28 Discussion 



higher oxidation state is very unstable, TPP yields both a one-equivalent and a two- 

 equivalent higher oxidation state that are appreciably more stable, i.e., 



one one 

 TPP ^ ^ TPP^ ^ ^ TPP" 



equiv. equiv. 



In phosphoric acid solution TPP is bright green, TPP' and TPP" are dull violet and 

 orange-brown respectively. These oxidations are completely reversible Uke the one- 

 equivalent oxidation of copper phthalocyanine, the addition of ascorbic acid, hydro- 

 quinone or ferrous salts regenerating the TPP. The structure of TPP" probably 

 corresponds to the removal of the two pyrrole hydrogen atoms with the introduction 

 of a new double bond into the ring system, as in the oxidation of reduced flavin. The 

 copper salt of TPP yields a one-equivalent higher oxidation state Uke the phthalocy- 

 anine derivative, but on addition of more oxidant a whole series of highly coloured 

 products are formed from which the original compound can no longer be recovered 

 by the addition of reducing agents (George and Goldstein, Abstracts \19th Meeting 

 Amer. chem. Soc, Dallas, K 16, p. 13, 1956; George, Ingram and Bennett, J. Amer. 

 chem. Soc. 79, 1870, 1957). 



Effects of Metal on Reactivity at Periphery 



Barrett: Concerning Dwyer's remarks on the effect of the introduction of metals into 

 porphyrins, and the consequent events occurring at the periphery of the molecule, I 

 would like to make this comment. Fischer and Bock (Hoppe-Seyl. Z. 255, 1, 1931) 

 exposed protoporphyrin in pyridine solution to light and obtained a substance with 

 a chlorin-like spectrum. The substance is not a true chlorin, or dihydroporphyrin, 

 but carries two or possibly three oxygen atoms. The addition of these oxygen atoms 

 results in the formation of a hydroxy group and a carbonyl group (Barrett, Nature^ 

 Loud. 183, 1185, 1959). A vinyl group is necessary for the formation of dioxy- 

 protoporphyrin. Pertinent to Dwyer's remarks is the observation that photo-oxidation 

 of the tetrapyrrole does not occur when complexed with a metal, e.g. Cu++, Fe+++, 

 or if irradiated in 1-10% hydrochloric acid. Could Dwyer comment on these effects: 

 the suppression of photo-oxidation by (1) the formation of a metal complex and (2) 

 the formation of the di-hydrochloride ? 



DwYER : One can anticipate a common effect as far as the peripheral charge is concerned 

 by either protonation or the formation of a metal complex. However, I feel that the 

 altered charge distribution is not per se the reason for the inhibition of photo- 

 oxidation, but rather the effect of the proton or the metal is on the fluorescence of 

 the protoporphyrin, and hence its ability to form active oxygen (or hydroxyl) which 

 is presumably the attacking agent. 



