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DOI: 10.1055/s-0039-1690744
Bioinspired Synthesis and Physical-Chemical Properties of a New 10-Methylpyrano-4′-hydroxyflavylium Chloride Salt
This research was supported by a research project grant (PTDC/OCE-ETA/31250/2017) with financial support from FCT/MEC through national funds and was co-financed by FEDER under the Partnership Agreement PT2020 (UID/QUI/50006/2019-POCI/01/0145/FERDER/ 007265, 2017). L.C. acknowledges an FCT research contract and V. G. acknowledges receipt of a Ph.D. grant from FCT (SFRH/BD/136556/2018).Publikationsverlauf
Received: 04. Oktober 2019
Accepted after revision: 24. Oktober 2019
Publikationsdatum:
07. November 2019 (online)
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Published as part of the Special Section 11th EuCheMS Organic Division Young Investigator Workshop
Abstract
A novel bioinspired 10-methylpyranoflavylium dye was synthesized. The dye was fully characterized by LC-DAD/ESI-MS and NMR 1D and 2D techniques, and its equilibrium constants towards pH variations were determined by UV–vis titration. These studies revealed the presence of three species (a flavylium cation AH+, a neutral quinoidal base A, and an anionic quinoidal base A–) in the pH range 1–12, driven by the two pK a values of the dye: pK a1 = 6.8 ± 0.1 and pK a2 = 10.8 ± 0.1. The first deprotonation of the dye occurs within the pH range for spoilage of many packed-food products, making the dye an excellent candidate for use as a pH sensor for real-time monitoring of the quality and freshness of foods.
Supporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0039-1690744.
- Supporting Information
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References and Notes
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- 19 HPLC-DAD analyses were performed on a Merck-Hitachi L-7100 apparatus with a 150 × 4.6 mm i.d. reverse-phase ODS C18 column (Merck, Darmstadt) at 25 °C. Detection was carried out using a L-7450A diode array detector (DAD). The eluents were A: H2O–HCO2H (9:1) and B: CH3CN. The gradient consisted of 0–35% B for 50 min at a flow rate of 0.5 mL/min. The column was washed with 100% B over 10 min and then stabilized to the initial conditions during another 10 min.
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- 21 The 1H NMR (600.13 MHz) and 13C NMR (125.77 MHz) spectra were recorded in 95:5 DMSO–TFA on a Bruker-Avance 600 spectrometer at 303 K, with TMS as an internal standard. 1H chemical shifts were assigned by using 2D NMR (COSY, NOESY) experiments, whereas 13C resonances were assigned by using 2D NMR techniques (gHMBC and gHSQC). The delay for the long-range C–H coupling constant was optimized to 7 Hz.
- 22 UV-Vis Titration A 0.3 mM pigment stock solution was prepared in 0.1 M HCl in 75:25 EtOH–H2O. A quartz cell was charged with Teorell and Stenhagen universal buffer23 at pH 1 (1 mL), 0.1 M aq NaOH (1 mL), and the pigment stock solution (1 mL). The final concentration of the pigment was 0.1 mM in 25% EtOH. Titrations were performed up to pH 12 by the addition of small volumes of 1 M aq NaOH. Successive spectra were recorded immediately after the addition of the base in a Thermo Scientific Evolution Array UV–Visible spectrophotometer at 25 °C. All titrations were performed in triplicate. Final volumes were appropriately corrected. All pH measurements were made with a Radiometer Copenhagen PHM240 pH/ion meter. The fitting for pK a determinations was carried out by using the Solver add-in program for Microsoft Excel.
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