640 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



Tables 21. VII and 21. VIII contain a summary of the experimental re- 

 sults. Considering the former table as a whole, one finds only a very 

 rough confirmation of Kundt's rule — it consists mainly in the fact that 

 ^max. values greater than 670 ran are found only with solvents whose refrac- 

 tive index is greater than 1.5. Between Ud = 1.33 and 1.5, the variations 

 of Xmax. appear small and irregular. 



However, if one sorts out solvents of the same chemical type, a more 

 regular shift at least of the red absorption maximum becomes apparent. 

 Figure 21.24 summarizes, as an example, the data of Katzand Wassinkand 

 Harris and Zscheile for nonpolar solvents (curve A) and for alcohols (curve 

 B). 



672 



670 



I" 668 



iLl 



666 



UJ 



664 



662 



660 



Carbon disulfide. 



NONPOLAR 

 SOLVENTS 



2-Et-l-hexanol 



POLAR SOLVENTS P l-Amylol 

 g JJ I'Butanol 

 '^2-Me-l- 



"Ethanol P^opanol 

 Methanol 



Xylene 

 Benzene 



^Toluene 



Pentane 



® 



©■iCyclohexane 



Hexane 



1.50 



1.60 



Fig. 21.24. Absorption maxima of chlorophyll a in solvents of different 



refractivity. 



The shift of the blue band remains irregular, even in selected series of 

 solvents. The highest value of X^ax. of the blue band was found in the 

 solvent with the lowest refractive index, methanol. (This fa(;t may per- 

 haps be attributed to a specific sensitivity of the blue-violet band to tautom- 

 erization— a hypothesis discussed on page 607; tautomerization equilibra 

 are known to be strongly affected by solvent changes.) 



Livingston and co-workers (1949) noted that, in the series of chlorophyll 

 a solutions in alcohols (from methanol to octanol), the relation between the 

 two peaks in the blue-violet region changes systematically. In methanol, 

 they are equally high and separated onl}' by a dip; in octanol, the short- 



