472 THE ACCESSORY PIGMENTS CHAP. 17 



made plausible by syntheses carried out by Karrer, Helfenstein, and 

 Widmer (1928). 



The carotenols are a more diversified class than the carotenes; there 

 are several groups of them with one to six oxygen atoms, and each has 

 numerous isomers. As mentioned above, the species most common in 

 green leaves is luteol, a 3,3'-dihydroxy-a;-carotene, with hydroxyl groups 

 in positions indicated by asterisks in formula 17.1. However, according 

 to Strain (1938), almost a dozen other carotenols are regularly found in 

 green leaves. One of these is zeaxanthol (dihydroxy-/3-carotene), which 

 is an isomer of luteol. It is the coloring matter of yellow corn. Table 

 15.IV showed a sample of the composition of the "leaf xanthophyll." 



All algae contain carotene, and almost all contain luteol. In addition, 

 a number of carotenols not encountered in the higher plants was isolated 

 from algae (c/. Table 15.V). The most important of them is fucoxanthol, 

 occurring mainly in brown algae and diatoms. The formula of fuco- 

 xanthol has been variously written as C40HB4O6 (Willstatter and Page 

 1914), C40H56O6 (Karrer 1929; Karrer, Helfenstein, Wehrli, Pieper, and 

 Morf 1931), and C40H60O6 (Heilbron and Phipers 1935). It contains, 

 according to Karrer, ten double bonds; of the six oxygen atoms, four are 

 hydroxyl oxygens (as shown by Zerewitinoff's test for "active" hydrogen 

 atoms). Heilbron and Phipers (1935) assumed that the two remaining 

 are carbonyl oxygens, although certain carbonyl tests gave negative 

 results. They suggested the accompanying formulation for the end- 

 groups of fucoxanthol, which they thought may account for the inactivity 

 of the carbonyl groups, as well as for the transformation of fucoxanthol 

 into zeaxanthol which takes place in dried algae. (Zeaxanthol has end 



Formula 17.11. End groups of fucoxanthol. 



groups similar to 17.11, but without the carbonyl oxygen, and with a 

 closed ionone ring.) Other algal carotenoids are, for example, the 

 "myxoxanthone" and "myxoxanthophyll " of blue-green algae. The first 

 one, C40H54O, contains 12 double bonds and one ionone ring, and one 

 carbonyl group; myxoxanthophyll contains seven oxygen atoms, that is, 

 one more than fucoxanthol (Heilbron, Lythgoe, and Phipers 1935; Heil- 

 bron and Lythgoe 1936). 



