Computational Chemistry Strategies to Investigate the Antioxidant Activity of Flavonoids-An Overview.

Despite several decades of research, the beneficial effect of flavonoids on health is still enigmatic. Here, we focus on the antioxidant effect of flavonoids, which is elementary to their biological activity. A relatively new strategy for obtaining a more accurate understanding of this effect is to leverage computational chemistry. This review systematically presents various computational chemistry indicators employed over the past five years to investigate the antioxidant activity of flavonoids. We categorize these strategies into five aspects: electronic structure analysis, thermodynamic analysis, kinetic analysis, interaction analysis, and bioavailability analysis. The principles, characteristics, and limitations of these methods are discussed, along with current trends.

Unraveling the Antioxidant Activity of 2R,3R-dihydroquercetin.

It has been reported that in an oxidative environment, the flavonoid 2R,3R-dihydroquercetin (2R,3R-DHQ) oxidizes into a product that rearranges to form quercetin. As quercetin is a very potent antioxidant, much better than 2R,3R-DHQ, this would be an intriguing form of targeting the antioxidant quercetin. The aim of the present study is to further elaborate on this targeting. We can confirm the previous observation that 2R,3R-DHQ is oxidized by horseradish peroxidase (HRP), with H2O2 as the oxidant. However, HPLC analysis revealed that no quercetin was formed, but instead an unstable oxidation product. The inclusion of glutathione (GSH) during the oxidation process resulted in the formation of a 2R,3R-DHQ-GSH adduct, as was identified using HPLC with IT-TOF/MS detection. GSH adducts appeared on the B-ring of the 2R,3R-DHQ quinone, indicating that during oxidation, the B-ring is oxidized from a catechol to form a quinone group. Ascorbate could reduce the quinone back to 2R,3R-DHQ. No 2S,3R-DHQ was detected after the reduction by ascorbate, indicating that a possible epimerization of 2R,3R-DHQ quinone to 2S,3R-DHQ quinone does not occur. The fact that no epimerization of the oxidized product of 2R,3R-DHQ is observed, and that GSH adducts the oxidized product of 2R,3R-DHQ on the B-ring, led us to conclude that the redox-modulating activity of 2R,3R-DHQ quinone resides in its B-ring. This could be confirmed by chemical calculation. Apparently, the administration of 2R,3R-DHQ in an oxidative environment does not result in 'biotargeting' quercetin.

Flavonoids Seen through the Energy Perspective.

In all life forms, opposing forces provide the energy that flows through networks in an organism, which fuels life. In this concept, health is the ability of an organism to maintain the balance between these opposing forces, which creates resilience, and a deranged flow of energy is the basis for diseases. Treatment should focus on adjusting the deranged flow of energy, e.g., by the redox modulating activity of antioxidants. A major group of antioxidants is formed by flavonoids, a group of polyphenolic compounds abundantly present in our diet. The objective here is to review how the redox modulation by flavonoids fits in the various concepts on the mode of action of bioactive compounds, so we can 'see' where there is overlap and where the missing links are. Based on this fundament, we should choose our research path aiming to 'understand' the redox modulating profile of specific flavonoids, so we can ultimately rationally apply the redox modulating power of flavonoids to improve our health.

Delocalization of the Unpaired Electron in the Quercetin Radical: Comparison of Experimental ESR Data with DFT Calculations.

In the antioxidant activity of quercetin (Q), stabilization of the energy in the quercetin radical (Q•) by delocalization of the unpaired electron (UE) in Q• is pivotal. The aim of this study is to further examine the delocalization of the UE in Q•, and to elucidate the importance of the functional groups of Q for the stabilization of the UE by combining experimentally obtained spin resonance spectroscopy (ESR) measurements with theoretical density functional theory (DFT) calculations. The ESR spectrum and DFT calculation of Q• and structurally related radicals both suggest that the UE of Q• is mostly delocalized in the B ring and partly on the AC ring. The negatively charged oxygen groups in the B ring (3' and 4') of Q• have an electron-donating effect that attract and stabilize the UE in the B ring. Radicals structurally related to Q• indicate that the negatively charged oxygen at 4' has more of an effect on concentrating the UE in ring B than the negatively charged oxygen at 3'. The DFT calculation showed that an OH group at the 3-position of the AC ring is essential for concentrating the radical on the C2-C3 double bond. All these effects help to explain how the high energy of the UE is captured and a stable Q• is generated, which is pivotal in the antioxidant activity of Q.

