Flavonols with a catechol or pyrogallol substitution pattern on ring B readily form stable dimers in phosphate buffered saline at four degrees celsius

Exploring the mechanism of flavonoids modification by dimerization strategies and their potential to enhance biological activity

Flavonoid dimers are being focused due to their particular structure that links two units through CC or C-O-C bonds. This paper provides a comprehensive and systematic overview of the reaction mechanism of flavonoid dimerization and discusses their synthesis process and methods to devise an ideal preparation scheme of flavonoid dimers. Given the polyphenolic hydroxyl groups of dimerized flavonoids as well as their unique bridging molecular structures, we preliminarily explored the link between conformation and function, and discovered their several reinforced bioactivities compared to flavonoid monomers, such as hypolipidemic, antidiabetic, and neuroprotective activities as well as other potential. In contrast to monomer, luteolin dimer demonstrated the remarkably higher activity in inhibiting α-amylase, α-glucosidase and the growth of A. niger (IC50: 0.86 μM), which is even comparable to acarbose and amphotericin B. Therefore, dimerization strategy represents a promising method for structural modification of flavonoid to potential applications in food supplements or pharmaceuticals.

In Vitro Comparative Analysis of the Effect and Structure-Based Influencing Factors of Flavonols on Lipid Accumulation

Flavonols represented by quercetin have been widely reported to have biological activities of regulating lipid metabolism. However, the differences in flavonols with different structures in lipid-lowering activity and the influencing factors remain unclear. In this study, the stability, transmembrane uptake ratio, and lipid metabolism regulation activities of 12 flavonol compounds in the 3T3-L1 cell model were systematically compared. The results showed that kaempferide had the highest cellular uptake ratio and the most potent inhibitory effect on adipogenesis at a dosing concentration of 20 μM, followed by isorhamnetin and kaempferol. They inhibited TG accumulation by more than 65% and downregulated the expression of PPARγ and SREBP1c by more than 60%. The other four aglycones, including quercetin, did not exhibit significant activity due to the structural instability in the cell culture medium. Meanwhile, five quercetin glucosides were quite stable but showed a low uptake ratio that no obvious activity was observed. Correlation analysis also showed that for 11 compounds except galangin, the activity was positively correlated with the cellular uptake ratio (p < 0.05, r = 0.6349). These findings may provide a valuable idea and insight for exploring the structure-based activity of flavonoids at the cellular level.

Thermal degradation of (2R, 3R)-dihydromyricetin in neutral aqueous solution at 100 ℃

In the field of thermal degradation of flavonoids, current studies mainly focused on flavonols. However, the thermal degradation of dihydroflavonols in aqueous solution has received limited attention compared to flavonols. The single C2-C3 bonds of dihydroflavonols, which differs from the C2-C3 double bond in flavonols, may cause different degradation mechanisms. Dihydromyricetin (DMY) is a typical dihydroflavonol with six hydroxyl groups, and possesses various health effects. We explored the thermal degradation of DMY in neutral aqueous solution (pH 7) at 100 ℃. Ultra-performance liquid chromatography combined with photodiode array and electrospray ionization quadrupole-time-of-flight tandem mass spectrometric detection (UPLC-PDA-ESI-QTOF-MS/MS) provided suitable platform for exploring DMY degradation pathways, and negative ion mode was applied. Thermal treatment led to a decline in DMY level with time, accompanied by the appearance of various degradation products of DMY. Degradation mechanisms of DMY included isomerization, oxidation, hydroxylation, dimerization and ring cleavage. The pyrogallol-type ring B of DMY might be initially oxidized into ortho-quinone, which could further attack another DMY to form dimers. In addition, hydroxylation is likely to occur at C-2, C-3 of DMY or DMY dimers, which then further yields ring-cleavage products via breakage of the O1-C2 bond, C2-C3 bond, or C3-C4 bond. The 3-hydroxy-5-(3,3,5,7-tetrahydroxy-4-oxochroman-2-yl) cyclohexa-3,5-diene-1, 2-dione (m/z 333.0244) and unknown compound m/z 435.0925 were annotated as key intermediates in DMY degradation. Four phenolic acids, including 3,4,5-trihydroxybenzoic acid (m/z 169.0136, RT 1.4 min), 2,4,6-trihydroxyphenylglyoxylic acid (m/z 197.0084, RT 1.7 min), 2-oxo-2-(2,4,6-trihydroxyphenyl) acetaldehyde (m/z 181.0132, RT 2.4 min), and 2,4,6-trihydroxybenzoic acid (m/z 169.0139, RT 2.5 min) were identified as the major end products of DMY degradation. In addition, 5-((3,5dihydroxyphenoxy) methyl)-3-hydroxycyclohexa-3,5-diene-1,2-dione (m/z 261.0399, RT 11.7 min) and unidentified compound with m/z 329.0507 (RT 1.0 min) were also suggested to be end products of DMY degradation. These results provide novel insights on DMY stability and degradation products. Moreover, the heat treatment of DMY aqueous solution was found to gradually reduce the antioxidant activities of DMY, and even destroy the beneficial effect of DMY on the gut microbiota composition.

