155Eu 3+ as a probe of metal ion and cationic drug binding sites on native and heat-denatured DNA

Abstract

A flow-dialysis apparatus suitable for the study of high affinity metal ion binding sites in macromolecules has been utilized to study 155Eu 3+ exchange processes, as a function of pH, in both ‘native’ and ‘heat-denatured’ DNA. ‘Free exchange’ of 155Eu 3+ was found to occur at a significantly faster rate at pH = 7.0 than at pH = 6.0 for both forms of DNA; while non-radioactive Eu 3+-induced ‘displacement’ of bound 155Eu 3+ occurred at a significantly faster rate at pH = 6.0 than at pH = 7.0 for both species of DNA. These results are consistent with a greater ‘entropic’ driving force for metal ion:DNA complexation at the lower pH value. The effect of ethidium bromide on 155Eu 3+ exchange was also examined as a function of pH. The intercalating agent was found to accelerate 155Eu 3+ displacement at pH = 6.0 and decelerate displacement at pH = 7.0. All three sets of experiments ( i.e., free- exchange of bound 155Eu 3+, Eu 3+-induced displacement of bound 155Eu 3+ and ethidium ion-induced displacement of bound 155Eu 3+) indicate that the 155Eu 3+ ion can serve as a useful probe of metal ion and drug binding sites in nucleic acid polymers, and constitutes a particularly sensitive probe at pH = 6.0.