The self-association of chicken-erythrocyte histones

The self-association of the separate histone fractions isolated from chicken erythrocytes has been studied in solution at a number of different pH values and ionic strengths. The apparent molecular weights of the histones were determined over a range of macromolecular concentrations using the techniques of osmotic pressure and sedimentation equilibrium. Histone F2c (H5) did not associate under any of the conditions investigated whereas the other histone fractions all appeared to undergo self-association forming dimers, dimers of dimers, etc. The degree of association increased with the pH and ionic strength of the medium. The tendency to aggregate increased in the order; histone F2c (H5) (non-aggregating), histone F2b (H2B), histone F2a2 (H2A), histone F3 (H3), histone F2a1 (H4) (highly aggregating). In the case of histone F2a2 (H2A) at pH 3.0 and ionic strength 0.1, the apparent weight-average molecular weight was determined at a number of macromolecular concentrations at five different temperatures. The self-association was analysed according to the method of Adams (published by Beckman Instruments Inc. in 1967) and shown to be a monomer-dimer-tetramer equilibrium. The association constants were evaluated at each of the temperatures studied and from their variation with temperature the values of the enthalpy and entropy of association were calculated. The intermolecular association was characterised by only a small change in enthalpy but a large, positive, change in entropy. This suggests that the association of histones at acid pH is due to hydrophobic interactions between the relatively uncharged segments of like polypeptide chains.