Terbium(3 +) as a probe of nucleic acids structure. Does it alter the DNA conformation in solution?

At low ionic strength, Tb³⁺ binding strongly alters the secondary structure of DNA. Circular dichroism and electro-optical techniques are more sensitive than fluorescence to study these alterations in double-stranded DNA, at low Tb³⁺/DNA phosphate ($\text{I}\text{P}$) ratios. Both techniques yield the following conclusion: as $\text{I}\text{P}$ is increased, native and sonicated DNA undergo a transition from the B- to ψ-form, the latter being a compact structure characteristic of aggregated DNA. Our study of alkylated DNA establishes that the accessibility of N-7 guanine to Tb³⁺ is clearly required for structural alterations in an aggregated state to occur. The chelation of the phosphate group and of the N-7 guanine by Tb³⁺ simultaneously alters the geometry of the sugar-phosphate backbone and the stacking interaction between the bases in double-stranded DNA.