HsC 



H3C 



CH=CH. 



CHj 



CHj 



H3C 



2*^5 



H3C 



C2oH3900C(CH2)2 



H— C— C=0 



I 

 COOCH3 



(b) 



Fig. 22. Tetrapyrole structures in biological pigments, (a) protoheme, com- 

 bined with protein in the red respiratory animal pigments hemoglobin and 

 erythrocruorin; (b) chlorophyll A, found in green plants. In each of these 

 complex structures the four bonds of the central metal atom represent a 

 resonant or highly reactive condition. (Adapted from Blum, Time's Arrow and 

 Evolution, Princeton University Press.) 



