NMR Spectrum of Quercetin

Quercetin (C₁₅H₁₀O₇), 3,3',4',5-7-pentahydroxyflavone, is a flavonol, and acts as a pigment in many fruits and vegetables, flowers, and leaves; the color of red onions and apples is caused by quercetin. 

The 82 MHz spectrum of a 9.8% (w/w; 0.36 M) solution of Quercetin in protonated DMSO acquired using the Thermo Scientific picoSpin 80 NMR spectrometer.

This spectrum reveals three types of proton signals: aromatic protons and hydroxy protons of the analyte, and a solvent proton signal. The parent structure of a flavonol is a flavone substituted at the 3 position on the benzopyrone with a hydroxy group, making it a 3-hydroxyflavone. The two aromatic protons on the benzopyrone, at C₆ and C₈, appear as doublet structures at 6.05 ppm and 6.30 ppm, respectively; the multiplicity arises from long-range ⁴J_HH W-coupling (2 Hz) across the aromatic π-system. The C_2’ (7.45 ppm) and C_6’ (7.6 ppm) protons on the catechol substituent appear as a doublet (³J_HH = 8.5 Hz) and singlet, respectively, with W-coupling further splitting each signal, whereas the C_3’ proton only couples to a single neighboring proton (C_2’) and appears as a doublet centered at 7.6 ppm.

Hydroxyl protons at C₃,_4’,₅,_5’ are lumped together in a broad singlet near 9.1 ppm, while the phenolic proton on C₇ appears as a well-defined singlet near 12.3 ppm. The chemical shift and shape of this hydroxyl proton are a consequence of intramolecular hydrogen bonding to the carbonyl oxygen atom (O₁₆). A 6-member ring is formed which affects the rate of dynamic proton exchange of the labile proton while also drawing electron density away, causing it to shift farther downfield. The DMSO solvent signal appears as a strong singlet at 2.5 ppm with two sharp ¹³C satellites (J_CH = 137 Hz), each at 0.55% of the main multiplet.

Flavonols belong to a large family of phytochemicals called flavonoids, non-nutritive polyphenol secondary metabolites that are responsible for fulfilling various plant functions, such as free radical scavenging, UV filtration, and cell cycle inhibition. Flavonoids are beneficial to human health because of their strong antioxidant activity and other health promoting effects. Quercetin has the highest antioxidant activity of the flavonol class, but also has health benefits due to its effectiveness as a histamine blocker. It is used to treat allergic conditions including asthma, hayfever, eczema and hives. Quercetin has additional beneficial health implications as an anti-inflammatory; it helps reduce inflammation and swelling caused by prostatitis, and flavonoid-rich diets may promote cardiovascular health, encourage blood flow, and protect against high cholesterol. Quercetin also has anti-microbial activity and it protects against certain forms of cancer. Quercetin rich foods are often rich in color, for example capers, dark berries, black plums, and red onions. It is also available as a dietary supplement.

An advantage of the picoSpin™ 45 and picoSpin™ 80 ¹H NMR spectrometers is the ability to acquire high-resolution spectra in protonated solvents. This has an obvious cost savings benefit since expensive NMR D-solvents are not required simply for the purpose for locking. Instead, automatic locking, signal alignment and drift control are managed through software coupled with micro-degree magnet temperature control. These simplifying spectrometer feature make sample preparation, testing and spectrometer operation easy, while also reducing chemical waste.

Chemical name: Quercetin (2- (3,4-dihydroxyphenyl) -3,5,7-trihydroxy-4H-chromen-4-one)
Concentration: 9.8% (w / w; 0.36 M) in DMSO
CAS: 117-39-5
Field: 82 MHz
Nuclear testing: ¹H
Applications: Nutraceuticals, pharmaceuticals, R & D

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