Xanthophyll

introduction image

Xanthophyll is a type of accessory pigment or phytochemicals which belongs to the class of “Carotenoids”. In many vascular plants and algae, xanthophylls act as the light harvesting protein complexes. Xanthophylls are rich in “Antioxidants” which prevents the cells from damaging.

In photosynthetic eukaryotes, the xanthophylls are usually bound to the chlorophyll molecules. Xanthophylls are the pigment molecules that present within the light-harvesting complex, which prevents the photosynthetic organism from the toxic effect of light.

Content: Xanthophyll

  1. Meaning of Xanthophyll
  2. Molecular Structure
  3. Occurrence
  4. Types
  5. Xanthophyll Cycle
  6. Functions
  7. Isolation of Xanthophyll

Meaning of Xanthophyll

Xanthophyll can define as the light harvesting accessory pigments which work coordinately with the chlorophyll-a. It can absorb light of a wavelength in a range of 425-475nm.

Xanthophylls are primarily of three types, namely lutein, zeaxanthin and cryptoxanthin. These are highly antioxygenic molecules which protect the cell from damage and ageing.

Xanthophyll is highly beneficial for eye health as it reduces the risk of eye cataract and macular degeneration. It also refers as “Phylloxanthins”. Xanthophylls are the yellow colour pigment which presents naturally in plants.

A xanthophyll can isolate from the plant on performing chromatography which separates xanthophyll in the form of a yellow colour band. There is a table given below, which shows the general properties of the xanthophyll.

PropertiesXanthophyll
Molecular formulaC40H56O2
Molecular weight568.886 g/mol
ChargeNeutral
Physical stateLiquid
ColourYellow
NaturePolar compound

Molecular Structure

Xanthophyll is the primary accessory pigment, and carotene is another pigment of the carotenoids. It consists of  C-40 terpenoid compounds which form as a result of condensation between the isoprene units.

molecular structure of xanthophyll

The molecular structure of xanthophyll and carotene is almost the same except the presence of oxygen atom. In xanthophyll, there is an oxygen atom present as “Hydroxyl group” whereas carotene lacks an oxygen atom and is a pure hydrocarbon.

Occurrence

Xanthophylls occur naturally in the plants which regulate the light energy and act as “Photochemical quenching agent” that deals with the exciting form of chlorophyll or triplet chlorophyll.

The triplet chlorophyll produces at a higher rate during the process of photosynthesis. Xanthophylls are also found in the body of humans and animals, which comes ultimately by the source of green plants.

Types

Xanthophylls mainly include accessory pigments like lutein, Zeaxanthin and cryptoxanthin.

Lutein: It is the most common xanthophyll, which is synthesized by the green plants itself. Spinach, kale, kiwi, green apples, egg yolk, corn etc. are the sources of lutein.

Lutein is a “Lipophilic molecule” which means it is insoluble in polar solvent like water. In plants, lutein is present as fatty acid esters in which one or two fatty acids attach with the two –OH groups.

Lutein mainly absorbs blue light, and thus it protects the eye from the blue light which can cause eye impairment.

Zeaxanthin: These commonly refer to as “Carotenoids alcohols”. Zeaxanthin can be synthesized naturally by the plants and certain microorganisms. It acts as a “Non-photochemical quenching agent”.

Zeaxanthin is an accessory pigment which gives characteristics colour to the corn, wolfberries etc. It consists of two chiral centres. Kale, spinach, turnip greens, mustard greens etc.

Cryptoxanthin: Its molecular structure is quite similar to the β-carotene but a hydroxyl group is present in addition. Cryptoxanthin found as red crystalline solid in its pure form. It also refers as “Provitamin-A” as in the xanthophyll cycle, it converts into vitamin A (Retinol).

Xanthophyll Cycle

The xanthophyll cycle occurs inside the thylakoid membrane of the chloroplast. Xanthophyll cycle can define as the interconversion of oxygenated carotenoids. There are many types of xanthophyll cycle, but violaxanthin and Diadino xanthin cycle are the most common.

Violaxanthin Cycle

First, violaxanthin, which contains two epoxide group converts into antheraxanthol, which includes one epoxide group. Antheraxanthol then further turns into Zeaxanthin which contains no epoxide group.

violaxanthin cycle

VDE, i.e. Violaxanthin de-epoxidase catalyses the conversion of violaxanthin to Zeaxanthin as a result of the de-epoxidation reaction. ZE, i.e. Zeaxanthin epoxidase is an enzyme which catalyses the conversion of Zeaxanthin into Violaxanthin as a result of epoxidation reaction.

The epoxidation reaction will occur at a low pH <5.8 and low light, whereas de-epoxidation reaction occurs at a high pH of 7.5 and high light source.

Diadinoxanthin Cycle

The diadinoxanthin cycle is another type of xanthophyll cycle commonly found in diatoms and eukaryotic algae. It is a reversible diadinoxanthin cycle is present which can covert diadinoxanthin into diatoxanthin and vice versa.  DEP, i.e. Diadinoxanthin epoxidase, catalyses the conversion of diadinoxanthin into diatoxanthin. DDE, i.e. Diatoxanthin de-epoxidase, catalyses the conversion of diatoxanthin into diadinoxanthin.

diadinoxanthin cycle

Some algae make the use of both violaxanthin and diadinoxanthin cycle.

Functions

Xanthophylls perform two central roles:

  • Harvesting of light
  • Dissipation of energy as heat

Xanthophylls are the accessory pigments which acts as a “Photosynthetic light-harvesting complexes” of algae and vascular plants.

Xanthophyll helps in photoprotection, i.e. it protects the photosynthetic apparatus from the photo-oxidative damage in the condition of excessive light by dissipating energy.

Isolation of Xanthophyll

A xanthophyll can be isolated by the method of chromatography. For this, prepare a plant extract by crushing the fresh leaves. Place a drop of leaf extract at one end above 1cm on chromatography paper will refer as a Stationary phase. Then take acetone ligroin mixture as a non-polar hydrophobic solvent which will run through the filter paper as Mobile phase.

Add solvent and hang the filter paper inside the chromatography chamber. Cover the chromatography chamber with a lid to prevent gas exchange. As the solvent mixture comes in contact with the leaf extract, it will help in the migration of different plant pigments at a different rate. The separation and the travelling distance of plant pigment are based on their solubility with the solvent used.

The different plant pigments like chlorophyll, xanthophyll and carotene will travel at different rates and appear as different bands. Chlorophyll being highly polar it will adhere to the polar surface of the paper. Thus, chlorophyll moves the shortest distance and appear as “Green band”.

Xanthophyll being less polar will move a shorter distance and appear as “Yellow band”. Carotene being non-polar will attract more strongly to the non-polar solvent and move along with it. Thus, carotene will move the longest distance and appear as “Orange band”.

isolation of xanthophyll

The solvent is allowed to run to the distance leaving 1cm distance from the top will refer as “Solvent front”. The formation of different bands on the chromatography paper commonly refers to as “Chromatogram”. The Rf value is calculated by the ratio of distance travelled by the pigment, and the total distance travelled by a solvent.

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