Helix-coil transition in nucleoprotein. Effect of ionic strength on thermal denaturation of polylysine-DNA complexes

Thermal denaturation of direct‐mixed and reconstituted polylysine–DNA complexes in 2.5 × 10−4 M EDTA, pH 8.0 and various concentrations of NaCl has been studied. For both complexes, increasing ionic strength of the solution raises Tm, the melting temperature of free base pairs. The linear dependence of Tm on log Na+ indicates that the concept of electrostatic shielding on phosphate lattice of an infinitely long pure DNA by Na+ can be applied to short free DNA segments in a nucleoprotein. For a direct‐mixed polylysine–DNA complex, the melting temperature of bound base pairs Tm′ remains constant at various ionic strengths. On the other hand, the Tm′ in a reconstituted polylysine–DNA complex is shifted to lower temperature at higher ionic strength. This phenomenon occurs for reconstituted complex with long polylysine of one thousand residues or short polylysine of one hundred residues. It is shown that such a decrease of Tm′ is not due to a reduction of coupling melting between free and bound regions in a complex when the ionic strength is raised. It is also not due to intermolecular or intramolecular change from a reconstituted to a direct‐mixed complex. It is suggested that this phenomenon is due to structural change on polylysine‐bound regions by ionic strength. It is suggested further that Na+ may replace water molecules and bind polylysine‐bound regions in a reconstituted complex. Such a dehydration effect destabilizes these regions and lowers Tm′. This explanation is supported by circular dichroism (CD) results.