Hg(II)-induced changes in DNA-circular dichroism: reversible transitions between right-handed and left-handed screwness.

Exposing native calf thymus DNA (in 0.1 M NaClO4, 5 mM cacodylic acid buffer, pH 6.81, 25 degrees C) to increasing concentrations of Hg(ClO4)2 produces dramatic changes in its circular dichroism (CD). Let r = [mol of added Hg(II)*/[mol DNA base]: the conservative CD spectrum of the DNA B-form, consisting of the 273 nm major (+) CD band, the 245 nm major (-) CD band, the 219 nm minor (+) CD band, and the 208 nm minor (-) CD band, becomes non-conservative in appearance at 0.01 less than r less than 0.12 and assumes the spectral characteristics of a left-handed DNA double helix at 0.12 less than r greater than 1.0. The presence of a number of isoellipticity points shows that well-defined equilibria exist between the various chiroptical forms of mercurated DNA. The CD changes are totally reversible upon the removal of Hg(II), at least up to r = 1.0, demonstrating that Hg(II) keeps all base pairs in register.