Changes in hydration of lanthanide ions on binding to DNA in aqueous solution

The interaction of the trivalent lanthanides Ce(III), Eu(III), and Tb(III) with sodium deoxyribonucleic acid (DNA) in aqueous solution has been studied using their luminescence spectra and decays. Complexation with DNA is indicated by changes in luminescence intensity. In the system terbium(III)-DNA, changes in luminescence with pH are suggested to be due to the protonation of phosphate groups. The degree of hydration of Tb(III) on binding to DNA is followed by luminescence lifetime measurements in water and deuterium oxide solutions, and it is found that the lanthanide ion loses at least one hydration water on binding to long double stranded DNA at pH 4.7 and pH 7. Rather different behavior is observed on binding to long or short single stranded DNA, where six water molecules are lost, independent of pH. It is suggested that in this case the lanthanide probably binds to the bases of the DNA backbone. The DNA conformation seems to be an important factor in the binding. In addition, the isotopic effect on terbium luminescence lifetime may provide a useful method to distinguish between single and double stranded DNA. DSC results are consistent with cleavage of the double helix of DNA at pH 9 in the presence of terbium.