628 



AnSORrTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



on the spectrum is less pronounced than that of the conjugated C=0 bond 

 in position 3. As stated on page 441, Vohime I, Fischer later concluded 

 from chemical degradation experiments that the two extra hydrogen atoms 

 are located in nucleus IV. A certain difference between the conjugation in 

 nuclei II and III exists, however, also in the latter structure (formula A, 

 page 608) — namely, the double bond 3-4 in nucleus II is part of the all- 

 round "aromatic" ring system, while the double band 5-6 in nucleus III is 



1 0.0x10^ 

 9.0 



Methyl chlorophyllide a 

 Methyl pheophorbide a 



< 



■A 



480 520 560 600 640 670 



WAVE LENGTH, m^ 



Fig. 21.18. Effect of magnesium on a porphin spectrum (in 

 dioxane) (after Stern and Wenderlein 1936). 



merely in "one-sided" conjugation with this system. In structure B, on 

 the other hand, bond 5-6 is part of the all-round conjugated system, and 

 bond 3-4 is in one-sided conjugation. Finally, in structure C, both bonds, 

 3-4 and 5-6, participate in all-round conjugation. Thus, a difference be- 

 tween the chromophoric effects of carboxyls in nuclei II and III appears 

 plausible in structures A and B, but not in structure C. We note further 

 that, according to Stern and Pruckner (1939), a carbonyl group in nucleus 

 I also has no strong effect on the spectrum. This indicates that, with re- 