The Flow of the Redox Energy in Quercetin during Its Antioxidant Activity in Water.

Most studies on the antioxidant activity of flavonoids like Quercetin (Q) do not consider that it comprises a series of sequential reactions. Therefore, the present study examines how the redox energy flows through the molecule during Q's antioxidant activity, by combining experimental data with quantum calculations. It appears that several main pathways are possible. Pivotal are subsequently: deprotonation of the 7-OH group; intramolecular hydrogen transfer from the 3-OH group to the 4-Oxygen atom; electron transfer leading to two conformers of the Q radical; deprotonation of the OH groups in the B-ring, leading to three different deprotonated Q radicals; and finally electron transfer of each deprotonated Q radical to form the corresponding quercetin quinones. The quinone in which the carbonyl groups are the most separated has the lowest energy content, and is the most abundant quinone. The pathways are also intertwined. The calculations show that Q can pick up redox energy at various sites of the molecule which explains Q's ability to scavenge all sorts of reactive oxidizing species. In the described pathways, Q picked up, e.g., two hydroxyl radicals, which can be processed and softened by forming quercetin quinone.

Connecting West and East.

Despite their similarities, Western medicine and Eastern medicine are very different because they are built on different fundamentals. The general idea has arisen that we will benefit by connecting Western and Eastern medicine. First, both the merits as well as the limitations of both types of medicine are discussed. It was concluded that to create a bridge, we should focus on similarities that inspire the further unravelling of the molecular mechanism of the mode of action and toxicity of Traditional Chinese Medicine. It is suggested that the energy perspective provides a basis to integrate Eastern and Western medicine.

Connecting Western and Eastern Medicine from an Energy Perspective.

Although Western medicine and Eastern medicine are worlds apart, there is a striking overlap in the basic principle of these types of medicine when we look at them from the perspective of energy. In both worlds, opposing forces provide the energy that flows through networks in an organism, which fuels life. In this concept, health is the ability of an organism to maintain the balance between these opposing forces, i.e., homeostasis (West) and harmony (East), which creates resilience. Moreover, strategies used to treat diseases are strikingly alike, namely adjusting the flow of energy by changing the connections in the network. The energy perspective provides a basis to integrate Eastern and Western medicine, and opens new directions for research to get the best of both worlds.

Slippage of a porphyrin macrocycle over threads of varying bulkiness: implications for the mechanism of threading polymers through a macrocyclic ring.

Threading of a polymer through a macrocyclic ring may occur directly, that is, by finding the end of the polymer chain, or by a process in which the polymer chain first folds and then threads through the macrocyclic ring in a hairpin-like conformation. We present kinetic and thermodynamic studies on the threading of a macrocyclic porphyrin receptor (H2 1) onto molecular threads that are blocked on one side and are open on the other side. The open side is modified by groups that vary in ease of folding and in bulkiness. Additionally, the threads contain a viologen binding site for the macrocyclic receptor, which is located close to the blocking group. The rates of threading of H2 1 were measured under various conditions, by recording as a function of time the quenching of the fluorescence of the porphyrin, which occurs when receptor H2 1 reaches the viologen binding site. The kinetic data suggest that threading is impossible if the receptor encounters an open side that is sterically encumbered in a similar way as a folded polymer chain. This indicates that threading of polymers through macrocyclic compounds through a folded chain mechanism is unlikely.

Molecular recognition and self-assembly special feature: Squaring cooperative binding circles.

The cooperative binding effects of viologens and pyridines to a synthetic bivalent porphyrin receptor are used as a model system to study how the magnitudes of these effects relate to the experimentally obtained values. The full thermodynamic and kinetic circles concerning both activation and inhibition of the cage of the receptor for the binding of viologens were measured and evaluated. The results strongly emphasize the apparent character of measured binding and rate constants, in which the fractional saturation of receptors with other guests is linearly expressed in these constants. The presented method can be used as a simple tool to better analyze and comprehend the experimentally observed kinetics and thermodynamics of natural and artificial cooperative systems.

A planar conformation and the hydroxyl groups in the B and C rings play a pivotal role in the antioxidant capacity of quercetin and quercetin derivatives.

