Reaction between hydrocyanic acid, cyanide ion and ferricytochrome c

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Abstract
A kinetic investigation of the formation of the ferricytochrome c-cyanide complex over the pH range 6-9 suggests that both un-ionized HCN and the free CN ion react:[image] Analysing the data according to this mechanism, the bimolecular constants kHCN and kCN- were found to be 5.43 x 10-2 and 15.2 [image] -1 sec. -1 respectively, at 24.6[degree]. From exptl. data at various temp., the activation energies were determined as 18.4 [plus or minus] 0.4 and 17.0 [plus or minus] 0.5 k cal., respectively. Using these velocity constants and equilibrium data obtained over the pH range 5.8-7.5, the dissociation velocity constants were calculated and found to be of the form[image] where k'' CN-and k'' HCN can be identified as the reverse of reactions (2) and (1), respectively. At 24.6[degree] k''CN- and k''HCN are 1.25 x 10-5 sec. -1 and 1.13 x 102 [image] -1 sec.-1 respectively and the activation energies 15.9 [plus or minus] 1.0 and 9.2 [plus or minus] 1.0 k cal. Combining these kinetic data gives the values for the equilibrium constants for reactions (1) and (2), K1 = 4.8 x 10-4 and K2 = 1.22 x 106 [image] -l. The corresponding changes in heat content and entropy are 9.2 [plus or minus] 1.4 k cal. and 15.7 [plus or minus] 4.4 e.u.; and 1.1 [plus or minus] 1.5 kg. cal. and + 31.3 [plus or minus] 4.7 e.u. respectively. For such reactions the relationship K1/K2 = Ka should hold, where Ka is the dissociation constant of HCN: this was found to be so within the exptl. error. Kinetic expts. in alkaline solns., pH 9-10 showed that the velocity constant for the formation of the complex decreases, which may be attributed to an ionization on the ferricytochrome c molecule such that the formation of the complex occurs predominantly by reaction of the acid form. The pK for this ionization was calculated as 8.75 at 24.6[degree] and the heat and entropy of ionization were found to be 2.9 [plus or minus] 0.7 k cal. and -30 [plus or minus] 3 e. u. respectively. The quantitative data given above support a model for ferricytochrome c in which the hematin iron atom is firmly bound on both sides to the protein.