PROPERTIES OF THE CAROTENOIDS 659 



of this subject, the reader is referred to the monographs of Zechmeister 

 and rhohioky.-^"^'^'" 



d. Use of Chromatography in the Study of Carotenoids. The impor- 

 tant contribution of chromatography to our knowledge of the carotenoids 

 is to be ascribed to the great variabiHty in adsorption affinities of various 

 members of the group. These differences are directly related to the chemi- 

 cal composition of the particular member involved. The order in which 

 the pigments will be deposited on the column (highest affinity at top to 

 lowest adsorption at bottom or in filti'ate) is governed by the following 

 groups : 



(a) Presence of the Carhonyl Group Conjuc/ated with a Double Bond. Thus, capsanthin 

 (1 — CO and 2 — OH) appears above zeaxanthin (2 — OH). 



(6) Those with the Highest \ umber of Alcohol Groups. Thus, zeaxanthin (2 — OH ) will 

 appear above cryptoxanthin (1 — OH). 



(c) Presence of a Hydroxtjl Group. Thus, cryptoxanthin (1 — OH) is more easily ad- 

 sorbed than is /3-carotene (0 — OH). 



(d) Occurrence of the Double Bonds in Conjugated Form. j8-Carotene with 11 conju- 

 fiated F is adsorbed higher than a-carotene (10 conjugated + 1 isolated F). 



(e) Presence of an Increased Number of Double F. Lycopene with 11 conjugated and 

 2 isolated F appears above 7-carotene (11 conjugated + 1 isolated F). 



The rules appear to apply, not only for the natural carotenoids, but also 

 for their decomposition products. The effect of the hydroxyl group is 

 ([uantitatively greater than is that of a conjugated double bond. Thus, 

 lutein with only 10 F and 2 OH groups occupies a higher position on the 

 adsorption column than does lycoxanthin with 11 F and only one OH 

 group. Rhodoxanthin with 12 F and 2 ketones but no hydroxyls is not as 

 well adsorbed as a polyene alcohol having only 11 F. 



When the hydroxyl groups are esterified, the adsorption of the resulting 

 ester is greatly depressed and compares in retention on the column with 

 that of the poorest adsorbed carotenoids, namely, the hydrocarbons. 

 Thus, physalien (zeaxanthin dipalmitate) is not only less easily adsorbed 

 than the free alcohol (zeaxanthin) but even less readily than the corre- 

 sponding monohydric carotenol, cryptoxanthin. 



The order of adsorption of the various carotenoids as determined by 

 Winterstein'^''- and more recently extended by Zechmeister and v. 

 C'holnoky'^^ is shown in Table 35. 



As noted in the earlier section, the stereochemical isomers of any single 

 carotenoid assume characteristic positions on the chromatographic column. 

 In most sets of carotenoids, the natural (all-^rans) form is present about 

 halfway down the column. The better known stereoisomers of the all- 

 trans fomi are located below it, i.e., are adsorbed less readily, although in 

 most cases some isomers which are more strongly adsorbed than is the all- 

 irans compound are also present in the isomeric mixture (see page 622) . 



