Theory of ion effects on DNA conformation: A high-salt model

Abstract

A theoretical model for the binding of alkali metal cations to DNA is proposed. At high ionic strengths a significant proportion of anionic phosphate sites become occupied by a direct site binding of immobilized counterions which distort the phosphate charge distribution. Calculations are presented which can explain the differences in binding among the alkali metals. The results are consistent with the observed order in which the alkali metals induce the transition from B form to C form DNA. The calculations are also consistent with the observed association constants for these cations and with the observed changes in melting temperature as a function of ionic strength. The results lead to the interpretation that all of the alkali metal cations will exert similar effects on DNA structure when the ionic strength is low and the binding is primarily an association of mobile counterions in the neighborhood of the polyanionic DNA. As the counterion concentration increases to the range of 1-2 M and above, this model assumes that a significant fraction of phosphate sites become occupied by an immobilized counterion. This occupancy of some phosphate sites by immobilized counterions at high ionic strengths can account for the specific differences within the alkali metal group in the relative ease with which transconformational reactions in DNA are induced.