title: Photoisomerization of 2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan, a yellow pigment in salted radish roots.
abstract: 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.
authors.affiliation: Department of Health and Nutrition, Takasaki University of Health and Welfare, Takasaki, USA. matsuoka@takasaki-u.ac.jp
authors.forename: Hiroki
authors.initials: H
authors.lastname: Matsuoka
authors.forename: Shuji
authors.initials: S
authors.lastname: Honzawa
authors.forename: Asaka
authors.initials: A
authors.lastname: Takahashi
authors.forename: Hiroko
authors.initials: H
authors.lastname: Yoshikawa
authors.forename: Emi
authors.initials: E
authors.lastname: Watanabe
authors.forename: Takeaki
authors.initials: T
authors.lastname: Watanabe
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Ozawa
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Yamada
authors.forename: Takeshi
authors.initials: T
authors.lastname: Iizuka
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Uda
mesh_terms: D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D008027:Light; D019906:Nuclear Magnetic Resonance, Biomolecular; D010777:Photochemistry; D010860:Pigments, Biological / Q000737:chemistry* / Q000302:isolation & purification; D018517:Plant Roots / Q000737:chemistry; D031224:Raphanus / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013871:Thiones; D013997:Time Factors; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000302:isolation & purification; D014466:Ultraviolet Rays
chemical_list: C472486:2-(3-(2-thioxopyrrolidin-3-ylidene)methyl)tryptophan; D010860:Pigments, Biological; D013871:Thiones; D012965:Sodium Chloride; D014364:Tryptophan