Xanthophylls from alfalfa (CAS 127-40-2)

Xanthophylls (originally phylloxanthins) are yellow pigments that form one of

two major divisions of the carotenoid group. The name is from Greek xanthos

 and phyllon due to their formation of the yellow band seen in early chromatography

 of leaf pigments. Their molecular structure is similar to carotenes, which form the

other major carotenoid group division, but xanthophylls contain oxygen atoms, while

 carotenes are purely hydrocarbons with no oxygen. Xanthophylls contain their oxygen

either as hydroxyl groups and/or as pairs of hydrogen atoms that are substituted by

oxygen atoms acting as a bridge (epoxide). For this reason, they are more polar than

the purely hydrocarbon carotenes, and it is this difference that allows their separations

from carotenes in many types of chromatography. Typically, carotenes are more orange

in color than xanthophylls.

Like other carotenoids, xanthophylls are found in highest quantity in the leaves of most

green plants, where they act to modulate light energy and perhaps serve as a non-photochemical

quenching agent to deal with triplet chlorophyll (an excited form of chlorophyll)[citation needed],

which is overproduced at high light levels in photosynthesis. The xanthophylls found in the bodies

of animals, and in dietary animal products, are ultimately derived from plant sources in the diet. For

example, the yellow color of chickenegg yolks, fat, and skin comes from ingested xanthophylls

(primarily lutein, which is often added to chicken feed for this purpose).

The yellow color of the human macula lutea (literally, yellow spot) in the retina of the eye comes

from the lutein and zeaxanthin it contains, both xanthophylls again requiring a source in the human

diet to be present in the eye. These function in eye protection from ionizing blue light, which they

absorb. These two specific xanthophylls do not function in the mechanism of sight, since they

cannot be converted to retinal (also called retinaldehyde or vitamin A aldehyde).

The group of xanthophylls includes (among many other compounds) lutein, zeaxanthin, neoxanthin, 

violaxanthin, and α- and β-cryptoxanthin. The latter compound is the only known xanthophyll to

contain a beta-ionone ring, and thus β-cryptoxanthin is the only xanthophyll that is known to

possess pro-vitamin A activity for mammals. Even then, it is a vitamin only for plant-eating

mammals that possess the enzyme to make retinal from carotenoids that contain beta-ionone

(some carnivores lack this enzyme). In species other than mammals, certain xanthophylls may

be converted to hydroxylated retinal-analogues that function directly in vision. For example,

with the exception of certain flies, most insects use the xanthophyll derived R-isomer of

3-hydroxyretinal for visual activities, which means that β-cryptoxanthin and other xanthophylls

(such as lutein and zeaxanthin) may function as forms of visual “vitamin A” for them, while

carotenes (such as beta carotene) do not.