Mercury-induced transitions between right-handed and putative left-handed forms of poly[d(A-T).d(A-T)] and poly[d(G-C).d(G-C)]

Polyd(A-T).d(A-T)] and polyd(G-C).d(G-C)], each dissolved in 0.1 M NaClO4, 5 mM cacodylic acid buffer, pH 6.8, experience inversion of their circular dichroism (CD) spectrum subsequent to the addition of Hg(ClO4)2. Let r identical to Hg(ClO4)2]added/DNA-P]. The spectrum of the right-handed form of polyd(A-T).d(A-T)] turns into that of a seemingly left-handed structure at r greater than or equal to 0.05 while a similar transition is noted with polyd(G-C).(G-C)] at r greater than or equal to 0.12. The spectral changes are highly cooperative in the long-wavelength region above 250 nm. At r = 1.0, the spectra of the two polymers are more or less mirror images of their CD at r = 0. While most CD bands experience red-shifts upon the addition of Hg(ClO4)2, there are some that are blue-shifted. The CD changes are totally reversible when Hg(II) is removed from the nucleic acids by the addition of a strong complexing agent such as NaCN. This demonstrates that mercury keeps all base pairs in register.