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

The effects of the highly charged herring protamines (clupeine) on the ther modynamics of thermal denaturation of herring sperm and calf thymus DNA hav e been studied by differential scanning calorimetry (DSC). The strong bindi ng affinity between the proteins and the polynucleotide is evidenced by the occurrence of a biphasic denaturation for the complex. The thermodynamic p arameters for the melting transitions have been measured at different prote in concentrations. Manning's theory of polyelectrolytes and McGhee and von Hippel's multiple-site exclusion approach were combined to obtain an evalua tion of the degree of binding of the protamines to DNA, experimentally diff icult 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-polynucleotid e complex. The restraint imposed by the experimental denaturation temperatu res provides the value of the clupeine binding site size. The binding const ant and the site size thus evaluated are in good agreement with previously reported values. The theoretical analysis of the thermodynamic parameters f rom DSC measurements herein presented provides a generally applicable, usef ul and direct tool to unravel complex systems with high binding affinities.