Research ArticleChemistry

A 1000-year-old mystery solved: Unlocking the molecular structure for the medieval blue from Chrozophora tinctoria, also known as folium

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Science Advances  17 Apr 2020:
Vol. 6, no. 16, eaaz7772
DOI: 10.1126/sciadv.aaz7772
  • Fig. 1 The molecule of this study, chrozophoridin.

    Left: Close-up of C. tinctoria fruits (collected in Alentejo, Portugal) and clothlets prepared with the juice of the fruits following the instructions in the Book of all color paints. Light green fruits were used in this study shortly after collection. Right: Molecular structures of the blue colorants, hermidin (from M. perennis), and chrozophoridin (from C. tinctoria). Photo credit: Paula Nabais, Universidade NOVA de Lisboa.

  • Fig. 2 Causes of color in hermidin extracted from M. perennis.

    Conversion of colorless hermidin into the blue hermidin quinone and formation of dimeric structures as proposed by Lorenz et al. (23, 24).

  • Fig. 3 Results of NMR (13C).

    Structure for the blue dye present in C. tinctoria fruits (shell), chrozophoridin, and 13C zoomed spectra (bottom).

  • Fig. 4 Analysis of the thermodynamically favored atropoisomers of chrozophoridin, in solution.

    Molecules are depicted as sticks and colored by atom type. (A) A ring, B ring, and glucose ring are colored in red, orange, and green, respectively. Both ω1 and ω2 dihedrals and hydrogen bonds are also indicated. (B) Superposition of the A ring of the two molecules. Colored by element (atropisomer 1) and green (atropisomer 2). (C) UV-VIS spectra (water, pH 7, blue) for chrozophoridin compared with the predicted spectra [Boese-Martin for kinetics (BMK) functional] for atropisomers 1 (black) and 2 (green).

  • Table 1 API-LC-ESI-MS/MS data for the major and minor compounds extracted from C. tinctoria fruits.

    Molecular ion and respective fragments MS2 and MS3 obtained by atmospheric pressure ionization (API)–LC–electrospray ionization (ESI)–MS/MS (positive ion mode) found in the blue extract of C. tinctoria fruits.

    Rt (min)Identity[M+][MS2][MS3]
    20.33Hermidin-derivative (blue dye)501339307; 279; 254
  • Table 2 Essential NMR data for the identification of chrozophoridin.

    1H and 13C chemical shifts of the blue dye present in C. tinctoria fruits, determined in DMSO-d6:D2O (9:1).

    Positionδ 1Η (ppm); J (Hz)δ 13C (ppm)*
    A ring
      1-N-CH33.12; s27.4/27.5
      2 C═O164.0/163.7
      4-OCH33.71; 3.78; s59.2/59.4
    B ring
      7-N-CH33.12; s27.3/27.5
      8 C═O161.7/161.6
      10-OCH33.75; 3.77; s60.8/61.0
      11 C═O172.6/172.3
      12 C=O157.0/156.9
    Glucose moiety
      1′4.32; d, 7.7/4.20; d, 7.5108.3/108.7

    *The carbon signals are duplicated, which indicates that at least two isomers are present.

    †Unresolved (superposition).

    Supplementary Materials

    • Supplementary Materials

      A 1000-year-old mystery solved: Unlocking the molecular structure for the medieval blue from Chrozophora tinctoria, also known as folium

      P. Nabais, J. Oliveira, F. Pina, N. Teixeira, V. de Freitas, N. F. Brás, A. Clemente, M. Rangel, A. M. S. Silva, M. J. Melo

      Download Supplement

      This PDF file includes:

      • Supplementary Materials and Methods (detailed)
      • Figs. S1 to S6
      • Tables S1 and S2

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