G30 



ABSORPTION SPKCTRA OF PIGMENTS IN VITRO 



CHAP. 21 



violet and ultraviolet all show the same pattern. As pointed out by Stern 

 (1939), neither the transition from porphin to chlorin nor the introduction 

 of magnesium has much effect on the intensity of the blue-violet absorption 

 band. This is shown by figures 21.19A and B. The blue-violet band is 

 shifted by hydrogenation toward shorter waves, but suffers no appreciable 

 change of intensity. 



Comparison of figure 21. 19 A and B shows that, while both magnesium 

 and the extra hydrogen atoms enhance the main red absorption band, 

 these two substituents have antagonistic effects on all the rest of the spec- 

 trum, below GOO m.fx. 



4. Some Theoretical Remarks on the Spectrum of Chlorophyll 



(a) The Term System 



In section 3 (c/. fig. 21.11) we interpreted the four absorption bands of 

 porphin in yellow and green as vibrational bands belonging to the same band 

 system (electronic transition X -^ A). A similar interpretation has been 

 suggested by Prins (1934) for the bands of chlorophyll in the red, orange, 

 yellow and green ; it is made plausible by the magnitude of the Ai* values 

 given in Table 21. VI. (c/. the infrared absorption frequencies in Table 

 21.IIA). The blue-violet and the two ultraviolet bands are best inter- 

 preted as separate electronic transitions. 



Table 21. VI 

 Chlorophyll a Bands 



According to this interpretation, the term scheme of figure 21.11 

 could apply also to chlorophyll. However, instead of a (more or less) luii- 

 form change in probability in the series of transitions X — > Ao, X -» Ai, 

 X ^ A2 (as revealed by a gradual increase in intensity of the corresponding 

 bands in porphin and its derivatives), one would have to postulate in the 

 case of chlorophyll a predominant probability of the transition X —> Ao 

 (to account for the outstanding intensity of the first absorption band at 

 660 m/i). Furthermore, the relationship between the spectra of porphins 

 and dihydroporphins, illustrated by figure 21.12, suggests a shift of the 

 bands in the hydrogenated compounds toward the shorter, rather than to- 

 ward the longer, waves. Therefore, if the red band of the dihydroporphins 

 is X -^ Ao, the first absorption band of the porphins must be interpreted as 



