Red Beet Fiber NMR

Published: 16 May 2022| Version 1 | DOI: 10.17632/9yph9rdkwc.1
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The data provided here include the 1D and 2D NMR spectra of four red beet fiber fractions, the pomace (PF), water-soluble (WSF), water-insoluble (WIF), and microwave-extracted pectin (10/80 min/˚C). The anomeric 1H-1H coupling constants and chemical shift resonances of the observed carbohydrate residues are reported. Additional analyses can be found in the associated paper "Structural characterization of red beet fiber and pectin” (Hotchkiss et al., 2022, https://doi.org/10.1016/j.foodhyd.2022.107549). The dominant anomeric resonances in the 1D-1H NMR spectrum of the WSF (Figure 1), as well as the PF (Figure 2) are at 5.42 ppm, with smaller intensity resonances at 5.23 and 5.09 ppm, followed by increasingly weaker resonances at 5.16, 5.26, 4.56, 5.31 and 5.13 ppm. The resonance at 5.42 ppm is an apparent doublet with a JH1H2 coupling constant of 2.8~3.42 Hz, which is consistent with H2 and H1 being in an axial-equatorial, or conformation. These peaks are assigned as α-Glc(1) and α-Glc(2). Similarly, the weak resonance at 5.26 ppm is a doublet with JH1H=3.15 Hz, which is also conformation, and is assigned as α-Glc(3). The weak doublet centered at 4.56 ppm has a JH1H2 = 7.51 Hz, which indicates an axial-axial orientation, or conformation, and is assigned as β-Glc(2). In the 1D-1H NMR spectrum of red beet WIS (Figure 3), the dominant resonance is at 4.65 ppm (doublet, JH1H2 = 8.1 Hz, conformation) and is assigned as β-Glc(1); this resonance is obscured by the water HOD peak in the water-soluble and pomace fractions. The next most intense peak in the water-insoluble fraction is at 5.24 ppm (doublet, JH1H2 = 3.8 Hz, conformation) which is assigned as α-Glc(3). Weaker peaks are found at 5.10 ppm (broad, JH1H2 = small, conformation), which is assigned as α-Ara, and at 4.52 ppm (doublet, JH1H2 = 7.5 Hz, conformation), which is assigned as β-Glc(2). The weakest anomeric peak observed is at 5.42 ppm (doublet, JH1H2 = 4.4 Hz, conformation), arising from α-Glc(2). All of the fractions contain many non-sugar resonances (0.5-3 ppm) of their 1D-1H spectra (Figures 1-3), that are primarily aliphatic, and comprise about 10% of the proton spectral intensity in WSF and PF, and about 13% of the proton spectral intensity in the WIF. Comparing the sugar resonance regions of the 1D-13C NMR spectra (Figure 4) for WSF, PF and WIF, it is seen that WSF and PF have very similar spectra, both in terms of resonances and intensities. The WIF 1D-13C spectrum, however, the resonances associated with Ara (~110 ppm) and Glc (~99 ppm) are not observed, as well as decreased sucrose resonances (Glc at ~95 ppm and Fru at ~84 ppm). All of these resonances can be weakly detected in the 2D spectra, however, indicating that while their concentration is diminished, they are still present.

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Sample Preparation. Red beet samples (pomace, water-soluble, water-insoluble and pectin) were prepared according to Hotchkiss et al. (2022). NMR Methods. The four polysaccharide fractions derived from red beet, PF, WSF, WIF and pectin, were dissolved in D2O with d4-trimethylsilylpropanoic acid (TMSP) added as an internal reference standard and sodium azide (NaN3) added as a preservative. Their NMR spectra were acquired at 40 °C and 75 °C. The 1D-1H NMR spectra were acquired with a relaxation delay of 1-2 s, a 45° pulse angle, and spectral widths of 6.5-10 ppm, and 32k data points. The 1D-13C spectra were acquired semi-quantitatively using proton decoupling only during the acquisition after a 45° pulse, an acquisition time of 0.87 s and a relaxation delay of 1 s. The spectral width was 250 ppm. The T1 relaxation times were not measured. The gradient-enhanced COSY and zTOCSY experiments were acquired with spectral widths of 5.8, 6.5 or 10 ppm in both dimensions, using 4k points in the directly-detected dimension, and 400 indirectly-detected increments. The zTOCSY experiments were acquired with mixing times of 22, 120, and130 ms. HSQC, HSQC-TOCSY, HMBC and H2BC heteronuclear experiments had spectral widths of either 6.5 or 10 ppm in the directly-detected (1H) dimension, while the indirectly-detected dimensions varied as follows: the HSQC and HSQC-TOCSY experiments covered 130 ppm, the HMBC experiments covered 240 ppm, and the H2BC experiments, 80-120 ppm. The signal averaging ranged from 64-128 transients per increment, with the exception of the HMBC and H2BC experiments which were averaged over 256-512 transients per increment. The indirect dimensions collected over 300-400 increments for most experiments, but the constant time H2BC used 108-178 increments. The HSQC spectra were multiplicity-edited to distinguish between carbons with odd or even numbers of attached hydrogens. The HSQC-TOCSY experiments had mixing times of 80-130 ms. HMBC experiments were acquired using C-H coupling constants of 5, 8, 10 and 16 Hz. All NMR spectra were processed using OpenVNMRJ (2020) and visualized using UCSF Sparky (Goddard & Kneller, 2008). References Hotchkiss, A.T., Chau, H.K., Strahan, G.D., Nuñez, A., Simon, S., White, A.K., Dieng, S., Heuberger, E.R., Yadav, M.P., & Hirsch, J. (2022). Structural characterization of red beet fiber and pectin. Food Hydrocolloids. https://doi.org/10.1016/j.foodhyd.2022.107549 OpenVnmrJ Version 2.1A, January 23, 2020, https://github.com/OpenVnmrJ/OpenVnmrJ/releases Accessed 3 February, 2020 Goddard, T. D., & Kneller, D. G. (2008). SPARKY 3. University of California, San Francisco. Retrieved from https://www.cgl.ucsf.edu/home/sparky/

Institutions

USDA Agricultural Research Service

Categories

Analytical Chemistry, Characterization of Food

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