The elusive indigo precursors in woad (Isatis tinctoria L.)--identification of the major indigo precursor, isatan A, and a structure revision of isatan B.
A metabolite-profiling study of shock-frozen leaves of Isatis tinctoria L., an old indigo dye plant and medicinal herb, revealed a complex pattern of indigo-forming compounds with higher polarities than the known indigo precursors isatan B and indican. These highly unstable compounds underwent rapid post-harvest transformation and were not detected in air-dried leaves. The major indigo precursor, named isatan A (4), was isolated by rapid normal-phase and gel chromatography, along with isatan B (3). A full spectral data set of 3 showed that the previous structure assignment as 'indoxyl-5-ketogluconate' has to be revised to 1H-indol-3-yl beta-D-ribohex-3-ulopyranoside. Isatan A (4) was identified as 1H-indol-3-yl 6'-O-(carboxyacetyl)-beta-D-ribohex-3'-ulopyranoside. In aqueous solution, glycosides 3 and 4 occur as hydrates and undergo rapid hydrolysis under very mild acidic or basic conditions.
Oberthür
Christine
C
Institute of Pharmacy, University of Jena, Semmelweisstrasse 10, D-07743 Jena.
Schneider
Bernd
B
Graf
Heidemarie
H
Hamburger
Matthias
M
D006027:Glycosides / Q000737:chemistry* / Q000302:isolation & purification; D007203:Indigo Carmine; D007211:Indoles / Q000737:chemistry* / Q000302:isolation & purification; D031219:Isatis; D010936:Plant Extracts / Q000737:chemistry / Q000302:isolation & purification; D018515:Plant Leaves
D006027:Glycosides; D007211:Indoles; D010936:Plant Extracts; C496761:indoxyl-3-O-(6'-O-malonyl-beta-D-ribohexo-3-ulopyranoside); C469389:isatan B; D007203:Indigo Carmine