The polyphenol quercetin (Q) that has a high antioxidant capacity is a lead compound in the design of antioxidants. We investigated the possibility of modifying quercetin while retaining its antioxidant capacity as much as possible. To this end, the antioxidant capacities of Q, rutin, monohydroxyethyl rutinoside (monoHER) and a series of synthesized methylated Q derivatives were determined. The results confirm that the electron donating effect of the hydroxyl groups is essential. It was also found that the relatively planar structure of Q needs to be conserved. This planar conformation enables the distribution of the electron donating effect through the large conjugated π-system over the entire molecule. This is essential for the cooperation between the electron donating groups. Based on the activity of the compounds tested, it was concluded that structural modification at the 5 or 7 position is the most optimal to retain most of the antioxidant capacity of Q. This was confirmed by synthesizing and testing Q5OMe (Q6) and Q7OMe (Q7) that indeed displayed antioxidant capacities closest to Q.

The chemical reactivity of (-)-epicatechin quinone mainly resides in its B-ring.

As one of the important dietary antioxidants, (-)-epicatechin is a potent reactive oxygen species (ROS) scavenger involved in the redox modulation of the cell. When scavenging ROS, (-)-epicatechin will donate two electrons and become (-)-epicatechin quinone, and thus take over part of the oxidative potential of the ROS. The aim of the study is to determine where this chemical reactivity resides in (-)-epicatechin quinone. When this reactivity is spread out over the entire molecule, i.e. over the AC-ring and B-ring, this will lead to partial epimerization of (-)-epicatechin quinone to (-)-catechin quinone. In our experiments, (-)-epicatechin quinone was generated with tyrosinase. The formation of (-)-epicatechin quinone was confirmed by trapping with GSH, and identification of (-)-epicatechin-GSH adducts. Moreover, (-)-epicatechin quinone could be detected using Q-TOF/MS despite its short half-life. To detect the epimerization, the ability of ascorbate to reduce the unstable flavonoid quinones into the corresponding stable flavonoids was used. The results showed that the reduction of the formed (-)-epicatechin quinone by ascorbate did not result in the formation of an appreciable amount of (-)-catechin. Therefore it can be concluded that the chemical reactivity of (-)-epicatechin quinone mainly resides in its B-ring. This could be corroborated by quantum chemical calculations. Understanding the stabilization of the (-)-epicatechin quinone will help to differentiate between flavonoids and to select the appropriate compound for a specific disorder.

The contribution of the major metabolite 4'-O-methylmonoHER to the antioxidant activity of the flavonoid monoHER.

The antioxidant flavonoid 7-mono-O-(ß-hydroxyethyl)-rutoside (monoHER) effectively protects against doxorubicin-induced cardiotoxicity in mice. Doxorubicin is a very effective anticancer drug. The clinical use of doxorubicin is limited by severe cardiotoxicity. Free radicals, i.e., hydroxyl and superoxide radicals play a crucial role in this toxicity. In this study the involvement of the major metabolite of monoHER, 4'-O-methylmonoHER (methylmonoHER) in the protective effect of monoHER is studied. MethylmonoHER displayed antioxidant activity i.e., TEAC, hydroxyl and superoxide radical scavenging activity; nevertheless monoHER appeared to be superior compared to methylmonoHER. As a result of scavenging, flavonoids are oxidized and display reactivity towards thiols. Oxidized methylmonoHER, is far less thiol reactive towards creatine kinase than monoHER, which indicates that methylmonoHER is less toxic towards thiol containing enzymes. The thiol-reactivity of oxidized methylmonoHER was also negligible towards KEAP1 compared to monoHER. These results indicate that methylmonoHER hardly protects against radical damage via scavenging or via activating the NRF2 defense system. Also in HUVECs, methylmonoHER provided far less protection against oxidative stress (EC50>100µM) than monoHER which was a very potent protector (EC50=80nM). The results indicate that the contribution of methylmonoHER to the protection against doxorubicin-induced cardiotoxicity by monoHER is relatively low.

Competition between ascorbate and glutathione for the oxidized form of methylated quercetin metabolites and analogues: tamarixetin, 4'O-methylquercetin, has the lowest thiol reactivity.

Quercetin (Q) is a bioactive compound with excellent antioxidant activity. However, the thiol reactivity of its oxidation product (oxQ) forms a disadvantage. The aim of the present study was to decrease this thiol toxicity. We found that methylated Q metabolites displayed lower thiol reactivity than Q. The most effective was tamarixetin, 4'O-methylquercetin (4'MQ), that has a corresponding oxidation product (ox4'MQ) with thiol reactivity 350 times lower than oxQ. The endogenous metabolism of Q to 4'MQ might be a physiological way to safely benefit from the antioxidant potential of Q in vivo. Our results were explained with Pearson's HSAB concept and corroborated by quantum molecular calculations that revealed a strong correlation between the relative thiol reactivity and the lowest unoccupied molecular orbital (LUMO). The polarity of the molecule and the π-π interaction between the AC- and the B-ring appeared to determine the LUMO and the thiol reactivity of the oxidation product.

