Entropy Changes in Aqueous Solutions of Non-polar Substances and in Bio-complex Formation

摘要

The entropy changes, ΔS _app, (i) for dissolution in water of non-polar substances and (ii) for protein-ligand complexation show linear dependences on the logarithm of the absolute temperature. For every compound, the slope m _(S)=ΔC _p for the line ΔS _app=f(ln T) depends on the size of the molecule and is exactly equal to the slope m _(H)=ΔC _p found in the diagram ΔH _app=f(T). This means that the slopes are rigorously proportional (with a ratio m _{(S) w}=C _p,w) where n _w is the number of involved water molecules as determined from the enthalpy change ΔH _app=f(T). It is also worth noting that the value of n _w is positive (as well as m _(S) and m _(H)) in the dissolution of non-polar substances, whereas it is negative (as well as m _(S) and m _(H)) in bio-complex formation and in micelle formation. The number n _w (n _ww water molecules (n _w<0) needed to fill the excess cavity return to the structure of the bulk solvent and release thermal energy, which compensates for the endothermic enthalpy. The affinity in the association processes is bound, for the most part, to the entropy produced by occupation of part of the cavity by condensation of water molecules. The association processes are therefore entropy driven.