Proton magnetic resonance analysis of terbium ion-nucleic acid complexes: Further evidence for two-site binding to polynucleotides

Terbium ion (Tb³⁺), like other rare earth lanthanides, has traditionally been viewed as binding nucleic acids at or near their ionized phosphate groups only. Here evidence is presented from ¹H NMR studies that confirms this mode of binding in Tb³⁺-mono-nucleotide complexes. However, in polynucleotides, we find that Tb³⁺ coordinately binds at two distinct sites, the phosphate moiety and electron donor groups on purine and pyrimidine bases. This two-site binding is best illustrated by complexes of Tb³⁺-polyuridylic acid, where the relative sensitivities of the uracil protons H5 and H6 to induced chemical shift and nuclear spin relaxation are the inverse of that seen in Tb³⁺-uridine monophosphate complexes. These data substantiate recently reported results derived from ultraviolet absorption and fluorescence spectroscopy (D. S. Gross and H. Simpkins, 1981, J. Biol. Chem.256, 9593-9598) that two-site binding is characteristic of the terbium(III)-polynucleotide interaction.