A thermodynamic study of herring protamine-DNA complex by differential scanning calorimetry.

The effects of the highly charged herring protamines (clupeine) on the thermodynamics of thermal denaturation of herring sperm and calf thymus DNA have been studied by differential scanning calorimetry (DSC). The strong binding affinity between the proteins and the polynucleotide is evidenced by the occurrence of a biphasic denaturation for the complex. The thermodynamic parameters for the melting transitions have been measured at different protein concentrations. Manning's theory of polyelectrolytes and McGhee and von Hippel's multiple-site exclusion approach were combined to obtain an evaluation of the degree of binding of the protamines to DNA, experimentally difficult to measure. A straightforward thermodynamic model was then adopted to reproduce, from the value of the estimated binding constant, the biphasic feature of the experimental melting transition of the protein–polynucleotide complex. The restraint imposed by the experimental denaturation temperatures provides the value of the clupeine binding site size. The binding constant and the site size thus evaluated are in good agreement with previously reported values. The theoretical analysis of the thermodynamic parameters from DSC measurements herein presented provides a generally applicable, useful and direct tool to unravel complex systems with high binding affinities.