Red Wine Inspired Chemistry: Hemisynthesis of Procyanidin Analogs and Determination of Their Protein Precipitation Capacity, Octanol-Water Partition, and Stability in Phosphate-Buffered Saline

Ten dimeric procyanidin (PC) analogs were hemisynthesized from catechin or epicatechin and from five different aldehydes using the same mechanism that produces the important acetaldehyde-mediated adducts of proanthocyanidins (PAs) and anthocyanins in red wine. Protein precipitation capacity (PPC), octanol-water partition coefficient (log P) and stability of the PC analogs were determined. The emphasis was on the PPC because it has been shown to correlate with anthelmintic activity against gastrointestinal nematodes in ruminants and with other beneficial bioactivities in animals, as well. The PPC of PC analogs was greatly improved compared to natural PC dimers, but the capacity was not as great as that of a PC trimer or epigallocatechin gallate. The log P of PC analogs varied from hydrophobic to hydrophilic depending on the intramolecular linkage. Great variation was observed in stabilities of PC analogs in phosphate buffered saline, and the mixtures of degradation products were characterized using high-resolution mass spectrometry.

Impact of thermal processing on dietary flavonoids

Flavonoids are a class of polyphenols which are widely distributed in natural products and foods. They have diverse bioactivities, including anti-inflammatory, anti-aging, and antioxidant activities. Generally, the foods rich in flavonoids are usually consumed after thermal processing. However, thermal stability of flavonoids is usually low, and thermal processing could cause either positive or negative influences on their stability and bioactivities. In this review, the effects of thermal processing on thermal stability and bioactivity of dietary flavonoids from different food sources are summarized. Then, strategies to improve thermal stability of dietary flavonoids are discussed and the effect of some promising thermal technologies are also preliminary clarified. The promising thermal technologies may be alternative to conventional thermal processing technologies.

Stability of polyphenols in food processing

In recent years, polyphenols have attracted considerable attention due to their diverse potential health-beneficial effects on humans. Polyphenols are widely distributed in natural plants, and therefore play an important role in human food. Thermal processing, irradiation, fermentation, high pressure, microwave, and drying are several popular food processing methods. However, polyphenols are instable in food processing, which easily degrade and react with other components because of their polyhydroxy characteristic. Traditional and advanced technologies have been used to characterize the stability of polyphenols. The main influence factors of stability of polyphenols such as pH, temperature, light, oxygen, enzymes, metal ions, as well as macromolecules, are summarized. Besides, thermal processing greatly promoted the degradation of polyphenols. Thermal degradation mechanisms and products of some polyphenols, such as quercetin and rutin, have been intensively demonstrated. Nevertheless, the structural changes of polyphenols caused by food processing, may lead to different bioactivities from the obtained results based on unprocessed polyphenols. Therefore, to maximize the beneficial effects of polyphenols ingested by human from processed food, the stability of polyphenols in food processing must be thoroughly investigated to assess their real bioactivities. In addition, some available technologies for improving the stability of polyphenols in food processing have been proposed.

