Cavity formation in water

Postma, J. Berendsen, H. Haak, J., Thermodynamics of cavity formation in water. A molecular dynamics study, Far. Symp. Chem. Soc. 1982,17, 55-67... 

For an atom in the enzyme or the substrate to interact with the solvent it must be able to form Van der Waals contact with water molecules. The accessible surface area of an atom is defined as the area on the surface of a sphere, radius R on each point of which the centre of a solvent molecule can be placed in contact with the atom without penetrating any other atoms of the molecule (Fig. 12). R is the sum of the Van der Waals radii of the atom and solvent molecule [27]. There is a linear relationship between the solubility of hydrocarbons and the surface area of the cavity they form in water [28]. It has been estimated that the hydrophobicity of residues in proteins is 100 J/mole/A of accessible surface area [29]. The surface tension of water is 72 dynes/cm so to form a free surface area of water of 1 A costs 435 J/mole/A. The implication is that the free energy of cavity formation in water to receive the hydrophobic group is offset by favourable interactions (dispersion forces) between the solute and water. 

Postrna J P M, H J C Berendsen and J R Haak 1982 Thermod)mamics of Cavity Formation in Water Faraday Symposium of the Chemical Society 17 55-67. 

Thermodynamics of Cavity Formation in Water, a Molecular Dynamics Study. 

As far as the enthalpies of hydration are concerned, the computations lead instead at small and positive values of H, due to the small values of the coefficient of thermal expansion Up for water, which do not agree with the negative and relatively large in magnitude enthalpy changes observed for the solution of non-polar gases in water(132). As an example Arakawa(132) has calculated for the methane (02 = 3.59 A) the following values of the thermodynamic functions for cavity formation in water at 25°C = 22.6 k J mol , ... 

Cavity Formation in Water and n-hexane Using the Widom Particle Insertion Method. 

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