Hopi red dye

Betalains are a class of red and yellow indole-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.

The name "betalain" comes from the Latin name of the common beet (Beta vulgaris), from which betalains were first extracted. The deep red color of beets, bougainvillea, amaranth, and many cactuses 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:

Plant physiologists are uncertain of the function that betalains serve in those plants which possess them, but there is some preliminary evidence that they may have fungicidal properties. Furthermore, betalains have been found in fluorescent flowers.

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 nitrogen whereas anthocyanins do not.

It is now known that betalains are aromatic indole 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 (betaxanthin derived of L-proline) are affected by dielectric microwave heating. Addition of TFE (2,2,2-trifluoroethanol) is reported to improve the hydrolytic stability of some betalains in aqueous solution. Furthermore, a betanin-europium(III) complex has been used to detect calcium dipicolinate in bacterial spores, including Bacillus anthracis and B. cereus.

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