290 6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



dii)ole contributions is approximate since the dielectric constant is difficult 

 to estimate in this situation and provides over-all energy differences which 

 could not be satisfactory for both chlorine and bromine derivatives. Some 

 factor, other than ion-dipole interaction, must be involved here. 



Dispersion Energy of Benzene Ring and Cholinesterase 



The inhibition constant K^ for H-N(CH3)3+ is 1.34 mM (Wilson, 1952a) 

 and for cp-N(CH3)3+ is 0.038 mM (Wilson and Quan, 1958). This 

 corresponds to an energy difference of 2.10 kcal/mole which probably 

 can be attributed to dispersion forces between the benzene ring and the 

 adjacent protein. Calculation of the average interaction distance can be 

 made using Eq. 6-116 and assuming the following values: R^y = 3.67, 

 R,,,, = 25.11, R,, = 1.65, 7?,,„, = 21.12, Z„. = 8, Z,,„, == 24, Z,, = 2, 

 ^i:rot = 8, and n = 4. It is probable that the benzene ring lies flat on the 

 protein surface and thus interacts with one side only. The distance d^ 

 is found to be 4.16 A, which is reasonable, inasmuch as the closest approach 

 would be 3.65 A and it is unlikely that there is exact fit between the ben- 

 zene ring and the protein. 



Effect of Substituent Position on Interaction Energy 



The inhibition constants for various substituted phenyltrimethylam- 

 monium ions were determined by Wilson and Quan (1958) and are shown 

 in Table 6-25 with the calculated free energy differences using the unsub- 

 stituted ion as a standard. The introduction of an hydroxyl group in the 

 3-position increases the binding energy by 2.84 kcal/mole, indicating the 

 formation of a hydrogen bond between this hydroxyl group and some 

 adjacent group on the protein. That this is a strong hydrogen bond rather 

 than a dispersion interaction is suggested by the magnitude of the energy 

 difference, the fact that the corresponding methyl and methoxy compounds 

 are much less tightly bound, and the poorer binding of the 2 — OH and 

 4— OH derivatives. The hydrogen bond could be formed with the hydroxyl 

 hydrogen atom in one of the following two positions: 



Z 



I 



H 



H 



N(CH3)3+ N(CH3)3^ 



(I) (II) 



