Using lanthanides as probes for polyelectrolyte-metal ion interactions. Hydration changes on binding of trivalent cations to nucleotides and nucleic acids

The interaction of the trivalent lanthanide ions Ce(III) and Tb(III) with ribonucleic acid (RNA) and guanosine 5′-triphosphate (GTP) in aqueous solution has been studied using their luminescence spectra and decays. Complexation is indicated by changes in luminescence intensity. With the system terbium(III)-RNA and terbium(III)-GTP, changes in luminescence with pH are probably related to conformational changes on the structure and also to the different degrees of protonation of phosphate groups and nucleotide bases. The degree of hydration of Tb(III) on binding to RNA and GTP is followed by luminescence lifetime measurements in water and deuterium oxide solutions, and at least one hydration water is lost from the lanthanide ion on binding to RNA or GTP at pH 4.7 and pH 7. Rather different behavior is observed on binding RNA at pH 9, where six water molecules are lost, possibly due to the lanthanide binding to the bases of the RNA backbone. This is similar to what has previously been seen with DNA, and is supported by 31P NMR spectral measurements, which confirm the possibility of lanthanide binding to both phosphate and bases in the nucleic acids. In the case of GTP at pH 9, two water molecules appear to be lost, probably due to terbium binding both to guanine and a phosphate group. Gd(III) EPR spectral measurements with DNA, RNA and GTP suggest decreased lanthanide ion mobility on binding.