Electrostatic effects on polynucleotide transitions. I. Behavior at neutral pH

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Abstract
An approximate analytical expression for the electrostatic free energy of a polynucleo‐tide in any of its possible ordered or random conformations is derived by integration of the screened‐Coulomb potential energy function over all charge pairs in the structure. The electrostatic free energy of any form is found to be a linear function of the logarithm of the monovalent counterion concentration, in the range of low salt concentrations. Hence the electrostatic free energy difference between ordered and disordered forms in a polynucleotide structural transition is a linear function of the logarithm of the monovalent counterion concentration. A free energy balance applied to a two‐state model for the transition then yields a linear dependence of the transition temperature Tm upon the logarithm of the counterion concentration. Calculation of the quantity dTm/d log M, where M is the monovalent counterion concentration, shows it to be a characteristic constant for a given transition, with a magnitude and sign proportional to the charge density difference between the ordered and disordered forms. Use of any one of several alternate, simple assumptions yields predicted dTm/d log M values in good agreement with experimental data for various polynucleotide transitions.