POLYELECTROLYTE COUNTERION CONDENSATION THEORY EXPLAINS DIFFERENTIAL SCANNING CALORIMETRY STUDIES OF SALT-INDUCED CONDENSATION OF CHICKEN ERYTHROCYTE CHROMATIN

The salt-induced chromatin condensation in chicken erythrocyte nuclei is studied by differential scanning calorimetry (DSC). The degree of c hromatin condensation is measured for condensation induced by monovale nt, divalent, trivalent, or tetravalent cations and by a mixture of so dium and magnesium. These last two cations show an evident competition effect. Salt-induced chromatin condensation is shown to be an entropy -driven process. A simple model of chromatin based on the polyelectrol yte counterion condensation theory is used in order to compute the cha rge neutralized by the cations in each chromatin domain. The degree of chromatin condensation is shown to be related to the weighed sum of t he square of the phosphate charge of each domain. The model predicts t he salt and the chromatin concentration dependence of the condensation and the effect of H1 removal.