ABSORPTION SPECTRA OF THE CAROTENOIDS 657 



on the state of the pigment and the surrounding medium. Table 21. IX 

 shows that the direction of the band shift in different solvents is the same 

 as for chlorophyll — i. e., they are displaced toward the longer waves with 

 increasing polarity and polarizability of the solvent. From ether to carbon 

 disulfide, the "red shift" amounts to 43 m/x for carotene /3 and 35 m/x for 

 luteol — as compared to only 10 m/z for chlorophyll. However, the effect of 

 transition from a nonpolar to a polar solvent of approximately the same 

 polarizability — e. g.,hom ether to ethanol — seems to be smaller for the carot- 

 enoids than for the more polar chlorophylls (c/. Table 22. IX). 



A still stronger displacement sometimes occurs in aqueous colloidal 

 solutions. According to Karrer and Strauss (1938), the maximum of 

 band I of carotene is shifted, from 480 m^ in hexane and ether, to 510 m/x, 

 or even 535 m^, in hydrosols. This, too, is a much wider shift than was ob- 

 served in chlorophyll colloids. (Because of this shift, some carotene sols 

 are red, while their molecular solutions are j^ellow.) 



In addition to a shift of the band maxima, changes of medium may also 

 cause a broadening of the carotenoid bands. This effect appears to be 

 particularly strong in the case of some algal carotenoids. For example, 

 the curves given for fucoxanthol spectrum by Strain, Manning and Hardin 

 (1944) show a considerable flattening of the absorption peaks and extension 

 of the absorption band toward the longer waves in ethanol as compared to 

 petroleum ether. The spectrum of peridinol, a pigment of the dinoflagel- 

 lates, shows a similarity strong solvent effect. 



The spreading of the absorption bands of the carotenoids into the green, 

 rather than a shift of their peaks, probably explains the color of broA\Ti algae 

 and diatoms. The striking difference between their color and that of green 

 plants appears inexplicable if one considers only the solution spectra of 

 fucoxanthol {cf. fig. 21.35A) or peridinol, since these are almost identical 

 with the spectra of the carotenols of green plants (e. g., luteol and zeaxan- 

 thol). 



Recently, Karrer and co-workers (1943, 1948) published an absorption 

 curve of fucoxanthol in hexane (fig. 21.36) which shows a comparatively 

 slow decline of absorption toward the longer waves. The absorption re- 

 mains marked up to 550 mju, while that of most other carotenoids drops to 

 zero at 500 m^u. The reason for the difference between this curve (which 

 togethei- with that of chlorophyll could explain the brown color of fucoxan- 

 thinol-bearing algae) and that given l)y Wald for the same solvent, remains 

 unexplained. 



The curves given by Wassinkand Kersten (1946) for the absorption spec- 

 trum of the "yellow" and the "orange" fraction of the carotenoids from the 

 diatom Nitzschia dissipata also show an extension of the absorption in the 

 second fraction (in methanol) to about 550 m/z, the peaks being situated 



