The Structure of Porphyrin a, Cryptoporphyrin a and Chlorin ag 347 



Vinyl alone does not cause a rhodo-type spectrum. This made Lemberg 

 and Falk (1951) postulate the presence of an acrylic acid side chain, which as 

 vinylogue of the carboxyl side chain has a strong rhodofying effect. How- 

 ever, later evidence has not confirmed this assumption. Cytodeuteroporphyrin 

 has two propionic acid side chains, and the infra-red spectrum of porphyrin a 

 ester shows only the 1737 cm~^ band due to the propionic acid side chains, 

 while the acrylic ester should have a band at about 1725 cm"^. 



It is, in fact possible to account for both the differences between porphyrin a 

 and chlorocruoroporphyrin on the basis of two rhodofying groups, formyl 

 and vinyl (or substituted vinyl) on opposite pyrrole rings, together with the 

 presence of an a-hydroxyalkyl group on the pyrrole ring in between (see 

 below). The average displacement of bands towards the infra-red caused by 

 replacement of an alkyl by an a-hydroxyalkyl is 1 m/<, in agreement with the 

 difference of band positions of porphyrin a and chlorocruoroporphyrin 

 except for band III (3-5 m/<). This band may be shifted further towards the 

 red in porphyrin a owing to overlapping with the high band II, which in 

 turn is shifted towards the blue in porphyrin a. 



Whereas the position of absorption bands is little influenced by the 

 relative position of the substituents, this is not so for band intensities. A 

 second rhodofying group increases the ratio III/IV very strongly, if it sub- 

 stitutes the pyrrole ring opposite to the first; it decreases it if it substitutes 

 a vicinal pyrrole ring, independent of whether the relative position is 1-4, 

 2-4 or 2-3 (cf. Table 2, No. 1-5). 



Table 2. Antirhodofying effects of rhodofytng groups on vicinal pyrroles 



No. 



Porphyrin in chloroform 



Relative position and nature 

 of groups 



Type of spectrum 



1 . Diformyldeutero 



2. 3-Desmethyl-3-formylrhodo 



3. Rhodinporphyrin gs 



4. Neorhodinporphyrin ga 



but 



5. Rhodoporphyrin gg 



2F 



3 F 

 3 F 

 2 V 



4F 



6 CO.,H 



6 CO,H (y—CH,) 



3 F (y-CH,) 



2 E 3 F (y-CU,) 



Actio 



Rhodo 



F = formyl V = vinyl 



2 3 

 7 6 



y = methene bridge. 



Positions 



Table 3 shows that the vinyl group has also a rhodofying effect if it substi- 

 tutes an opposite pyrrole (cf. 7 with 6, 10 with 9, Table 3) and an anti-rhodo- 

 fying effect if it substitutes a vicinal pyrrole (cf. 3 with 1, Table 3, or 4 with 5, 

 Table 2). Thus it raises R III/IV from 1-29 to 1-89 opposite to a carboxyl, 



H.E. — VOL. I — Y 



