Helix–coil transition and conformational studies of protamine–DNA complexes

Protamine–DNA complexes prepared by the method of direct and slow mixing in 2.5 × 10 −4 M EDTA, pH 8.0, have been studied by thermal denaturation and circular dichroism. The complexes show biphasic melting with T m at about 50 °C corresponding to the melting of free DNA regions and T m ′ at about 92 °C corresponding to the melting of protamine‐bound regions. In protamine‐bound regions there are 1.38 amino acid residues per nucleotide, indicating a nearly completely charge neutralization. T m is increased but T m ′ is not when the ionic strength of the buffer is raised. This also supports a full charge neutralization in protamine‐bound regions. The circular dichroism of the complexes can be decomposed into two components, Δ ε0 of free DNA regions in B‐form conformation and Δ εb of protamine‐bound regions in a characteristic conformation neither that of B‐ nor C‐form but somewhere between them.