Effect of mercuric and silver ions on cesium sulfate equilibrium buoyant densities of synthetic polydeoxyribonucleotides

The effect of Hg²⁺ and Ag⁺ on the buoyant density (ρ{variant}) of four synthetic DNA polymers, poly[d(A-T)]; poly(dA) · poly(dT); poly[d(G-C)]; and poly(dG) · poly(dC), was investigated. The buoyant density of poly[d(A-T)] in Cs₂SO₄ increased dramatically after complexing with Hg²⁺, but little change in the buoyant density of other polymers resulted except at very high molar ratios of Hg²⁺ DNA-P (r_f) Hg²⁺ raised the thermal transition temperature (T_m) of alternating polymers and lowered the T_m of homopolymers. Measurements in the preparative ultracentrifuge indicated that lowered T_m correlated with Hg²⁺-induced strand separation of one homopolymer [poly(dA) · poly(dT)], but strand separation was not observed with another homopolymer [poly(dG) · poly(dC)] complexed with Hg²⁺. When Ag⁺ was mixed with the polymers, the buoyant density of poly(dG) · poly(dC) increased most markedly. A substantial increase in the buoyant density of poly[d(A-T)] and a small increase in the buoyant density of poly[d(G-C)] were also observed. The T_m changes induced by Ag⁺ were not related in any obvious way to buoyant density changes. These findings indicate that nucleotide sequence as well as overall base composition is of importance in understanding the buoyant density changes induced by metal ions. Although these data do not allow construction of a detailed molecular model of polymer-metal ion interactions, they may be used to explain much of the behavior of naturally occurring DNA sequences, such as heterochromatic satellite sequences and 5 S and rRNA sequences, in Hg²⁺/Cs₂SO₄ and Ag⁺/Cs₂SO₄ gradients.