Xanthophylls (originally phylloxanthins) are yellow

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

s that occur widely in nature and form one of two major divisions of the

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

group; the other division is formed by the

carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the ex ...

s. The name is from Greek: (), meaning "yellow", and (), meaning "leaf"), due to their formation of the yellow band seen in early

chromatography In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it ...

leaf A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leav ...

pigments.

Molecular structure

As both are carotenoids, xanthophylls and carotenes are similar in structure, but xanthophylls contain

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

atoms while carotenes are ''purely

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

s'', which do not contain oxygen. Their content of oxygen causes xanthophylls to be more polar (in molecular structure) than carotenes, and causes their separation from carotenes in many types of

. (Carotenes are usually more orange in color than xanthophylls.) Xanthophylls present their oxygen either as

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

s and/or as hydrogen atoms substituted by oxygen atoms when acting as a bridge to form

epoxide In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...

Occurrence

Like other carotenoids, xanthophylls are found in highest quantity in the

leaves A leaf (: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, ...

of most green

plants Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...

, where they act to modulate light energy and perhaps serve as a

non-photochemical quenching Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal conv ...

agent to deal with triplet

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

(an excited form of chlorophyll), which is overproduced at high light levels in photosynthesis. The xanthophylls found in the bodies of animals including humans, and in dietary animal products, are ultimately derived from plant sources in the diet. For example, the yellow color of chicken

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

s, fat, and skin comes from ingested xanthophylls—primarily

lutein Lutein (;"Lutein"

macula lutea The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal av ...

(literally, ''yellow spot'') in the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

of the human eye results from the presence of

lutein Lutein (;"Lutein"

zeaxanthin Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji ( wolfberries) ...

. Again, both these specific xanthophylls require a source in the human diet to be present in the human eye. They protect the eye from ionizing light (blue and ultraviolet light), which they absorb; but xanthophylls do not function in the mechanism of sight itself as they cannot be converted to

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

(also called retinaldehyde or

vitamin A Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...

aldehyde). Their physical arrangement in the macula lutea is believed to be the cause of Haidinger's brush, an

entoptic phenomenon Entoptic phenomena (), occasionally and incorrectly referred to as entopic phenomena, are visual effects whose source is within the human eye itself. In Helmholtz's words: "Under suitable conditions, light falling on the eye may render visible, ...

that enables perception of polarizing light.

Example compounds

The group of xanthophylls includes (among many other compounds)

lutein Lutein (;"Lutein"

, neoxanthin, violaxanthin, flavoxanthin, 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.

Xanthophyll cycle

The xanthophyll cycle involves the enzymatic removal of epoxy groups from xanthophylls (e.g. violaxanthin, antheraxanthin, diadinoxanthin) to create so-called de-epoxidised xanthophylls (e.g. diatoxanthin,

). These enzymatic cycles were found to play a key role in stimulating energy dissipation within light-harvesting antenna proteins by

- a mechanism to reduce the amount of energy that reaches the photosynthetic reaction centers. Non-photochemical quenching is one of the main ways of protecting against

photoinhibition Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as ...

. In higher plants, there are three carotenoid pigments that are active in the xanthophyll cycle: violaxanthin, antheraxanthin, and zeaxanthin. During light stress, violaxanthin is converted, i.e. reduced, to zeaxanthin via the intermediate antheraxanthin, which plays a direct photoprotective role acting as a lipid-protective anti-oxidant and by stimulating non-photochemical quenching within light-harvesting proteins. This conversion of violaxanthin to zeaxanthin is done by the enzyme violaxanthin de-epoxidase ( ECbr>1.23.5.1
, while the reverse reaction, i.e. oxidation, is performed by zeaxanthin epoxidase ( ECbr>1.14.15.21
. In

diatoms A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

and

dinoflagellates The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

, the xanthophyll cycle consists of the pigment diadinoxanthin, which is transformed into diatoxanthin (diatoms) or dinoxanthin (dinoflagellates) under high-light conditions. Wright et al. (Feb 2011) found that, "The increase in zeaxanthin appears to surpass the decrease in violaxanthin in spinach" and commented that the discrepancy could be explained by "a synthesis of zeaxanthin from beta-carotene", however they noted further study is required to explore this hypothesis.

Food sources

Xanthophylls are found in all young leaves and in etiolated leaves. Examples of other rich sources include

papaya The papaya (, ), papaw, () or pawpaw () is the plant species ''Carica papaya'', one of the 21 accepted species in the genus '' Carica'' of the family Caricaceae, and also the name of its fruit. It was first domesticated in Mesoamerica, within ...

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

es,

prune A prune is a dried plum, most commonly from the European plum (''Prunus domestica'') tree. Not all plum species or varieties can be dried into prunes. Use of the term ''prune'' for fresh plums is obsolete except when applied to varieties of ...

s, and squash, which contain lutein diesters.

Kale Kale (), also called leaf cabbage, belongs to a group of cabbage (''Brassica oleracea'') cultivars primarily grown for their Leaf vegetable, edible leaves; it has also been used as an ornamental plant. Its multiple different cultivars vary quite ...

contains about 18 mg lutein and zeaxanthin per 100g,

spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to Central Asia, Central and Western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common vegetable consumed eit ...

about 11 mg/100g,

parsley Parsley, or garden parsley (''Petroselinum crispum''), is a species of flowering plant in the family Apiaceae that is native to Greece, Morocco and the former Yugoslavia. It has been introduced and naturalisation (biology), naturalized in Eur ...

about 6 mg/100g,

peas Pea (''pisum'' in Latin) is a pulse or fodder crop, but the word often refers to the seed or sometimes the pod of this flowering plant species. Peas are eaten as a vegetable. Carl Linnaeus gave the species the scientific name ''Pisum sativum ...

about 3 mg/110g, squash about 2 mg/100g, and

pistachios The pistachio (, ; ''Pistacia vera'') is a small to medium-sized tree of the Anacardiaceae, cashew family, originating in Iran. The tree produces nut (fruit)#Culinary definition and uses, seeds that are widely consumed as food. In 2022, world ...

about 1 mg/100g.

References

*Demmig-Adams, B & W. W. Adams, 2006. Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation, New Phytologist, 172: 11–21.

External links

* {{Carotenoids Carotenoids Fatty alcohols