Betalains are a class of red and yellow tyrosine-derived pigments found in plants of the Caryophyllales, where they replace anthocyanin pigments. Betalains also occur in some higher order fungi. They are most often noticeable in the petals of flowers, but may color the fruits, leaves, stems, and roots of plants that contain them. They include pigments such as those found in beets.
Description
The name "betalain" comes from the Latin name of the common beet, from which betalains were first extracted. The deep red color of beets, bougainvillea, amaranth, and many cacti results from the presence of betalain pigments. The particular shades of red to purple are distinctive and unlike that of anthocyanin pigments found in most plants. There are two categories of betalains:
Betacyanins include the reddish to violet betalain pigments. Among the betacyanins present in plants include betanin, isobetanin, probetanin, and neobetanin.
Betaxanthins are those betalain pigments which appear yellow to orange. Among the betaxanthins present in plants include vulgaxanthin, miraxanthin, portulaxanthin, and indicaxanthin.
The physiological function of betalains in plants is uncertain, but there is some evidence that they may have fungicidal properties. Additionally, betalains have been found in fluorescent flowers, though their role in these plants is also uncertain.
Chemistry
It was once thought that betalains were related to anthocyanins, the reddish pigments found in most plants. Both betalains and anthocyanins are water-soluble pigments found in the vacuoles of plant cells. However, betalains are structurally and chemically unlike anthocyanins and the two have never been found in the same plant together. For example, betalains contain nitrogenwhereas anthocyanins do not. It is now known that betalains are aromaticindole derivatives synthesized from tyrosine. They are not related chemically to the anthocyanins and are not even flavonoids. Each betalain is a glycoside, and consists of a sugar and a colored portion. Their synthesis is promoted by light. The most heavily studied betalain is betanin, also called beetroot red after the fact that it may be extracted from red beet roots. Betanin is a glucoside, and hydrolyzes into the sugar glucose and betanidin. It is used as a food coloring agent, and the color is sensitive to pH. Other betalains known to occur in beets are isobetanin, probetanin, and neobetanin. The color and antioxidant capacity of betanin and indicaxanthin are affected by dielectric microwave heating. Addition of TFE is reported to improve the hydrolytic stability of some betalains in aqueous solution. Furthermore, a betanin-europium complex has been used to detect calcium dipicolinate in bacterial spores, including Bacillus anthracis and B. cereus. Other important betacyanins are amaranthine and isoamaranthine, isolated from species of Amaranthus.
Taxonomic significance
Betalain pigments occur only in the Caryophyllales and some Basidiomycota, for instance Hygrophoraceae . Where they occur in plants, they sometimes coexist with anthoxanthins, but never occur in plant species with anthocyanins. Among the flowering plant order Caryophyllales, most members produce betalains and lack anthocyanins. Of all the families in the Caryophyllales, only the Caryophyllaceae and Molluginaceae produce anthocyanins instead of betalains. The limited distribution of betalains among plants is a synapomorphy for the Caryophyllales, though their production has been lost in two families.
Betanin extracted from the red beet was used as starting material for the semisynthesis of an artificial betalainic coumarin, which was applied as a fluorescent probe for the live-cell imaging of Plasmodium-infected erythrocytes.
