Osmotic Pressure of DNA Solutions and Effective Diameter of the Double Helix

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Abstract
A simple osmometer with nuclear filters (polymer films with pores of a preset diameter) were used to measure the osmotic pressure of Col El plasmid DNA solutions in the concentration range of 1–4 mg/ml DNA. Linear and open circular DNA forms proved to have the same osmotic pressure within the experimental accuracy. The results of the measurements were used for calculating the second virial coefficient A 2 of the solution of DNA segments and the effective chain diameter d eff in the ionic strength range of 10−2-0.1 M, As the ionic strength is lowered from 0.1 to 10−2 M the effective diameter of DNA increases from 80 to 220 A. The results are in rather good agreement with theory and with other experimental data.