Evidence for two-site binding in the terbium(iii)-nucleic acid interaction

The interaction of terbium(III) (Tb3+) with synthetic and native nucleic acids, including circular supercoiled DNA, has been studied. Fluorescence spectroscopy has revealed that: (a) the intrinsic fluorescence of Tb3+ is greatly enhanced upon binding either xanthine or guanine residues; (b) slight chemical modification of the nucleoside bases abrogates energy transfer; (c) binding affinity increases with chain length; (d) protonation of the N-7 site of xanthine or guanine obviates energy transfer in Tb3+-homopolynucleotide complexes; and (e) unimpaired base residues in supercoiled DNA are capable of measurably enhancing Tb3+ fluorescence. In addition, equilibrium dialysis experiments with 160Tb3+ show that double helical polynucleotides (which do not enhance Tb3+ emission) bind more cation/molar nucleotide residue than do single-stranded polynucleotides. Melting profiles demonstrate that at low Tb3+: base ratios, the lanthanide destabilizes helical DNA and prevents reannealing of strands following thermal denaturation. These data suggest the existence of two Tb3+ binding sites on the unpaired residues of polynucleotide chains: the phosphate moiety and electron donor groups on the nucleoside bases.