442 CHLOROPHYLL CHAP. 16 



similar to that of the vinyl group). The location of this "semi-isolated" 

 double bond distinguishes formulations A, B, and C. Each of these for- 

 mulae represents two structures, analogous to the two Kekule structures 



C N N^— C- 



HC< \{ ^CH HC^ Mg ^( 



H r-c/ ET I I m ^C— CH3 H3C-Ce ej I j H j,C-CH3 





.c T c 



yi I ^ H^i 



H L. . ..'10 9 1 o 



Y 



CH2 HC C 2 .;>^ 



fP»,w»ol1 ^n (Phytol) 1 O 



^nXoOC-iH, i00CH3 ° k.'c,,OOC-CH, COOCH3 



A. Semi-isolated double bond in B. Semi-isolated double bond in 



nucleus III; Mg bound to nuclei I and nucleus II; Mg bound to nuclei I and 



II. ni. 



H2C=:CH CH3'^ 



' H ' 



y\/\ 



C— "N N C 



HC 5 Mg fi CH 



\_, \_/ 



H,c-;Ce H I I m «-CH, 



H I l|o «■ 

 CHj HC C 



(Phy+ol) I I ^O 



H3,C2oOOC — CHj COOCH3 



C. Semi-isolated double bond in 

 nucleus I; Mg bound to nuclei II and 

 III. 



Formula 16.111. 

 Chlorophyll a structure according to Hans Fischer. A, B, and C are three isomeric 

 (or mesomeric) structures distinguished by the routing of the all-round conjugated ring 

 system (heavy line), and the position of the "semi-isolated" double bond and the direc- 

 tions of the Mg — N bonds which depend on this routing. 



The asterisk designates the position of a carbonyl group in chlorophyll h. 



of benzene. These pairs of structures are "mesomeric," i. e., they 

 correspond to the same spacial arrangement of atomic nuclei, and differ 

 only m the distribution of electrons. In such cases, the true structure 



