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 × 10−4M EDTA, pH 8.0, have been studied by thermal denaturation and circular dichroism. The complexes show biphasic melting with Tm at about 50 °C corresponding to the melting of free DNA regions and Tm′ 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. Tm is increased but Tm′ 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, Δε0 of free DNA regions in B‐form conformation and Δεb of protamine‐bound regions in a characteristic conformation neither that of B‐ nor C‐form but somewhere between them.