SEQUENCE-SELECTIVE INTERACTION BETWEEN MERCURY(II) IONS AND THE DNA DODECAMER [D(GCCGATATCGGC)](2) STUDIED BY H-1-NMR SPECTROSCOPY

The palindromic dodecamer [d(GCCGATATCGGC)](2) has been titrated by Hg (ClO4)(2) in order to study sequence dependent Hg-II ion interactions. The titration pattern as monitored by 1D and 2D H-1 NMR is consistent with a transition to a new conformer of the dodecamer induced by Hg-I I ions, At intermediate stages of the titration, the proton signals fr om the new conformer coexist with those of the original one, indicatin g slow exchange between the two forms on the NMR timescale. The data c learly show that there is no major alteration in the secondary structu re, e.g. B-->Z-form or duplex-->hairpin transition. The intra- and int er-residue sequential walk is completed except for a break between T6 and A7. At a concentration level r = [Hg-II]/[nucleobase] < 0.20 all f our central imino signals are present. This definitely excludes thymin e N3 as a possible mercuration site. In the imino region of the spectr a Hg-II ions induce a large upheld shift of the thymine imino resonanc e T8-N3H, while the other thymine resonance T6-N3H is unperturbed. The guanine imino signal G4-N1H shows a large downfield ring current shif t caused by major conformational changes in the duplex. The complete t itration experiment indicates that mercury, initially, binds selective ly to the A5'-T8 base pair. A tentative model is proposed where mercur y is cross-linking the two strands via thymine T8-O4 and the deprotona ted amino group of the complementary adenine base A5'.