Effect of ionic strength on the chromatin conformational transitions. A differential scanning calorimetry study

Abstract

The thermal denaturation of hen erythrocyte chromatin at low ionic strength (0.25 m m EDTA, pH 8.0 (A), and 0.25 m m EDTA, pH 8.0, 5 m m NaCl (B)) was studied by high sensitivity differential scanning microcalorimetry and absorbance at 260 nm. The melting profiles recorded by the two methods were decomposed into sums of three (case A) and two (case B) Gauss curves and were found to be of identical shape. In case A, the three individual peaks at 61, 72, and 82°C have enthalpies 4.1, 8.4 and 12.5 kcal/mol base pairs −1 , respectively. The enthalpies of pure DNA melting at these temperatures are 6.2, 6.9 and 7.6 kcal/mol base pairs −1 , respectively. The smaller in comparison to pure DNA enthalpy of the peak at 61°C indicates a decreased thermal stability of the chromatin portion melting during this transition. The disappearance of this transition at increased ionic strength (case B) is accompanied by a transfer of its enthalpy to the peak at 72°C and its hyperchromicity to the peak at 82°C. During this complex transformation the overall melting enthalpy remains unchanged (9.5±0.5 kcal/mol base pairs −1).