446 



CHLOROPHYLL 



CHAP. 16 



In Rothemund's method (c/. also Aronoflf and Calvin 1943), pyrrole and formalde- 

 hyde are condensed in methanol: 



(16.1) 4 C4H5N + 4 H2CO > C20HUN4 + 4 H2O + 6 {H ) 



The product contains 11 double bonds, as against eight in the four pyrrole nuclei; 

 condensation is thus accompanied by the removal of six hydrogen atoms. Of the 11 

 double bonds in porphin, nine form a closed conjugated system, and two are "semi- 

 isolated." This allows of several structures, which may be mesomeric or isomeric, 

 depending on the position of the imino hydrogens. 



In the porphin formula (16. VI), the two structures, A and B, are 

 isomeric — one is a "lateral," another a "diagonal." Each consists of 

 two mesomeric "K6kul6 structures." It may be, however, that the two 

 central hydrogen atoms are situated symmetrically between two adjacent 

 nitrogen atoms. This would make structures A and B mesomeric. 



A B 



Formula 16. VI. 

 The "diagonal" and "lateral" forms of porphin. 



It is interesting that the structure with nine conjugated double 

 bonds arises in preference to the more symmetric structures with eight 

 or ten conjugated double bonds, represented by 16. VII A and 16. VII B. 



The derivatives of porphin are called porphyrins; hemin is an iron 

 complex of a porphyrin; protochlorophyll is a magnesium complex of 

 another porphyrin. Chlorophyll is derived (as mentioned above) from 



A B 



Formula 16.VII. 

 Ring systems C20N4H12 (A, with eight conjugated double bonds), and C20N4H8 (B, 

 with ten conjugated double bonds), which appear to be less favorable than the porphin 

 system C20N4H10 with nine double bonds. 