Stability profiling and degradation products of dihydromyricetin in Dulbecco's modified eagle's medium

Dihydromyricetin has shown many bioactivities in cell level. However, dihydromyricetin was found to be highly instable in cell culture medium DMEM. Here, the underlying degradation mechanism was investigated via UPLC-MS/MS analysis. Dihydromyricetin was mainly converted into its dimers and oxidized products. At lower temperature, dihydromyricetin in DMEM showed higher stability. Vitamin C increased the stability of dihydromyricetin in DMEM probably due to its high antioxidant potential.

A multifaceted review on dihydromyricetin resources, extraction, bioavailability, biotransformation, bioactivities, and food applications with future perspectives to maximize its value

Natural bioactive compounds present a better alternative to prevent and treat chronic diseases owing to their lower toxicity and abundant resources. (+)-Dihydromyricetin (DMY) is a flavanonol, possessing numerous interesting bioactivities with abundant resources. This review provides a comprehensive overview of the recent advances in DMY natural resources, stereoisomerism, physicochemical properties, extraction, biosynthesis, pharmacokinetics, and biotransformation. Stereoisomerism of DMY should be considered for better indication of its efficacy. Biotechnological approach presents a potential tool for the production of DMY using microbial cell factories. DMY high instability is related to its powerful antioxidant capacity due to pyrogallol moiety in ring B, and whether preparation of other analogues could demonstrate improved properties. DMY demonstrates poor bioavailability based on its low solubility and permeability with several attempts to improve its pharmacokinetics and efficacy. DMY possesses various pharmacological effects, which have been proven by many in vitro and in vivo experiments, while clinical trials are rather scarce, with underlying action mechanisms remaining unclear. Consequently, to maximize the usefulness of DMY in nutraceuticals, improvement in bioavailability, and better understanding of its actions mechanisms and drug interactions ought to be examined in the future along with more clinical evidence.

A Comprehensive Review on the Interaction of Milk Protein Concentrates with Plant-Based Polyphenolics

Functional properties and biological activities of plant-derived polyphenolic compounds have gained great interest due to their epidemiologically proven health benefits and diverse industrial applications in the food and pharmaceutical industry. Moreover, the food processing conditions and certain chemical reactions such as pigmentation, acylation, hydroxylation, and glycosylation can also cause alteration in the stability, antioxidant activity, and structural characteristics of the polyphenolic compounds. Since the (poly)phenols are highly reactive, to overcome these problems, the formulation of a complex of polyphenolic compounds with natural biopolymers is an effective approach. Besides, to increase the bioavailability and bioaccessibility of polyphenolic compounds, milk proteins such as whey protein concentrate, sodium caseinate, and milk protein concentrate act as natural vehicles, due to their specific structural and functional properties with high nutritional value. Therefore, milk proteins are suitable for the delivery of polyphenols to parts of the gastrointestinal tract. Therefore, this review reports on types of (poly)phenols, methods for the analysis of binding interactions between (poly)phenols-milk proteins, and structural changes that occur during the interaction.

Stability and antioxidant capacity of epigallocatechin gallate in Dulbecco's modified eagle medium

Though the instability of polyphenols in cell culture experiment has been investigated previously, the underlying mechanism is not completely clear yet. Therefore, in this study, the stability of epigallocatechin gallate (EGCG) in cell culture medium DMEM was investigated at 4 °C and 37 °C via UPLC-MS-MS analysis followed by determination of the antioxidant capacity of EGCG. EGCG was instable in DMEM and formed various degradation products derived from its dimer with increasing incubation time with many isomers being formed at both temperatures. The dimer products were more stable at 4 °C than at 37 °C. The structure and formation mechanism of five products were analyzed with four unidentified. Ascorbic acid significantly improved the stability of EGCG by protecting EGCG from auto-oxidation in DMEM, particularly at 4 °C. The antioxidative activity of EGCG in DMEM was determined by DPPH, ABTS and FRAP assay. The antioxidative properties of EGCG continuously decreased over 8 h in DMEM, which was consistent with its course of degradation.

