(618d) Quantitative Analysis of the Binding of Monovalent Cations by Oligonucleotides

The interaction of monovalent cations with small nucleotides and single and double-stranded DNA oligomers has been measured by capillary electrophoresis. Two of the analytes, cAMP and single-stranded DNAs, do not bind monovalent cations, and the free solution mobility decreases linearly with the square root of ionic strength. For the other analytes, AMP, ADP, ATP and double-stranded DNA oligomers, the dependence of the mobility on ionic strength is determined by the type of cation. If the cation is a tetraalkylammonium ion, the mobility decreases linearly with the square root of ionic strength, as observed for the non-binding analytes. However, non-linear mobility plots are observed if the cation is Tris, Li, Na or K, suggesting that AMP, ADP, ATP and double-stranded DNAs bind these cations. Analysis of the dependence of the mobility on cation concentration by the variable ionic strength method indicates that the apparent dissociation constants are in the range of 10-100 mM, in agreement with other values in the literature. Binding constants can also be obtained at fixed ionic strength by replacing a non-binding ion with a binding ion in the background electrolyte. Somewhat larger apparent dissociation constants obtained by this method, because electrostatic interactions are weaker at high ionic strengths.