Abstract

By working at very low Na concentrations (1mM and less), the number of bound Mg(2+), cobalt hexamineC3+), and spermine(4+) necessary to induce the B-Z transition of poly (dG-ra dC) has been directly measured. The results show that if as little as 1 cobalt hpxamine(3+) or spermine(4+) is bound per 40-50 nucleotides the transition will occur. A greater fraction of bound Hg(2+) is required, 1 bound Mg(2+)/10 nucleotides. The dependence of the transition midpoint concentrations of oligovalent ions on Na concentrations suggests that specific ion binding energies, not included in counterion condensation theory, are responsible for the transition.