Orcein and Litmus

Abstract

Orcein was separated into 14 dyes by partition chromatography. Their constitutions were determined mainly by spectroscopy and led to formulae that are derived from 7-amino-2-phenoxazone, 7-hydroxy-2-phenoxazone, and 7-amino-2-phenoxazime, and that were confirmed by syntheses. The major constituent of litmus is assembled polymerically from 7-hydroxy-2-phenoxazone chromophores. The mechanism of formation is elucidated.

Small-molecule conversion of toxic oligomers to nontoxic β-sheet–rich amyloid fibrils

“…1b). Orcein is a compound mixture consisting of 14 related small molecules 23 that has been used as a food coloring since the Middle Ages. As the orcein components might have different biological activities, we searched for pure chemicals with similar properties, which lead to the identification of the small molecule O4 (2,8-bis-(2,4-dihydroxy-phenyl)-7-hydroxy-phenoxazin-3-one) that very efficiently stimulated A42 polymerization.…”

“…1b) 23 . In order to identify a pure compound that stimulates A42 aggregation, we searched for orcein-related small molecules in compound libraries and databases.…”

Organic dyes in illuminated manuscripts: a unique cultural and historic record

“…The central rings depicted in table 1 were shown to be the main chromophores and their UVvis spectra were not relevantly influenced by the phenyl/methyl substituents [32]. It was also determined that litmus, an acid-base indicator extracted from the same lichen species by a similar process, is essentially based on 7-hydroxy-2-phenoxazone chromophores assembled in a polymeric structure [31] (table 1).…”

“…This work would be further expanded in the 1961 Angewandte Chemie paper [33], which appeared in an English translation in 2003 [31]. As stated in the summary 'Orcein was separated into 14 dyes by partition chromatography.…”

Light‐harvesting and light‐protecting pigments in simple life forms

“…These lichens contain orcinol depsides such as lecanoric acid (21) and extraction followed by a slow anaerobic fermentation process in aqueous ammonia, yields the red to purple orcein dye or, if reacted under more alkaline conditions, the acid/base indicator, litmus, which is a trimer of orcein. These colours are classed as Natural Red 28 (red at low pH and blue when alkaline) but are complex mixtures derived from 7-amino-2-phenoxazone (hydroxy-and amino-orcein are illustrated in structure 22) [99].…”