Study of Tetrahydroxylated Anthraquinones—Potential Tool to Assess Degradation of Anthocyanins Rich Food

Abstract

Degradation of anthocyanins involves scission of the flavonoid skeleton yielding 2,4,6-trihydroxybenzaldehyde (phloroglucinaldehyde, PGA) and a phenolic acid. However, the process is not finished with the formation of PGA, as the consequent condensation of two PGA molecules providing colored hydroxylated anthraquinones was observed for the first time. This process was studied using a combination of preparative column chromatography, nuclear magnetic resonance, liquid chromatography/high resolution tandem mass spectrometry (LC/HRMS2), and quantum calculations using density functional theory. 1,3,5,7-tetrahydroxyanthraquinone (anthrachrysone) and its isomers were found to rise during heating (95 °C) in a buffered PGA model solution (phosphate buffer, pH 7). These compounds were detected in heated red wine after an increase of its pH value. The concentration of the identified anthrachrysone in the red wine reached 0.01 mg·L−1. Presence of those compounds could therefore indicate involvement of certain steps in the processing of plant materials rich in anthocyanins (e.g., utilization of a higher temperature and/or reduction of acidity) or long-term transformation of anthocyanins (potentially, for instance, in archaeological findings such as wine or fruit residues). Additionally, measurement of wine–soil suspensions proved an increase of their pH to the values suitable for anthocyanin cleavage (neutral to slightly alkaline; reached using soil from archaeologically well-known Bull Rock Cave). Although not found in artificially prepared samples (imitations) or authentic materials so far, according to our results the above mentioned conditions are suitable for the formation of tetrahydroxylated anthraquinone derivatives and their monitoring would be beneficial.

New Potential Pharmacological Targets of Plant-Derived Hydroxyanthraquinones from Rubia spp.

“…Based on these results, further studies could be designed to explore more comprehensively the mechanistically relevant binding behavior of this class of naturally-occurring compounds in the two target complexes. Given the fact that these hydroxyantraquinones have been known as constituents of orally applied traditional medicinal preparations [ 14 ], and even commonly consumed beverages in some regions [ 15 , 16 ], an in silico investigation of the potential metabolites of these compounds might be performed as a next step in this modelling workflow. Another intriguing aspect that deserves further attention is the influence of the protonation states of the ligands and their targets on the ligand-receptor interactions upon different pH-related settings.…”

Stressing the differences in alizarin and purpurin dyes through UV-visible light absorption and 1 H-NMR spectroscopies

“…From the 1 H-NMR perspective, 3 main types of protons exist in HAQ, depending on the nuclei to which they are bound: (i) sp 2 carbon participating to an extended conjugated π electronic system, (ii) hydroxy oxygen atoms involved in intra-molecular hydrogen bonds and (iii) hydroxy oxygen atoms involved in intermolecular hydrogen bonds with the HAQ surroundings. It must be immediately noted that HAQ molecules feature acidic protons and tautomer forms, possibly complicating their chemical spaces, which in turn complicates the analysis and the interpretation of HAQ 1 H-NMR spectra 9,10 . From previous works in which experimental and theoretical NMR spectra of HAQ systems are confronted, it turns out that Density Functional Theory (DFT) is most of the time accurate enough for reaching a 5 to 10% error with respect to the experimental chemical shifts of protons bound to sp 2 carbon atoms.…”

Stressing the differences in alizarin and purpurin dyes through UV-visible light absorption and 1 H-NMR spectroscopies

“…From the 1 H-NMR perspective, 3 main types of protons exist in HAQ, depending on the nuclei to which they are bound: (i) sp 2 carbon participating in an extended conjugated π electronic system, (ii) hydroxy oxygen atoms involved in intra-molecular hydrogen bonds and (iii) hydroxy oxygen atoms involved in intermolecular hydrogen bonds with the HAQ surroundings. It must be immediately noted that HAQ molecules feature acidic protons and tautomer forms, possibly complicating their chemical spaces, which in turn complicates the analysis and the interpretation of HAQ 1 H-NMR spectra 18,19 . From previous works in which experimental and theoretical NMR spectra of HAQ systems are confronted, it turns out that Density Functional Theory (DFT) is most of the time accurate enough for reaching a 5 to 10% error with respect to the experimental chemical shifts of protons bound to sp 2 carbon atoms.…”