Kinetic and equilibrium data for the binding of imidazole to horse heart ferricytochrome c at temperature range of 303-319 K were determine at pH = 7.04 by using 1H NMR double resonance method. From van't Hoff and Arrhenius equations, the observed thermodn. parameters for imidazole binding are DH = 48.5 kJ/mol, DS = 184 J/mol/K, Ea = 159 kJ/mol and were compared with those of other ferric hemoproteins and a model complex. The reason of lower affinity of cytochrome c for imidazole was discussed. Saturation transfer experiments were used to assign the resolved hyperfine-shifted resonances of ferricytochrome c imidazole ring methyls. That the imidazole binds to iron by displacing the axial coordinated Met 80 was confirmed by NMR.

Key words: REACTION MECHANISM, IMIDAZOLE, PROTON MAGNETIC RESONANCE SPECTROMETRY, CYTOCHROME, THERMODYNAMIC FUNCTION