Photoisomerization of 2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan, a yellow pigment in salted radish [Raphanus sativus] roots

• Published in: Bioscience, biotechnology, and biochemistry Vol. 72; no. 9; pp. 2262 - 2268
• Main Authors: Matsuoka, H.(Takasaki Univ. of Health and Welfare, Gunma (Japan)), Honzawa, S, Takahashi, A, Yoshikawa, H, Watanabe, E, Watanabe, T, Ozawa, Y, Yamada, Y, Iizuka, T, Uda, Y
• Format: Journal Article
• Language: English
• Published: Tokyo Japan Society for Bioscience, Biotechnology, and Agrochemistry 01.09.2008; Japan Society for Bioscience Biotechnology and Agrochemistry; Oxford University Press
• Subjects: Biological and medical sciences; ENCURTIDOS; ESTABILIDAD; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; IRRADIACION ULTRAVIOLETA; IRRADIATION ULTRAVIOLETTE; ISOMERISATION; Isomerism; ISOMERIZACION; ISOMERIZATION; Light; Nuclear Magnetic Resonance, Biomolecular; Photochemistry; photochromic compound; photoisomerization; PICKLES; PIGMENT; PIGMENTOS; PIGMENTS; Pigments, Biological - chemistry; Pigments, Biological - isolation & purification; Plant Roots - chemistry; RABANO; RADIS; RADISHES; Raphanus - chemistry; RAPHANUS SATIVUS; salted radish roots; Sodium Chloride - chemistry; Spectrophotometry, Ultraviolet; STABILITE; STABILITY; Temperature; tetrahydro-β-carboline derivative; Thiones; Time Factors; Tryptophan - analogs & derivatives; Tryptophan - chemistry; Tryptophan - isolation & purification; ULTRAVIOLET IRRADIATION; Ultraviolet Rays; yellow pigment; photochromic compound; tetra- hydro-β-carboline derivative; Carboline derivatives; Root; salted radish roots; Photoisomerization; yellow pigment; Pigments; Tryptophan
• ISSN: 0916-8451; 1347-6947
• DOI: 10.1271/bbb.80092

The photostability of (E)-2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan ((E)-TPMT), the main yellow pigment in salted radish, was studied. First we analyzed the photoproduct generated from (E)-TPMT under longwave UV irradiation. On the basis of NMR spectroscopy, the photoproduct was identified as Z-configurated TPMT, and isomerization from the Z- to the E-form was reversibly induced by Vis-light irradiation. The optimum wavelength for isomerization from the E- to the Z-form was 360-380 nm, and that for isomerization from the Z- to the E-form was 440-460 nm. The E/Z-ratios in the photostationary state under UV- and Vis-light irradiation conditions were approximately 0.95:1 and 26:1 respectively. The (Z)-isomer was more sensitive to light irradiation than the (E)-isomer in the quantum yield measurement. Yellowing was dependent on the ratio of the (Z)-isomer, because the b* and chroma value rose with increases in the (Z)-isomer by the colorimeters. Hence, it is possible that the formation of the (Z)-isomer contribute to the yellow color of takuan-zuke during long salting and fermentation.