52 R. J. P. Williams 



course this by itself does not eliminate the possibility that electron transport 

 occurs across both the porphyrin and the imidazole in cytochrome c. 



NOTES 



1. The position of the Soret band moves in the order of increasing wavelength with 

 ligand ammonia, pyridine, carbon monoxide and oxygen, cyanide, water. This order, 

 saturated bases < unsaturated bases < water is similar though not exactly the same as 

 that found for the a and |3 bands and the explanation which we offer is that given earlier 

 (Williams, 1956). 



2. We imply here that the ferrous ion is more stabilized than the ferric ion by 7r-electron 

 acceptors. The evidence is given elsewhere (Tomkinson and Williams, 1958). Thus in 

 Fig. 2 we expect that the maximum will be more sharply defined in a series of pyridine 

 complexes of increasing pyridine basicity than in a series of ammines. Imidazoles will 

 occupy an intermediate position while for a series of oxygen anion-donors there need be no 

 maximum as the ferric ion may well go over into the strong-field complex the more readily. 



SUMMARY 



An account is given of the properties of iron-porphyrin complexes of 

 biological interest which is largely developed from a consideration of the 

 properties of simpler iron complexes. Spectroscopic criteria for distinguishing 

 between high- and low-spin complexes are suggested. New features of the 

 inter-relationship of different cytochromes are proposed, based upon their 

 redox potentials and their chemical reactions. Some comments are made 

 upon the reactions of haemoglobin and their pH dependences. A discussion 

 of the model iron complexes which are autoxidizable as opposed to those 

 which can carry oxygen leads to a discussion of autoxidation and oxygenation 

 of haem complexes. 



A cknowledgement 



I would like to acknowledge the help of the late B. A. Jillot, and of J. M. F. 

 Drake and D. Croft, who have done all the experimental work connected 

 with this paper. 



REFERENCES 



Coryell, C. D. & Pauling, L. (1936). Proc. nat. Acad. Sci. Wash. 22, 159. 



Croft, D. & Williams, R. J. P. Unpublished observations. 



Gibson, J. F. (1959). Disc. Faraday Soc. 29. 



James, B. R. & Williams, R. J. P. Unpublished observations. 



Jillot, B. A. & Williams, R. J. P. (1958). J. chem. Soc, A61. 



Leberman, R. &. Rabin, B. R. (1959). Nature, Lond. 183, 746. 



Lemberg, R. & Legge, J. W. (1949). Hematin Compounds and Bile Pigments, Chap. 5 & 



6, Interscience, New York. 

 Morton, R. K. (1958). Rev. pure appl. Chem. 8, 161. 

 Scheler, W., Schoffa, G. & Jung, F. (1957). Biochem. Z. 329, 232. 

 Sekuzu, I., Takemori, S., Yonetani, T. & Okunuki, K. (1959). J. Biochem. Tokyo 46, 43. 

 Theorell, H. (1942). Ark. Kemi. Min. Geol. 16A, No. 3. 

 Tomkinson, J. & Williams. R. J. P. (1958). J. chem. Soc, 2010, 



