A flow-dialysis apparatus suitable for the study of high affinity metal ion binding sites in macromolecules has been utilized to study155Eu3+exchange processes, as a function of pH, in both 'native' and 'heat-denatured' DNA. 'Free exchange' of155Eu3+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 Eu3+-induced 'displacement' of bound155Eu3+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 on155Eu3+exchange was also examined as a function of pH. The intercalating agent was found to accelerate155Eu3+displacement at pH = 6.0 and decelerate displacement at pH = 7.0. All three sets of experiments (i.e., free- exchange of bound155Eu3+, Eu3+-induced displacement of bound155Eu3+and ethidium ion-induced displacement of bound155Eu3+) indicate that the155Eu3+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. © 1986.
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