20 to 30 kcal/mole
||Chothia C, Janin J. Principles of protein-protein recognition. Nature. 1975 Aug 28 256(5520):705-8. p.707 left column top paragraphPubMed ID1153006
||"Using available atomic coordinates and a computer program, [researchers] calculated the total area accessible to water for each protein complex and for the isolated subunits. The surface area buried in the complex is then defined as the accessible surface area of one subunit plus that of the other subunit minus that of the complex (Table 1)."
||"...[researchers] find that the entropy corresponding to three translational and three rotational degrees of freedom lost when two molecules associate amounts to 70 to 100calorie/degree/mole, equivalent to a free energy ?G[S][D] of 20 to 30 kcal/mole at 27°C (in the standard state)."