214 Discussion 



A cknowledgement 



The author is indebted to Professor H. Tuppy for laboratory facilities and for 

 helpful and friendly discussion. The John Simon Guggenheim Foundation provided 

 financial support. 



REFERENCES 



Fischer, H. & Orth, H. (1937). The Chemistry of the Pyrroles. Leipzig. 

 Heikel, T., Lockwood, W. H. &. Rimington, C. (1958). Nature, Lond. 182, 313. 

 Mauzerall, D. & Granick, S. (1958). /. biol. Chem. 131, 1141. 

 Morell, D. B. & Stewart, M. (1956). Aust. J. e.xp. Biol. med. Sci. 34, 211. 

 Neilands, J. B. & Tuppy, H. (1960). Biochim. biophys. Acta 38, 351. 

 Orlando, J. (1958). Doctoral dissertation. University of California. Berkeley. 

 Paleus, S. & Neilands, J. B. (1950). Acta chem. Scand. 4, 1024. 

 Pauling, L. & Coryell, C. D. (1936). Proc. nat. Acad. Sci. Wash. 21, 159. 

 Zeile, K. & Meyer, H. (1939). Hoppe-Seyl. Z. 262, 178. 



Metal Incorporation in Model Systems 



Phillips: I should like to make three comments on Neilands' very interesting contribution. 



1 . The slowing down of metal ion incorporation into porphyrin c by the addition 

 of detergents could well be due to solubilization removing the porphyrin from the 

 aqueous (reactive) environment, rather than to an interaction between the detergent 

 and the metal ion. 



2. It is of interest to note the differential incorporation of Cu++ and Zn++ on the 

 one hand as compared with the other metal ions (Co++, Fe++ and Mn++). This is 

 similar to our results in detergent solution. We have recently attempted to check our 

 hypothesis that incorporation proceeds more readily when the metal ion is tetra- 

 co-ordinated by studying the incorporation of Co++ into dimethyl protoporphyrin 

 ester under various conditions. Co++ has the useful experimental advantage of being 

 pink in the octahedral configuration and blue in the tetrahedral configuration. Pre- 

 liminary experiments suggest that the rate of incorporation of Co++ is a direct function 

 of the tetrahedral character of the Co++ ion, a result which appears to apply to both 

 aqueous and non-aqueous media. 



3. With respect to the mechanism of the rapid incorporation of Fe++ into copro- 

 porphyrin in the presence of Na-amalgam, there is a possible alternative explanation 



Fig. 1. The hexahydroporphyrin structure of maximum double bond 

 conjugation. 



to that suggested by Neilands. It is simply this: as the porphyrin is progressively 

 reduced the degree of conjugation throughout the nucleus is markedly decreased; 

 indeed a critical stage is reached when 6 H atoms have been added. The classical 

 structure demands a break in the cyclic conjugation, but this can be avoided by a 

 tautomeric shift of the two hydrogen atoms from the ring nitrogens to the beta pyrrole 

 carbon atoms, to give the structure shown in Fig. 1. Such a structure would not 



