The Role of the Metal in Porphyrin Complexes Tl 



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DISCUSSION 



Higher Oxidation States 



Lemberg : While it is certainly correct that we have strongly interacting systems in which 

 both the metal and the ligand are in unique electronic states because of their combina- 

 tion, I feel that Dwyer has somewhat overstated his case. In such instances as the 

 RO2H or HjOa-complexes of peroxidase, catalase and ferrimyoglobin, one may well 

 be in doubt about the exact valency state of the iron, but in most other haemoprotein 

 compounds there is little doubt about the valency of the iron. 



George: Gibson and Ingram {Nature, Lond. 178, 871, 1956) demonstrated the presence 

 of a free radical in the oxidation of ferrimyoglobin by H2O2 by paramagnetic resonance 

 absorption measurements, and identified this with the higher oxidation state, Mb'^. 

 However, in more recent experiments, Gibson, Ingram and NichoUs {Nature, Lond. 

 181, 1398, 1958) have shown the radical to be present in much lower concentration 

 than the oxidation state IV of the prosthetic group, invalidating the previous 

 conclusion. 



It is not unexpected that radicals can be detected in such a system because there is 

 already ample chemical evidence for the production of a radical in the formation 

 reaction, i.e. 



Mb"' -h H2O2 -> Mb^ -f- radical 



and furthermore, in the reduction of Mb"", which occurs spontaneously and more 

 rapidly the higher the concentration, radical species must again be formed, since 

 Mb'^ is a one-equivalent oxidation product of Mb"'. (George and Irvine, Biochem. 

 /. 52, 511, 1952.) 



Some years ago it was shown that a simple radical structure of the type that was 

 proposed by Gibson and Ingram would not account for the hydrogen ion participation 

 in Mb'^ reactions, whereas the "ferryl ion" structure or an isomer of this structure 

 is in accord with the experimental data (George and Irvine, Sympos. on Coordination 

 Compounds, Danish chem. Soc, p. 135, 1954; Biochem. J. 60, 596, 1955). 



It should be emphasized that although paramagnetic resonance absorption provides 

 an excellent technique for the detection of free radicals, other evidence must also be 

 considered in discussing possible structures for intermediates in oxidation-reduction 

 reactions. The same is true of the mechanism of oxidation-reduction reactions, since 

 radical species could be formed in side reactions, and not necessarily be involved in 

 the principal reaction path. 

 George: Another example where higher oxidation states are formed, somewhat similar 

 to that of the Cu", Co", Zn" and Al"' phthalocyanines, is that of a-/3-}'-5-tetraphenyl- 

 porphin (TPP). Whereas with the metal-free phthalocyanine the one-equivalent 

 H.E. — vol. 1 — D 



