INTERACTIONS OF AROMATIC COMPOUNDS 



299 



for the case of DPN or TPN where W represents the total number of water 

 molecules in contact with the reactants. When 40 A^ was taken as the area 

 of interaction, the following energy values were calculated: 



steroid-W -^ steroid + W 



coenzyme-W -^ coenzyme + W 

 steroid + coenzyme ^ steroid-coenzyme 

 W + W -^ W-W 



+ 5.3 kcal/mole 

 + 7.6 

 - 5.7 

 -13.4 



— 6.2 kcal/mole 



Values around — 6 kcal/mole were obtained experimentally but they 

 point out that the close agreement is prol^ably fortuitous inasmuch as the 

 energy calculations are only approximate. However, two important con- 

 clusions may be drawn from these results. In the first place, it would in- 

 dicate that the major contribution to such complexes is dispersion with 

 regard to the forces between the reactants but that the largest factor is the 

 recombination of the displaced water molecules. Secondly, it points out 

 the necessity of taking interactions between inhibitor and coenzyme into 

 account in the inhibitions by steroids of the /?-hydroxysteroid dehydro- 

 genase reported by Marcus and Talalay (1955), since this enzyme is DPN- 

 dependent. 



The dispersion interaction between the benzene ring and various alkyl 

 groups has been treated by T. L. Brown (1959) who calculated the energies 

 when the groups were attached to the ring (see tabulation below). 



The polarizability of the benzene ring normal to its axis was taken as 12.3 X 

 10"^* ml. The amount of dispersion energy is thus quite appreciable and this 

 must play a major role in inductive effects on the ring. When these aromatic 

 molecules are in solution, it w^ould be expected that the solvent would 

 reduce somewhat the interaction energy. 



