Polyelectrolyte Counterion Condensation Theory Explains Differential Scanning Calorimetry Studies of Salt-Induced Condensation of Chicken Erythrocyte Chromatin

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Abstract
The salt-induced chromatin condensation in chicken erythrocyte nuclei is studied by differential scanning calorimetry (DSC). The degree of chromatin condensation is measured for condensation induced by monovalent, divalent, trivalent, or tetravalent cations and by a mixture of sodium 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 polyelectrolyte counterion condensation theory is used in order to compute the charge neutralized by the cations in each chromatin domain. The degree of chromatin condensation is shown to be related to the weighed sum of the square of the phosphate charge of each domain. The model predicts the salt and the chromatin concentration dependence of the condensation and the effect of H1 removal.