STRUCTURAL FORMULA 441 



atoms and four nitrogen atoms would not be stable enough; but subse- 

 quent experiments have shown that this structural unit occurs not only 

 in hemin and chlorophyll, but also in many other natural compounds, 

 and, with a slight alteration (nitrogen atoms instead of CH — groups), 

 in the very stable synthetic dyes of the phthalocyanin class. 



We cannot discuss here the development of Kiister's scheme into a 

 complete structural formula of chlorophyll. Formula 16. Ill is the 

 latest formulation of Hans Fischer (1940), accepted by most workers in 

 the field of chlorophyll chemistry. The three alternative structures. A, 

 B and C, differ only in the routing of the eighteen-membered, "aro- 

 matic" {i. e., all-around conjugated) system of alternating single and 

 double bonds (designated by heavy lines) and in the direction of mag- 

 nesium-nitrogen bonds (which is determined by this routing). 



In formula 16.III, the four pyrrole nuclei numbered I, II, III and 

 IV, are linked by CH — bridges a, jS, 7, 5. An additional homocyclic 

 (cyclopentanone) ring, V, carries a carbonyl oxygen in position 9, and a 

 methanol-esterified carboxyl group in position 10; nucleus I carries a 

 vinyl group, and nucleus IV a propionic acid side chain esterified by 

 phytol. Other substituents are ethyl and methyl groups. In chlorophyll 

 6, the methyl group in position 3 is replaced by methoxyl. 



An important characteristic of formula 16.III is the presence of two 

 hydrogen atoms in positions 7 and 8 in nucleus IV. Partial hydrogena- 

 tion of the double-bond system in chlorophyll was first suspected by 

 Conant (on the strength of spectroscopic analogies), as well as by Stoll, 

 and confirmed by Fischer by the observation of the optical activity of 

 chlorophyll and its derivatives, which proves the presence of an asym- 

 metric carbon atom. In chlorophyll itself, this could be atom 10, but 

 activity is retained also by chlorophyll derivatives deprived of the side 

 chain in position 7; thus, the asymmetry must lie in one of the pyrrole 

 nuclei. This means that at least one ring carbon must be free from 

 double bonding. 



The presence of two hydrogen atoms in nucleus IV reduces the 

 number of double bonds in the nuclear system of chlorophyll to ten (as 

 against 11 in the porphyrins); but an eleventh C=C bond is present in 

 the vinyl side chain. In Fischer's earlier work (1937), the two "extra" 

 hydrogen atoms were placed in positions 5 and 6 in nucleus III. (Spec- 

 troscopic data which favored this assignment will be given in Volume II, 

 Chapter 21); but later (1940), Fischer decided, on the basis of observa- 

 tions on the oxidative degradation of chlorophyll, that the hydrogenated 

 nucleus must be nucleus IV. 



Of the ten double bonds in the ring system of chlorophyll, nine form 

 a closed conjugated system of the "aromatic" type (indicated by heavy 

 lines), while the tenth is in a "one-sided" conjugation with it (a position 



