| Abstract
| - Hg(II) interacted site selectively with only one of threedeoxyribooligonucleotides examined; these “oligos”eachhad a different number of unmatched T residues. Thus, Hg(II)formed an intrastrand T−Hg−T cross-link betweenthe first and fourth T residues of the hairpin, d(GCGCTTTTGCGC)(T4). The DNA strand formed a loop aroundthe Hg, as if the Hg atom had been lassoed. The interactions ofHg(II) with two other oligos, d(ATGGGTTCCCAT)(T2) and d(GCGCTTTGCGC) (T3), were less specific.Previously, we found that at high DNA and saltconcentrations, T2 was a mixture of hairpin and duplex formswhile T3 and T4 had the hairpin form;modelingstudies showed that in the free T4 hairpin the two T's atthe ends of the (T)4 loop form a T·T wobble basepair.Only in T4 are the T residues positioned to form anintrastrand cross-link readily. The Hg(II)−oligoadductsformed as a function of added Hg(II) were investigated bytitrations monitored by UV, CD, and 1H NMRspectroscopy. The appearance of a new set of 1Hsignals with the concomitant decay of the free oligo 1Hsignalsindicated that 1:1 Hg(II):T2, 1.5:1Hg(II):T3, and 1:1 Hg(II):T4 adductswere formed with Hg(NO3)2. InH2O,these adducts all had spectra with very downfield signals for theexchangeable TN(3)H and GN(1)H groups, acharacteristic of base-paired regions. All upfield N(3)H signalsfrom the (T)2 and (T)3 sequences of thefreeoligo disappeared in the spectra of the 1:1 Hg(II):T2and 1.5:1 Hg(II):T3 adducts. The disappearance ofthe NHsignals, the UV spectral changes, and the stoichiometries (1:1Hg(II):T2 and 1.5:1 Hg(II):T3) indicatethat theseadducts are duplexes containing two and three T−Hg−T interstrandcross-links for T2 and T3,respectively.The 1H and 13C signals of the 1:1Hg(II):T4 adduct in D2O were nearlycompletely assigned by 2D NMRspectroscopy. The spectrum of the adduct in H2O hadonly two of the four original TN(3)H signals from the(T)4sequence present in the spectrum of T4; this result isconsistent with the presence of a TN3−Hg−TN3 cross-link. The 13C chemical shift changes upon Hg(II)binding indicated that the TN3−Hg−TN3 cross-link wasbetweenthe T's at each end of the (T)4 loop. The NOESY, CD,and UV spectra were all consistent with a hairpinconformation for the 1:1 Hg(II):T4 adduct. Ahairpin conformation also appeared reasonable frommolecularmodeling calculations. In conclusion, the length of the central(T)n sequence influenced the type ofT−Hg−Tcross-link formed and, in turn, the conformation of the adducts.For (T)2 and (T)3, interstrand T−Hg−Tcross-linking favored the duplex form. In contrast, for (T)4,intrastrand T−Hg−T cross-linking stabilized thehairpinform.
- Hg(II) interacted site selectively with only one ofthree deoxyribooligonucleotides, each with a different number ofunmatched T's. Hg(II) formed an intrastrand T−Hg−T cross-linkbetween the first and fourth T's of the hairpin, d(GCGCTTTTGCGC).The DNA strand formed a loop around the Hg, as if the Hg atom hadbeen lassoed. Hg(II) formed interstrand T−Hg−T cross-linkswith two other oligos, d(ATGGGTTCCCAT) and d(GCGCTTTGCGC), thusstabilizing the duplex form.
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