798 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



the [Chi] region where self-quenching of the short-lived state Chi* cannot 

 be significant (cf. Table 18. Ill, page 513, Vol. I). This "deactivation" of 

 tautomeric chlorophyll by normal chlorophyll can perhaps occur via dis- 

 mutation : 



(23.20) tChl + Chi > oChl + rChl 



(as suggested, e. g., in equation 18.42b, page 519, Vol. I) followed by back 

 reaction : 



(23.21) oChl + rChl » 2 Chi 



B. Fluorescence of Carotenoids and Phycobilins 



in Vitro* 



1. Fluorescence of Carotenoids 



Carotenoids are usually described as nonfluorescent. According to 

 Willstatter and Stoll (1918), this is true both of the leaf carotenoids and of 

 the carotenoids of brown algae (e. g., fucoxanthol) . However, de Rogovski 

 (1912) asserted that he has observed a fluorescence of carotene in petroleum 

 ether, in the region 505-600 m/x, and Dhere and Castelli {cf. Dhere 1939) 

 stated that, at —180°, three separate fluorescence bands can be observed 

 in carotene solution in xylene. Klein and Linser (1930) mentioned a 

 green fluorescence of carotene solutions in alcohol. Strain (1936) found, 

 in the chromatograms of leaf extracts in petroleum ether, a fluorescent 

 layer situated below that of carotene a, and belonging to an unknown color- 

 less substance, probably a hydrocarbon without sharp absorption bands in 

 the visible or near ultraviolet. Zechmeister and co-workers (see, for ex- 

 ample, Zechmeister and Sandoval 1945, 1946) found a fluorescent, colorless 

 polyene hydrocarbon with sharp absorption bands at 331, 348 and 367 

 m^u (in petroleum ether) to be present in great abundance in extracts from 

 various plant organs (fruits, stems etc., but not chlorophyllous organs, 

 such as grass, leaves or green needles). This hydrocarbon, called phyto- 

 fluene (probably C40H64), may be a hydrogenation product (or precursor) 

 of the carotenes; it contains seven double bonds, with probably only five 

 of them conjugated. 



Absence of fluorescence indicates, according to page 799, that the exci- 

 tation energy of carotenoid molecules in solution is dissipated within less 

 than 10~i^ sec. 



* Bibliography, page 804. 



