A circular dichroism study on the structure of DNA and the nucleosomal core particle using Hg(II) and Ag(I)

CD studies of known sequence DNA-like polynucleotide-Hg2+ complexes have established a cross-strand binding of Hg2+ to the two thymines of the base-paired first-neighbor unit TpA, as the type I binding mode of poly[d(A-T) x d(A-T)] and other polynucleotides with high TpA incidence. The amino group of adenine is the most probable Hg2+ site for poly(A) and for the binding mode II of poly[d(A-T) x d(A-T)]. The binding of Hg2+ to the natural DNAs shows two binding modes with two different degrees of CD perturbation. The two proposed binding modes are substantiated by Hg2+ binding studies with the DNa-poly(L-lysine) complexes. The increase of sedimentation coefficient of the nucleosomal core particle-heavy metal ion complexes can be accounted for by the binding of heavy metal ions. From the CD experiments with Hg2+ and Ag+, it is estimated that 80 +/- 10% of the DNA in the nucleosomal core particle is involved in the wrapping around the histone core. An out-of-plane transition of the DNA-Ag+ binding mode II is proposed as the cause for the large CD perturbation at 270 nm, but the origin of optical activity of the DNA-Hg2+ complex is still unknown.