DNA is a polyelectrolyte, a long-chain polymer with many ionizable sites. According to polyelectrolyte theory, over limited distances along the DNA chain, electrical charges are modeled as a linear array of a well-defined charge density. By definition, if the axial charge density parameter of a molecule becomes greater than 1, counterions will condense on the polyelectrolyte to lower the effective value of the charge density parameter to 1. The preferential interaction parameter (PIP) is a parameter that is proportional to the axial charge density of the polymer chain. We report PIP values for a series of oligonucleotides of varying lengths. Equilibrium dialysis, UV spectroscopy, and capillary electrophoresis of solutions of 14, 20, 28, 34, and 160 base single strands and duplexes in 1.0 mM Na+ were performed to determine PIP as a function of DNA length. Results show that as the oligonucleotide length increases, the value of the average axial charge density increases as well. This observation is consistent with predictions based upon counterion condensation theory.
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