| To characterize the thermodynamics of formation of a large-interface, wrapped protein-DNA complex analogous to the nucleosome, we have investigated the specific interaction of binding 34 bp H'DNA with the E. coli DNA-remodeling protein Integration Host Factor (IHF). Isothermal titration calorimetry and fluorescence resonance energy transfer are applied to determine effects of salt concentration (KCl, KF, KGlutamate (KGlu)), on the binding thermodynamics. The binding constant Kobs, enthalpy change (DeltaH° obs), and heat capacity change (DeltaCp°obs) depend strongly on [salt] and anion identity.;The thermodynamics of formation of the wrapped complex were detailed at 20°C, where binding is enthalpy-driven, especially at low [salt] (e.g. DeltaH° obs = -20+/-2 kcal·mol-1 in 0.04 M KCl). DeltaH° obs increases linearly with [salt] with a slope (dDeltaH° obs/d[salt]) which is much larger in KCl (38+/-3 kcal·mol -1M-1) than in KF (14+/-4 kcal·mol -1M-1) or KGlu (9+/-1 kcal·mol-1 M-1). At 0.33 M [salt], Kobs is 20-fold larger in KGlu or KF than in KCl, and the [salt] derivative SKobs ≡ dlnKobs/dln[salt] is almost twice as large in magnitude in KCl (-8.4+/-1) as in KF (-4.7+/-1) and KGlu (-4.6+/-1).;Complex formation is also characterized by a large, negative and strongly [salt]-dependent DeltaCp°obs (-1.4+/-0.1 kcal·mol -1K-1 in 0.23 M KCl). The [salt]-dependence of DeltaCP° obs is larger in KCl than KF, such that dDeltaCp°obs/d[KCl] = 1.3+/-0.6 kcal·mol-1K-1M -1 and dDeltaCp°obs/d[KF] = 0.7+/-0.4 kcal·mol -1K-1M-1.;A novel analysis of the large effects of anion identity on Kobs , SKobs, DeltaH°obs, and on DeltaCp° obs dissects coulombic, Hofmeister and osmotic contributions to these quantities. This analysis attributes anion-specific differences in K obs, SKobs, and DeltaH°obs to (i) displacement of a large number of waters of hydration (ca. 1000) from the 5400 A 2 of biopolymer surface buried in complex formation, and (ii) significant local exclusion of F- and Glu- from this hydration water, relative to the situation with Cl-, which we propose is randomly distributed. |
