SPECTRUM AND STRTICTItrE OF PORPIIIN DERIVATIVES 



027 



different positions of the bhie-violet bands: That of chlorophyll a is con- 

 fined to the violet and ultraviolet regions, allowing free transmission of blue 

 light, whereas the a])S()rption peak of the 6 compound is situated in the 

 blue, so that this compound transmits only green light. The arrangement 

 of the weaker bands in the middle of the visible ii^gion is also affected by 

 the carbonyl in position 5, to such an extent that Stern classified the spectra 

 of chlorophyll b and its derivatives as a separate "rhodin" type (fig. 21.10, 

 5) distinct from the "chlorin" type (fig. 21.10, 4). 



480 520 560 600 640 



WAVE LENGTH, m>i 



Fig. 21.16. Effect of esterification on 

 chloriri spectrum (after Stern and Mol- 

 vig 1937). 



670 480 



520 560 600 



WAVE LENGTH, m/i 



640 670 



Fig. 21.17. Effect of closure of car- 

 bocyclic ring (transition chlorin — *■ 

 phorbin) on chlorin spectrum in di- 

 oxane (after Stern and Wenderlein 

 1935). 



The other carbonyl group of chlorophyll, the one in the carbocyclic 

 ring (in position 9), has no pronounced influence on the spectrum of chlorin 

 derivatives. This is striking because, in the case of porphyrins, a carbonyl 

 group in a similar position affects the spectrum to a considerable degree. 

 Stern (c/. Fischer and Stei-n 1940, page 343) suggested an explanation of 

 this difference, based on Fischer's earlier assignment of the two extra hy- 

 drogen atoms to nucleus III. By this assignment, the C=0 double bond 

 in position 9 was removed from conjugation with other double bonds in the 

 molecule (cf. Volume I, page 441), and this could explain why its effect 



