Orcein

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Orcein ( CI Natural Red 28) is an organic plant pigment made from lichen and a mixture of at least 14 substances .

Occurrence and extraction

Lichen of the genus ( Roccella fuciformis ).

The mixture of substances is obtained by alcoholic extraction from Orseille , a dye obtained from lichens of the genus Roccella .

Composition and properties

At least 14 components can be isolated from orcein by chromatography. These fall into three phenoxazine - chromophores divided:

  • 7-hydroxy-2-phenoxazone
    (hydroxy-orcein).

  • 7-amino-2-phenoxazone
    (amino-orcein).

  • 7-amino-2-phenoxazime
    (hydroxy-orceimine).

Orcein is a brownish red, microcrystalline powder that is practically insoluble in water, benzene, chloroform or ether, is soluble in alcohol, acetone or glacial acetic acid with a red color and in dilute alkali solutions with a blue-violet color. In this respect, an alcoholic (ethanolic) solution of orcein such as litmus is suitable as an indicator for alkaline (blue-violet), neutral (red-violet) and acidic solutions (red).

use

In ancient times, in the Middle Ages and in the early modern period, orcein, like Brazil wood, was an important supplier of the red coloration of fabrics. Similar to Brazil wood, the color faded very quickly. However, due to the lack of suitable coloring agents, orcein has been widely used. After the collapse of the Roman Empire, the use of orcein was largely forgotten in Europe. This fabric was only used for further dyeing in the Middle East. Around 1300 the use of orcein as a dye was rediscovered by a Florentine merchant and again played a more important role as a dye in Europe in the following centuries.

Virgin wool dyed with orcein

In 1890 orcein was introduced as a dye in histology by Paul Gerson Unna . The basic dye is used today (like the carmine dye ) among other things as an acetic acid solution for coloring chromosomes or chromatids in microscopic specimens. Colorless wool can be dyed with a solution of orcein in a sodium carbonate solution, which has a weakly alkaline effect (blue-violet color). After washing with water, the wool has an intense red-violet color. However, the wool dyed in this way is not washable.

gallery

  • α-amino-orcein

  • α-hydroxy-orcein

  • β-amino-orcein

  • β-hydroxy-orcein

  • β-amino-orceinimine

  • γ-amino-orcein

  • γ-hydroxy-orcein

  • γ-amino-orceinimine

Individual evidence

  1. ↑ H. Beecken, E.-M. Gottschalk, UV Gizycki, H. Krämer, D. Maassen, H.-G. Matthies, H. Musso , C. Rathjen, UI Záhorszky: “Orcein and litmus”, in: Angewandte Chemie , 1961 , Vol. 73, No. 20, pp. 665–673; doi : 10.1002 / anie.19610732002 .
  2. ↑ Entry on Orcein. In: Römpp Online . Georg Thieme Verlag, accessed on July 23, 2014.
  3. ^ Amy Butler Greenfield: A Perfect Red - Empire, Espionage and the Qest for the Color of Desire , HarperCollins Publisher, New York 2004, ISBN 0-06-052275-5 , p. 29.
  4. ↑ HC Cook: Origins of ... tinctorial methods in histology. In: Journal of clinical pathology. Volume 50, Number 9, September 1997, pp. 716-720, PMID 9389971 . PMC 500167 (free full text).

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