1. Haem c was synthesized and purified. It was shown unequivocally that the method gives a product with the cysteine residues on the -carbon atoms at the 2 and 4 positions of the haem.2. Redox potentials of haem c in the presence of 2.5 M pyridine were determined in the pH range 1.5–13; it was found necessary to add cetyl trimethyl ammonium bromide (CTAB) to prevent precipitation in the acid range below about pH 4. The Em vs pH curve shows three slopes (−dE/dpH) of value, 0.18, 0.01 and 0.06 with points of inflexion at pH 3.8 and 10.6. The potentials are intermediate between those of protohaem and mesohaem obtained under similar conditions. 3. With constant haem c concentration (a) 10−4 M and (b) 10−5 M and varying pyridine concentration (0.12–5 M) it was found at pH 9.0 that Em values increased as the pyridine concentration was increased and there was a tendency to reach a plateau value. The explanation appears to be that pyridine binds more firmly to ferroporphyrin c than to ferriporhyrin c. 4. When the pyridine concentration was kept constant (2.5 M) and the haem c concentration was varied in the range 7 · 10−4–7 · 10−6 M, it was found that a decrease in haem c concentration brought about an increase in redox potential. The results are explained as being due to dimerization of the oxidized form. 5. The results are discussed in comparison with a number of related haem systems. |