SEQUENCE-SELECTIVE METAL-ION BINDING TO DNA OLIGONUCLEOTIDES

Metal ion titrations of several DNA oligonucleotides, 10 dodecamers an d one decamer have been monitored by H-1 NMR spectroscopy in order to elucidate metal ion binding patterns. Also, the effects of paramagneti c impurities on resonance linewidths and NOESY cross-peak intensities have been reversed by EDTA back-titration experiments. H-1 ID NMR spec tra were recorded after successive additions of aliquots of different metal salts to oligonucleotide samples. Paramagnetic manganese(II) sal ts were used in most cases, but a few samples were also titrated with diamagnetic zinc(II). From this study, we conclude that there exists a sequence-selective metal ion binding pattern. The metal ions bind pre dominantly to 5'-G in the contexts 5'-GG and 5'-GA. The order of prefe rence seems to be GG greater-than-or-equal GA > GT > > GC. No evidence of metal ion binding to 5'-G in 5'-GC steps or to non-G residues was found. The H6 or H8 resonances on preceding (5'-) bases were affected by the adjacent bound paramagnetic metal ion, but no effect was observ ed on the protons of the succeding (3'-) base. The metal binding site in the duplexes is most likely at G-N7, as manifested by the pronounce d paramagnetic line broadening or diamagnetic shift of the G-H8 signal . This sequence selectivity may be qualitatively explained by a sequen ce-dependent variation in the molecular electrostatic potentials of gu anine residues (MEPs) along the oligonucleotide chain.