial drug fluoroquine [7‐fluoro‐4‐(diethyl‐amino‐1‐methylbutylamino)quinoline] with DNA, tRNA, and poly(A) has been investigated by optical absorption, fluorescence, and ^19^F‐nmr chemical‐shift and relaxation methods. Optical absorption and fluorescence experiments indicate that fluoroquine binds to nucleic acids in a similar manner to that of its well‐known analog chloroquine. At low drug‐to‐base pair ratios, binding of both drugs appears to be random. Fluoroquine and chloroquine also elevate the melting temperature (T~m~) of DNA to a comparable extent. Binding of fluoroquine to DNA, tRNA, or poly(A) results in a downfield shift of about 1.5 ppm for the ^19^F‐nmr resonance. The chemical shift of free fluoroquine depends on the isotopic composition of the solvent (D~2~O vs H~2~O). The solvent isotope shift is virtually eliminated by fluoroquine binding to any one of the nucleic acids. ^19^F‐nmr relaxation experiments were carried out to measure the spin‐lattice relaxation time (T~1~), ^19^F{^1^H} nuclear Overhauser effect (NOE), off‐resonance intensity ratio (R), off‐resonance rotating‐frame spin‐lattice relaxation time (T), and linewidth for fluoroquine in the nucleic acid complexes. By accounting for intramolecular proton‐fluorine dipolar and chemical‐shift anisotropy contributions to the fluorine relaxation, all of the relaxation parameters for the fluoroquine–DNA complex can be well described by a motional model incorporating long‐range DNA bending on the order of a microsecond and an internal motion of the drug on the order of a nanosecond. Selective NOE experiments indicate that the fluorine in the drug is near the ribose protons in the RNA complexes, but not in the DNA complex. Details of the binding evidently differ for the two types of nucleic acids. This study provides the foundation for an investigation of fluoroquine in intact cells.