Conformational transitions of ferricytochrome c in strong inorganic acids

Conformational transitions of horse heart ferricytochrome c (ferricyt c) have been investigated in the presence of strong inorganic acids and their salts by optical absorption spectroscopy, magnetic circular dichroism and circular dichroism. In the presence of acids (HClO4 or H2SO4, pH 2) or their salts (1 M NaClO4 or Na2SO4, pH 2, 25 degrees C), the three ligation states of ferricyt c heme were identified. One is the high-spin state: His18-Fe-H2O (40 - 50%), and two are the low-spin states: His18-Fe-Met80 (30 - 25%) and His18-Fe-His (30 - 25%). Under these conditions low temperatures facilitate native heme coordination of ferricyt c. Transition from low-spin to high-spin heme coordination of ferricyt c is complete in 1 M HClO4 or 3 M H2SO4. At the concentration of HClO4 and H2SO4 above 3 M, different behavior in spectral transitions of ferricyt c near the heme is observed. High-spin pentacoordinated ferricyt c with the heme ligand of His18-Fe is formed in 8 M H2SO4. This state is unstable at higher concentration of H2SO4 and porphyrin ferricyt c is formed. At HClO4 concentration higher than 3 M, the new, until this time not observed heme coordination structure of ferricyt c originates.