An essential difference in the reactivity of the glutathione adducts of the structurally closely related flavonoids monoHER and quercetin.

During the scavenging of free radicals flavonoids are oxidized to electrophilic quinones. Glutathione (GSH) can trap these quinones, thereby forming GSH-flavonoid adducts. The aim of this study was to compare the stability of the GSH-flavonoid adduct of 7-mono-O-(ß-hydroxyethyl)rutoside (monoHER) with that of quercetin. It was found that GSH-quercetin reacts with the thiol N-acetyl-L-cysteine (NAC) to form NAC-quercetin, whereas GSH-monoHER does not react with NAC. In addition, the adduct of the monoHER quinone with the dithiol dithiothreitol (DTT) is relatively stable, whereas the DTT-quercetin adduct is readily converted into quercetin and DTT disulfide. These differences in reactivity of the thiol-flavonoid adducts demonstrate that GSH-monoHER is much more stable than GSH-quercetin. This difference in reactivity was corroborated by molecular quantum chemical calculations. Thus, although both flavonoid quinones are rapidly scavenged by GSH, the advantage of monoHER is that it forms a stable conjugate with GSH, thereby preventing a possible spread of toxicity. These findings demonstrate that even structurally comparable flavonoids behave differently, which will be reflected in the biological effects of these flavonoids.

An essential difference between the flavonoids monoHER and quercetin in their interplay with the endogenous antioxidant network.

Antioxidants can scavenge highly reactive radicals. As a result the antioxidants are converted into oxidation products that might cause damage to vital cellular components. To prevent this damage, the human body possesses an intricate network of antioxidants that pass over the reactivity from one antioxidant to another in a controlled way. The aim of the present study was to investigate how the semi-synthetic flavonoid 7-mono-O-(ß-hydroxyethyl)-rutoside (monoHER), a potential protective agent against doxorubicin-induced cardiotoxicity, fits into this antioxidant network. This position was compared with that of the well-known flavonoid quercetin. The present study shows that the oxidation products of both monoHER and quercetin are reactive towards thiol groups of both GSH and proteins. However, in human blood plasma, oxidized quercetin easily reacts with protein thiols, whereas oxidized monoHER does not react with plasma protein thiols. Our results indicate that this can be explained by the presence of ascorbate in plasma; ascorbate is able to reduce oxidized monoHER to the parent compound monoHER before oxidized monoHER can react with thiols. This is a major difference with oxidized quercetin that preferentially reacts with thiols rather than ascorbate. The difference in selectivity between monoHER and quercetin originates from an intrinsic difference in the chemical nature of their oxidation products, which was corroborated by molecular quantum chemical calculations. These findings point towards an essential difference between structurally closely related flavonoids in their interplay with the endogenous antioxidant network. The advantage of monoHER is that it can safely channel the reactivity of radicals into the antioxidant network where the reactivity is completely neutralized.

Characterization of the glutathione conjugate of the semisynthetic flavonoid monoHER.

Flavonoids protect against oxidative stress by scavenging free radicals. During this protection flavonoids are oxidized. The oxidized flavonoids formed are often reactive. Consequently, protection by flavonoids can result in the formation of toxic products. In this study the oxidation of 7-mono-O-(beta-hydroxyethyl)rutoside (monoHER), which is a constituent of the registered drug Venoruton, was studied in the absence and presence of glutathione (GSH). MonoHER was oxidized by horseradish peroxidase/H(2)O(2). Spectrophotometric and HPLC analysis showed that in the presence of GSH, a monoHER-GSH conjugate was formed, which was identified as 2'-glutathionyl monohydroxyethylrutoside by mass spectrometric analysis and (1)H NMR. Preferential formation of this glutathione adduct in the B ring at C2' was confirmed by molecular quantum chemical calculations. This conjugate was also detected in the bile fluid of a healthy volunteer after iv administration of monoHER, demonstrating its formation in vivo. These results indicate that in the process of offering protection against free radicals, monoHER is converted into an oxidation product that is reactive toward thiols. The formation of this thiol-reactive oxidation product is potentially harmful. Thus, the supposed beneficial effect of monoHER as an antioxidant may be accompanied by the formation of products with an electrophilic, toxic potential.