626 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



The "c'hlorin type" spectrum remains almost unaffected by all trans- 

 formations leading from the parent substance chlorin to chlorophyll a. 

 This is in accordance with general experience as far as the introduction of 

 methyl and ethyl groups is concerned. It is noteworthy, however, that 

 the introduction of an unsaturated (vinyl) substituent in nucleus I also 

 causes only a slight shift of the bands, as illustrated by figure 21.15. In 

 other words, a difference of two hydrogen atoms in a side chain is almost 

 without influence on the spectiaim, while a similar difference in the nucleus 

 has a strong effect. 



The spectroscopic effect of carboxyl groups in chlorophyll also is small. 

 The alcohols esterifying these carboxyls have a certain influence on the 



6.4 X 10* 

 62 



5.6 



Rhodochlorin dimethyl ester 



Mesorhodoctilorin dimethyl 



ester 



(both in dioxane) 



o 

 o 



E 



470 490 520 550 580 610 640 670 

 WAVE LENGTH, m^ 



Fig. 21.15. Effect of vinyl group on chlorin spectrum in di- 

 oxane (after Stern and Molvig 1937). 



intensity of the absorption bands (but none on their position) : The shorter 

 the alcohol, the sharper the absorption peak. As an example, figure 21.16 

 shows the extinction curve of phytyl pheophorbide (pheoph^^tin), together 

 with that of methyl pheophorbide. 



It is further noteworthy that closure of the carbocyclic ring, i. e., the 

 transition from chlorin to phorbin, also hardly affects the spectrum at all, 

 as shown by figure 21.17. The carhonyl group in nucleus II, whose pres- 

 ence distinguishes chlorophyll b and its derivatives from the corresponding 

 compounds of the a series, has a much stronger effect on the spectrum: 

 The two chlorophylls have distinctly different colors — one blue-green and 

 the other yellow-green. Figure 21.1 shows that this difference is caused by 



