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Thermodynamic Studies of the Core Histones: pH and Ionic Strength Effects on the Stability of the (H3−H4)/(H3−H4)2 System

“…It is clear that the T m of the thermal unfolding (shown in Figure 8b) increases from 51.5 to 61.7 °C as the protein concentration is increased from 10 to 120 µM. This result is in agreement with a system that undergoes dissociation upon unfolding (Freire, 1989); in this case, an increase in protein concentration leads to higher levels of association and, therefore, to an increase in the stability and the melting temperature of the system.…”

“…The unfolding transition is coupled to the dissociation of the two subunits of the H3-H4 dimer, as indicated by the concentration dependence of the transition temperature, described in a following paragraph. This effect is also reflected by the asymmetry (slight) of the heat capacity function which in all cases is skewed toward the low-temperature side of the transition, as expected for a transition coupled to dissociation (Freire, 1989).…”

“…This figure illustrates first, at the lower side of the temperature range, a gradual increase of the heat capacity, next the transition region, and last the characteristic increase of the unfolded state at high temperatures. It is evident that the heat capacity of the high-temperature state of the H3-H4 dimer is very similar to the one expected for a completely hydrated, unstructured polypeptide of the same amino acid composition (Privalov & Makhataze, 1990;Freire, 1994;Gomez et al, 1995), suggesting that the H3-H4 dimer in this chemical environment becomes maximally unfolded and hydrated upon thermal denaturation. The experimental results also indicate that at 20 °C the heat capacity of the native state of the H3-H4 dimer is 0.425 cal/K g, a value significantly higher than that of 0.35 cal/K g reported for most globular proteins.…”