Interaction of Calf-Thymus DNA With Trivalent La, Eu, and Tb Ions. Metal Ion Binding, DNA Condensation and Structural Features

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Abstract
The interaction of calf-thymus DNA with La3+, Eu3+ and Tb3+ has been investigated in aqueous solution at pH 6.5, using metal/DNA(P) molar ratios (r) 1/80, 1/40, 1/20, 1/10, 1/4 and 1/2. Correlations between FTIR spectral changes and DNA structural properties have been established. At low metal/DNA(P) (r) 1/80, the metal ions bind mainly to the PO− 2 groups of the backbone, resulting in increased base-stacking interaction and duplex stability. At (r) 1/40 and 1/20, metal ion binding to the PO− 2 and the guanine N-7 site (chelation) predominates with minor perturbations of the A-T base pairs. Evidence for this comes from the displacement of the band at 1712 cm−1 (T,G) towards a lower frequency and the PO− 2 antisymmetric band at 1222 cm−1 towards a higher frequency. At higher metal/DNA(P) ratio, r> 1/20, DNA begins to condensate and drastic structural changes occur, which are accompanied by the shift and intensity changes of several G-C and A-T absorption bands. No major departure from B-DNA conformation was observed before and after DNA condensation eventhough some local structural modifications were observed. A comparison with the Cu-DNA complexes (denaturated DNA) shows some degree of helical destabilizition of the biopolymer in the presence of lanthanide ions.