Abstract
Several classes of flavonoids, such as anthocyanins, flavonols, flavanols, and flavones, undergo a slow H/D exchange on aromatic ring A, leading to full deuteration at positions C(6) and C(8). Within the flavanol class, H-C(6) and H-C(8) of catechin and epicatechin are slowly exchanged in D2O to the corresponding deuterated analogues. Even quercetin, a relevant flavonol representative, shows the same behaviour in a D2O/DMSOd6 1:1 solution. Detailed kinetic measurements of these H/D exchange processes are here reported by exploiting the time-dependent changes of their peak areas in the 1H-NMR spectra taken at different temperatures. A unifying reaction mechanism is also proposed based on our detailed kinetic observations, even taking into account pH and solvent effects. Molecular modelling and QM calculations were also carried out to shed more light on several molecular details of the proposed mechanism.
Highlights
Flavonoids are polyphenolic compounds that are diverse in both chemical structure and properties
The antioxidant activity of flavonoids depends upon the arrangement of functional groups on the molecular skeleton [3,4]
Assuming the well-known and largely accepted mechanism involving keto-enol tautomerism of 1,3,5 activated aromatic systems [24], we propose that deuterated keto-analogues (4ab and 4ac) could be the putative transient intermediates leading to the detectable intermediates 4b (k1) and 4c (k2) in the two competitive divergent processes 4a → 4b and 4a → 4c, respectively
Summary
Flavonoids are polyphenolic compounds that are diverse in both chemical structure and properties. Since flavonoids are naturally present in fruits, vegetable, and tea, they are an integral part of the human diet. It is generally accepted that diary flavonoids have important biological effects [1,2]. Flavonoids often demonstrate phytoalexin and antioxidant properties, important for promoting abiotic and biotic stress tolerance. The antioxidant activity of flavonoids depends upon the arrangement of functional groups on the molecular skeleton [3,4]. Flavonoids are divided into several classes of compounds based on substituents and the functional groups pattern on the shared backbone (Scheme 1)
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