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

Terbium ion (Tb 3+), 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 Tb 3+- mono-nucleotide complexes. However, in polynucleotides, we find that Tb 3+ 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 Tb 3+-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 Tb 3+-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.