254 



6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



Vquad to qiiaclrupole interactions, (p^ to induced dipoles and miiltipoles, cp^ 

 to dispersion forces, 99 ^ to lateral interactions between the tetrahedral 

 water molecules, cp^ to the interactions with water molecules outside the 

 hydration layer, q)/^ to the energy required to make a hole in the water 

 structure, and <p^ to the repulsion energy arising from atomic overlap. These 

 individual values were calculated for several ions and compared to the 

 experimental hydration energies; the treatment generally overestimates the 

 energy by 12-15 kcal/mole, and this was attributed to the rigidity of the 

 assumed water structure. However, the neglect of the repulsion correction 

 may account for this difference also, inasmuch as it was assumed that cp^ 

 is zero unless the interacting surfaces are in contact; actually the repulsion 

 forces do contribute appreciably in hydration. This factor was included in 

 the equations used in the previous calculation of the hydration energy. 



Brady (1958) calculated the hydration energy for K^ assuming four wa- 

 ter molecules oriented with their two centers of negative charge towards 

 the ion. The existence of two such negative centers in water is probably 

 questionable. The hydration energy was written as: 



2r 



1 

 1) 



+ wP 



(6-78) 



The first term is the Born expression for the energy required to charge a 

 sphere of radius r = r,o,j + 2r^^,^tg^ in a medium of dielectric constant D. 

 The second term is the product of the number of water molecules by the 

 potential energy of each in the hydration layer, P being given by: 



fie' 



G/i- 



K'^ion I 'water) 



(6-79) 



which represents the coulombic and the dipolar interactions, /?e being the 

 fractional charge on the H or atoms and G a geometrical factor dependent 

 on the hydration number. The third term takes account of the energy lost 



