The binding of sodium n-dodecyl sulphate (SDS) to calf thymus histone H2B was studied in the pH range 3.2–10 by equilibrium dialysis at 27 and 37°C. The binding data have been used in terms of the Schatchard equation showing unusual plots with minima. No theoretical model gives Scatchard plots with such conditions, except for a combination of two types of binding with large differences in the Hill coefficients and binding affinity, i.e. a combination of negatively and positively cooperative binding sites.

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