PHILIP GEORGE 



magnitudes of ki and k[. The cyanide complex is formed by 

 reaction of CN~ just a little more readily than by HCN, whereas 



TABLE III 



Kinetic Data for the Ferrimyoglobin-Fluoride Reaction at 20 °G. and 



I = 0.40 (16,18)" 



ki = 1.7 X 10-» 

 Y-Fe+(H20) + F- =± Y FeF + HjO (1) 



*-i= 4.6 X 10-4 



ki' = 1.4 X 104 

 Y-Fe+(H20) + HF =± Y FeF + H3O+ (1') 



k-i' = 5.7 X IC 

 " For units see Table IL 



the formation of the fluoride complex by reaction with HF is about 

 a million times faster than the reaction with F~. A possible 

 explanation lies in the relative magnitudes of the solvation 

 energies of the ions. The fluoride ion is more highly solvated 

 than the cyanide ion, and in forming a complex, one, at least, 

 of the water molecules must be removed from the solvation shell. 

 The energy required to do this, reflected in a high activation 

 energy for the F" reaction, could thus account for the result. 



These data provide a quantitative background against 

 which the effect of pH on the biologically important hemo- 

 protein reactions can be judged. For example, in the oxy- 

 genation of hemoglobin (the Bohr effect) it is small in magnitude 

 as with the ferrimyoglobin reactions that involve neutral mole- 

 cules, viz., HCN and H2O. The equilibrium constant for 

 combination with oxygen is about 8 times greater in alkaline 

 solution as a consequence of an 8-fold decrease in the dissociation 

 velocity constant, the formation velocity constant being sub- 

 stantially the same (8,29). The marked contrast between these 

 reactions and those involving ionic species, e.g., CN~ and 

 H3O+, where the ionization of the heme-linked groups affects 

 the velocity constants far more, suggests that the same kind of 

 theoretical treatment (27) which has proved so successful in 



350 



