Effect of Hg(II) on d(GCGCATATGCGC)2 conformation: UV absorption and circular dichroism studies

Exposing the palindromic dodecamer d(GCGCATATGCGC)2, dissolved in the neutral-salt solvent 0.1 M (identical to low salt) or 3.0 M (identical to high salt) NaC1O4, to increasing amounts of Hg(C1O4)2 (identical to Hg(II)) produces major changes in its chirality in the low-salt medium but not high-salt. Let r identical to [Hg(C1O4)2]added/[DNA-P]. At low salt, the conformational changes are noted as follows: in absence in Hg(II), the near-conservative CD spectrum of the dodecamer is that of a B-type DNA. In the presence of Hg(II), the long-wavelength positive Cotton band (central location near 280 nm) undergoes inversion at r > 0.2 and reaches maximum negative extension at r = 0.5-0.6. A new minor positive CD peak appears around 315 nm. The long-wavelength negative Cotton band (central location near 250 nm) experiences a slight red-shift by about 10 nm but remains otherwise rather unaffected by Hg(II). Major CD changes are also noted in the short-wavelength region near 220 nm. The inverted CD spectrum is in general more complex in structure than the spectrum of the untreated oligomer. By contrast, the CD changes seen in the high-salt medium are not as complex, and the CD scripture of a right-handed B-type helix is retained. It is stressed that all CD changes are fully reversible: both the low-salt and high-salt CD spectra of the untreated dodecamer are regained subsequent to the removal of Hg(II) with the help of a complexing agent such as sodium cyanide. The (ordinary) UV absorption spectra of the dodecamer are altered by Hg(II) in less spectacular fashion although differences in progression in the two-salt media are noted. We interpret the CD changes as signaling (right-handed) B<-->(left-handed) non-B transitions of the oligodeoxyribonucleotide at low-salt levels but not at high-ionic strength. The presence of isodichroic (isosbestic) points in the various spectra shows that the Hg(II)-induced conformational B<-->Z transitions is unclear at present.