Enthalpy electrostatic interactions, estimates

Molecular mechanics (AMBER, Singh et al., 1986) was employed by the authors to study the effect of the whole system (protein + solvent) on the scissile bond in the Michaelis complex and in the TI. They find that interaction enthalpy is more favorable in the TI due to stabilization by the catalytic triad, by the oxyanion hole and by the environment, a total of 26 kcal/mol. The breakdown to interaction components demonstrates the importance of electrostatic stabilization of the TI. More reliable estimates of these interactions require free energy perturbations or related free energy methods. 

Michaeli and Overbeek used the Debye-Huckel approach to estimate the enthalpy of the PE-PE and PE-solvent interaction [23]. In this model, the Flory entropy is used as the entropic term of the PE chain, and the electrostatic free enthalpy is calculated from the electrostatic energy of a strong PE [23]. By thermodynamic considerations, Michaeli and Overbeek showed that for small univalent counterions no coacervation (complex-formation between PE) occurs... 

The water molecule has a dipole moment, fx, of 1.84 X 10 esu-cm (esu = electrostatic unit). If water dipoles in ice are separated by 2.76 A, estimate the potential intermolecular attraction energy (in kcal/mole) due to oriented dipole-dipole interaction. Compare this quantity with the enthalpy of sublimation of ice (11.3 kcal/mole), which represents the amount of energy needed to break these bonds. Why might this calculation underestimate the actual energy of water molecule interaction ... 

---