CHLOROPHYLL FORMATION 399 



By analogy with the use of the term "chlorophyll," "protochlorophyll" 

 will be used in this article to designate the precursor of chlorophyll except 

 when it is necessary to distinguish explicitly between its natural and 

 derived states. 



3. PROPERTIES OF PROTOCHLOROPHYLL 



The assumption that protochlorophyll is the immediate precursor of 

 chlorophyll provides a starting point for the analysis of the greening 

 process in leaves. Much of what is said subsequently in this article is 

 directed toward the establishment of this hypothesis and an analysis of 

 its implications. 



3-1. CHEMICAL NATURE OF PROTOCHLOROPHYLL 



In order to understand the biosynthesis of chlorophyll, it is necessary 

 to know the chemical relations that exist between it and its precursor. 

 For this reason a resume of the chemistry of protochlorophyll in its 

 relation to chlorophyll will be given. 



Protochlorophyll has been isolated in solid form from two sources: 

 the inner seed coats of pumpkin seeds (Noack and Kiessling, 1929) and 

 etiolated barley leaves (Koski and Smith, 1948). For the details of 

 preparation, the original papers must be consulted. 



Noack and Kiessling (1929) determined that the protochlorophylls 

 from pumpkin seed coats and from etiolated leaves possess the same 

 absorption spectra and the same basicity reactions. From these simi- 

 larities they concluded that the protochlorophylls from the two sources 

 are identical. Inasmuch as the content of protochlorophyll is greater in 

 seed coats, they proceeded to carry out studies on the chemical structure 

 of protochlorophyll from this source. They established that protochloro- 

 phyll is a complex magnesium compound from which magnesium can be 

 removed with acid and reintroduced by means of the Grignard reagent. 

 The magnesium-free compound they termed protopheophytin. They 

 believed the pigment to be a diester of phytol and methanol, similar to 

 chlorophyll. Later (1930), however, they questioned the presence of 

 phytol, a question that subsequent investigators have also raised (Stoll 

 and Wiedemann, 1938; Granick, 1950). They concluded that proto- 

 chlorophyll was closely related to chlorophyll, because they could obtain 

 the same product, protophytochlorintrimethyl ester [vinylchloroporphy- 

 rin-ee (Fischer and Stern, 1943, p. 323)], from protopheophytin and from 

 pheophytin a. 



Stoll and Wiedemann (1938) were the first to suggest a structural for- 

 mula for protochlorophyll. They suggested that it was the magnesium 

 complex of the phytyl methyl diester of 2-vinylpheoporphyrin-a5. 



The analytical and synthetic work of Fischer and his collaborators 

 (Fischer, Oestreicher, and Albert, 1939; Fischer, Mittenzwei, and Oes- 



