214 6. INTEKACTIONS OF INHIBITORS WITH ENZYMES 



the mutual orientation polarization they exert, in which case the potential 

 energy is: 



SkTd'D' 



(6-20) 



These expressions may be derived from the electrical field around a dipole, 

 the orientation such a field produces on a freely rotating dipole, and the 

 interaction of the field with the average moment induced. The energy is 

 always negative since orientation tends towards the most stable state. 

 Equations of type (p = — A fj.^fi^jkTd^D^, such as 6-19 and 6-20, may 

 be written: 



9, = - 350 A ''" 'l^" kcal/mole (6-21) 



when the //'s are in debyes and (/ in A. Thus the potential energy of two 

 freely rotating dipoles of fi — 2 debyes at a distance of 5 A in a vacuum 

 would be — 0.24 kcal/mole. For this treatment to be valid, the potential 

 energy must be less than the thermal energy A'T. for otherwise the interaction 

 would be strong enough to orient the dipoles in relatively fixed positions 

 and equations of type 6-18 would be applicable. The dependence of the 

 potential energy of two dipoles upon distance will thus be related to the 

 magnitudes of the moments and the ability to approach close enough to 

 produce a partially rigid configuration. 



Hydrogen Bonding 



Hydrogen bonds are said to occur when electronegative atoms are bonded 

 by a hydrogen atom between them. The bond energy is less than that in- 

 volved in covalent bonding and is usually of the order of 2-10 kcal/mole. 

 Such a bond between nitrogen and oxygen is usually written as 0— H ••• N 

 where the — H bond is essentially covalent and the dots indicate the hy- 

 drogen bond. Hydrogen cannot form two covalent bonds and it is not a 

 matter of resonance of the hydrogen atoms between the two electronegative 

 atoms because the bonds are asymmetrical. The explanation is probably 

 to be sought in the electrostatic interaction of bond dipoles. If A and B 

 are electronegative atoms, they will usually possess a fractional negative 

 charge when bonded to other atoms, due to the ability of such atoms to 

 distort the electronic configuration. Thus the bond A— H is a dipole and 

 may be written A~— H^, these charges being fractions of electronic and 

 protonic charges. The bond B— R will similarly be a dipole and may be 

 written B~— R"^. The interaction of these two bonds in the appropriate 

 head-to-tail position can be represented as follows: 



A~-H+ B~-R+ 



