Ligand enthalpic contributions

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

Force-field-based scoring functions use arbitrary empirical estimates of interaction energies obtained by molecular mechanics energy functions. This simple approximation, which takes into account only enthalpic contribution often correlates well with the experiment. Solvent effects are described by atom-based solvation parameters, which are computed for the surface of both ligand and receptor which is buried upon complexation. DOCK-chemical27 and CHARMm scoring functions represent this class. 

The enthalpic contributions to the ethereal ligand exchange processes are similar for Et20 and TFIF, but the entropy associated with the dissociation of THF from... 

According to the equation (21.2), ligand-receptor interactions are characterized by enthalpy-entropy compensation in which one term favors and the other disfavors binding. While enthalpic contributions include electrostatic, hydrogen bond, and Van der Waals interactions, entropic contributions arise from several sources. On one hand, the loss of flexibility upon binding has an important entropic cost, counterbalanced on the other hand by the displacement of ordered water molecules. This will be discussed in the next section as well as the various types of drug-receptor interactions. 

The primary contribution to the hydrophobic effect comes from the favorable increase in system entropy upon the release of ordered first-shell waters from nonpolar surface areas on both the macromolecule and the ligand upon ligand binding there are also secondary enthalpic contributions. 

Assuming that the metal-ligand, metal-metal and ligand-ligand interactions are identical in HHH-[R2(L30)3] " and HHT-[] 2(L30)3] , we can easily deduce that the enthalpic contribution to equilibrium... 

Thermodynamic effects on ligand binding affinity can roughly be divided into two classes enthalpic contributions, which can be derived from electrostatic and van der Waals potentials by examining the distances between interacting groups, and entropic contributions, which arise from the dynamics of theses groups. 

---