I. INTRODUCTION 



Theoretically this system is not the most unsaturated one, the 

 latter being that represented by Figure 5. So far no derivative of 

 the latter ring system (Fig. 5) has been discovered, and it appears 

 doubtful whether it exists. 



TABLE I 

 TetrapyiTolic Compounds 



I. Closed ring 



II. Open ring 



Porphyrins 



Free porphyrins 

 Iron complexes (hematins) 

 Linked to protein 

 Hematin enzymes 

 Hemoglobin 



Dihydro- and tefrahydroporphyrins 

 Chlorins, rhodins 



Magnesium complexes (chlorophylls, 

 bacteriochlorophyll) 

 Esterified with phytol 

 Linked to protein.^ 



C. Bile pigments 



Bilirubin, biliverdin, urobilins 

 (Zn, Cu, Fe complexes) 

 Linked to protein (metal-free) 



Phycoerythrin 



Phycocyanin 



Porphyrins are found free in nature, but are far more important 

 in the form of iron complexes (hematins), in which the iron replaces 

 the two central hydrogen atoms of Figure 4, but is bound equally by 

 all four nitrogen atoms. In the biologically important hematin com- 



Figure 4 



Figure 5 



pounds — hemoglobin and the hematin enzymes — we find this 

 complex as the prosthetic group of a hemoprotein. 



The second group of closed ring compounds comprises those with 

 ring systems containing two (chlorophylls) or four (bacteriochloro- 

 phyll) hydrogen atoms more than the porphin system. Chlorins and 



