Table I. Thermodynamic parameters involved in interaction of Drosophila HP1 with methyl-K9 H3 peptide.
| Temperature (°C) | KD (fluorescence µM) | KD (ITC µM) | ΔH (ITC kcal/mol) | N (ITC) | ΔG (kcal/mol) | TΔS (kcal/mol) |
|---|---|---|---|---|---|---|
| 25 | 120 ± 12 | 105 ± 24 | –11.7 ± 2.4 | 0.90 ± 0.11 | –5.4 | –6.3 |
| (133 ± 11) | ||||||
| 15 | 80 ± 8 | 59 ± 8 | –10.6 ± 0.5 | 0.96 ± 0.02 | –5.6 | –5.0 |
| (91 ± 5) |
Binding results were obtained from fluorescence anisotropy (fluorescence) and isothermal titration calorimetry (ITC). Fluorescence anisotropy of the fluorescein-labeled peptide was used to determine the apparent dissociation constant KD (when a 1:1 binding stoichiometry is assumed) for the chromo domain and intact HP1 (values in parentheses) at 15 and 25°C. Using ITC, the single site binding constant KD, the heat of binding ΔH and the number of sites N were independently measured variables at 15 and 25°C. The free energy and entropy changes for binding were then calculated using the following relationships: ΔG = –RT ln KD and ΔG = ΔH – TΔS, respectively. In each case, parameters are reported as the mean (± average deviation from the mean) obtained from two independent titration experiments.