I would like to know the salt (Nacl) effect at different concentrations on dH and dS of protein ligand interactions in Isothermal Titration Calorimetry. And how pH of buffer (both acidic and basic) effect the same (dH and dS)?
If there is any binding sites for Na+ or Cl- atoms at the interaction site, the enthalpy of the interaction would be modified by the salt concentration.
Otherwise, an increase of the salt concentration increase the hydrophobic interactions, and would most likely increase the affinity throught the entropic contribution.
The pH influence is directly related to the presence of residues which can capture or release a proton within the interaction site: Glu, Asp, His (especially important for variation around neutral pH), Cys, Tyr, Arg, Lys. It can affect both enthalpy and entropy, bust mostly the former.
In addition to the points articulated very elegantly by David Teze, one should also keep in mind that if there are electrostatic forces or polar interactions facilitating the binding then the presence of salt will weaken the binding.
Regarding pH effect, one should also consider the possibility of the ligand also having protonatable groups, which would undergo protonation/deprotonation depending on the change in pH. Such changes also will have an effect on the enthalpy and entropy of binding.
The effect of NaCl concentration on the binding thermodynamics of a protein-ligand complex could be of two types: i) specific binding of Na+ or Cl- ions and/or ii) general screening effect on electrostatic interactions. When charge-charge interactions are present in the formed complex, the ionic strength of the solution will have an impact on the binding enthalpy, for instance by diminishing attractive interactions. An example of this effect has been documented in: J. Mol. Biol. (2011) 405: 158-172; doi:10.1016/j.jmb.2010.10.018
Protein-ligand interactions are modulated by the environmental pH, which dictates the protonation state of the ionizing residues according to the pKa of the interfacial ionizing residues in the separate molecules and in the complex and thus also dictates the electrostatic binding energy. This is illustrated for a protease-inhibitor complex analyzed in a 3.5-10.0 pH range: Protein and Peptide Letters (2015) 22: 239-247;
the salt contribute the charge -charge interaction to the free energy of binding and also alter the binding affinity as a result of conformational change.
Entropy: A concept that is not a physical quantity
https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity
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