



CHAPTER XIV 



PYRROLE PIGMENTS IN EVOLUTION 



1. INTRODUCTION 



The color of substances arises from their abihty to absorb selec- 

 tively energy of certain wavelengths in the visible spectrum. Physico- 

 chemically this is due to resonance in the absorbing molecule. The 

 pyrrole pigments are at the same time the most highly resonant and 

 the most deeply colored biological compounds, and their absorption 

 reaches far into the region of the longer wavelengths of the red, in 

 bacteriochlorophyll even into the infrared. 



For the functional importance of hematin derivatives and other 

 catalysts in respiration, only the resonance, not the color, is of funda- 

 mental importance {1685). The resonance confers on the molecule 

 sufficient stability to permit the existence of free radicals in aqueous 

 solution. In conjunction with the monovalent change of the iron 

 valency, this is of fundamental importance for the catalysis of 

 respiration {191^2,2516; cf. Chapter VIII), for the activation of hydro- 

 gen by hydrogenase and probably also for the action of chlorophyll 

 as hydrogen donor in photoassimilation {94.0). Though neither the 

 role of bivalent iron in hydrogenase nor that of magnesium in chloro- 

 phyll is as yet understood, it would appear that the most important 

 property which led to the selection of these pyrrole pigments for 

 biological catalysis was their high degree of resonance. 



At the present time, indeed, pyrrole pigments are found almost uni- 

 versally, in the most advanced as well as in very primitive organisms. 



In the green plant a highly complex pyrrole pigment system exists 

 — chlorophylls a and b as well as probably a hematin enzyme in the 



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