Ferrihaemoprotein Hydroxides 



133 



simplest interpretation of this result is that for the Fej]^+++ — OD formed in 

 heavy water the fraction of the low-spin form is about 6 % higher than the 

 fraction for Fejj^+++ — OH under similar conditions. Using the data given 



Table 5. The fractions of the low-spin and 

 high-spin forms of ferrimyoglobin hydroxide, 

 a and (1 — a) respectively, at different 

 temperatures, calculated from the temper- 

 ATURE VARIATION OF Xm FOR FERRIMYOGLOBIN 

 HYDROXIDE, FLUORIDE, AND CYANIDE 



Kg is the equilibrium constant for the conversion 



high-spin form ^^ low-spin form 

 and is given by a/(l — a). 



in Table 5 for the hydroxide, an approximate value of the equilibrium con- 

 stant for the reaction 



high-spin deuteroxide ^^ low-spin deuteroxide 



(18) 



is found to be 0-55 at 25°C, compared to 0-41 for the hydroxide. Substitution 

 of hydrogen by deuterium thus favours the conversion by about 0-2 kcal/mole 

 in units of free energy. It is probable that the bulk of this difference arises 

 via the water of solvation and not from any effect on the ligand field. The 

 change of mass of the hydrogen nucleus affects the free energy of 'crystalliza- 

 tion' around the iron ion directly but the ligand field only very indirectly 

 through the effect of the change of vibrational amplitudes for the OH group 

 on the mean ligand field. 



GENERAL REMARKS 



The experiments with heavy water offer particularly direct evidence for the 

 existence of a thermal mixture in ferrimyoglobin hydroxide. Further, 

 because the fundamental difference between the high- and low-spin form lies 

 in the electronic structure of the iron ion, they show that water molecules 

 (or those protons which are exchangeable with those of water) play an 

 essential part in determining the free energy change. We would naturally 

 guess that the water molecules which are 'crystallized' around the iron ion 

 (and also the hydrogen of the OH~ group) are the ones concerned here. 