Relationships between Structure and Antioxidant Capacity and Activity of Glycosylated Flavonols

The antioxidant capacity (AC) and antioxidant activity (AA) of three flavonols (FLV), aglycones and their glycosylated derivatives were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays in various solvents. Findings confirmed that the glycosylation at the 3-position (3-glycosylation) always decreased the AC under most conditions due to substitution of the 3-position hydroxyl group and glycoside disruption in the molecular planarity. The 7-glycosylated derivatives did not have the above effects, thus generally exhibited ACs similar to their aglycones. Glycosylation decreased the AA of kaempferol and isorhamnetin for both assays in methanol, 3-glycosylation inhibited quercetin AA in the ABTS assay. In the DPPH assay, the AA of 3-glycosylated quercetin was significantly higher than quercetin. Using LC-MS/MS analysis, we found that quercetin and quercetin-7-glucoside underwent dimerization during the antioxidant reaction, potentially leading to a decline in AAs. However, 3-glycoside substitution may have hindered dimer formation, thereby allowing the FLVs to retain strong free radical scavenging abilities.

Objective measures of greengage wine quality: From taste-active compound and aroma-active compound to sensory profiles

This study is sought to identify the components in greengage wine that predict the sensory properties. Taste-active compounds and aroma-active compounds of 20 commercially available greengage wines from different regions were characterized. The relationship between these compounds, wine samples and sensory attributes was modeled by partial least squares regression. The regression analysis indicated the taste-active compounds, alanine, leucine, proline, glutamic acid, lysine, malic acid, citric acid, sucrose, glucose, gallic acid, caffeic acid and tannin made a great contribution to the characteristic taste or mouthfeel of greengage wine. Meanwhile, the aroma-active compounds, including ethyl acetate, ethyl butanoate, ethyl hexanoate, ethyl octanoate, ethyl decanoate, 3-methylbutanol, 5-hydroxymethylfurfural, octanoic acid and benzaldehyde, modeled well with the flavor characteristic of greengage wine. The study revealed new insights into the relationship between chemistry and wine sensory characters, which has implications for developing an objective measurement system for determining greengage wine quality.

Investigation of new products and reaction kinetics for myricetin in DMEM via an in situ UPLC–MS–MS analysis

Myricetin, with its pyrogallol B‐ring, is evidently instable in DMEM (Dulbecco's modified Eagle's) medium at 37°C. However, the underlying mechanism of this instability is not clear yet. Herein, the reaction products of myricetin in DMEM were investigated via an in situ UPLC–MS–MS analysis. Myricetin was mixed with prewarmed DMEM in an autosampler screw top vial for UPLC. Immediately, a 50‐µl sample was automatically injected every 50 min. Myricetin was highly stable in dimethyl sulfoxide (DMSO) when kept at 37°C for 2 weeks. Myricetin was very unstable in DMEM at 37°C and largely converted to new products A (m/z = 635.07) and B (m/z = 633.05) within 1 min. Another new product, C (m/z = 469.04), was detected after the fourth injection (150 min). Myricetin was much more stable in DMEM at 4°C as compared to 37°C. There was a minor conversion of myricetin to the new product A (m/z = 635.07) within 1.0 min, but it subsequently increased until the sixth injection (250 min), and disappeared after the fifteenth injection (700 min). In summary, myricetin first formed a dimer A (m/z = 635.07) and then was oxidized to B (m/z = 633.05), which was finally degraded to C (m/z = 469.04) in DMEM. This process was significantly affected by the temperature, and low temperature is helpful to detect the unstable products.