The antiradical activity of some selected flavones and flavonols. Experimental and quantum mechanical study

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.

Comparison of binding sites and affinity of flavonol-Cu(II) complexes with the same parent nucleus: Synthesis, DFT prediction, and coordination pattern

This study explores the coordination dynamics between dietary polyphenols, specifically kaempferol, quercetin, and myricetin, and Cu ions in aqueous environments. A novel synthesis method for flavonol-Cu(II) coordination compounds is introduced, effectively reducing interference from free metal ions. Our results reveal consistent binding patterns of Cu ions with flavonols (2:1 ratio of flavonol to Cu(II)), predominantly at the 4,5 sites. Various analytical techniques are used to validate these coordination ratios and sites. The binding affinity of the flavonols for Cu ions follows a descending sequence: myricetin > quercetin > kaempferol. Notably, coordination with Cu ions enhances the free-radical scavenging activities of these flavonols. These findings hold substantial importance for food chemistry, biology, and medicine, providing crucial insights into the way dietary flavonols form stable structures in environments similar to human body fluids and their interactions with metal ions, opening new possibilities for their application and understanding in diverse scientific domains.

Molecular Structure, Antioxidant Potential, and Pharmacokinetic Properties of Plant Flavonoid Blumeatin and Investigating Its Inhibition Mechanism on Xanthine Oxidase for Hyperuricemia by Molecular Modeling

Hyperuricemia, which usually results in metabolic syndrome symptoms, is increasing rapidly all over the world and becoming a global public health issue. Xanthine oxidase (XO) is regarded as a key drug target for the treatment of this disease. Therefore, finding natural, nontoxic, and highly active XO inhibitors is quite important. To get insights into inhibitory potential toward XO and determine antioxidant action mechanism depending on the molecular structure, plant flavonoid blumeatin was investigated for the first time by Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT), ADME/Tox (absorption, distribution, metabolism, excretion, and toxicity) analysis, and molecular docking study. Theoretical findings indicated that blumeatin has high radical scavenging activity due to its noncoplanarity and over twisted torsion angle (-94.64°) with respect to its flavanone skeleton could explain that there might be a correlation between antioxidant activity and planarity of blumeatin. Based on the ADME/Tox analysis, it is determined that blumeatin has a high absorption profile in the human intestine (81.93%), and this plant flavonoid is not carcinogenic or mutagenic. A molecular docking study showed that Thr1010, Val1011, Phe914, and Ala1078 are the main amino acid residues participating in XO's interaction with blumeatin via hydrogen bonds.

Rice Byproduct Compounds: From Green Extraction to Antioxidant Properties

Currently, rice (Oryza sativa L.) production and consumption is increasing worldwide, and many efforts to decrease the substantial impact of its byproducts are needed. In recent years, the interest in utilizing rice kernels, husk, bran, and germ for the recovery of different molecules, from catalysts (to produce biodiesel) to bioactive compounds, has grown. In fact, rice byproducts are rich in secondary metabolites (phenolic compounds, flavonoids, and tocopherols) with different types of bioactivity, mainly antioxidant, antimicrobial, antidiabetic, and anti-inflammatory, which make them useful as functional ingredients. In this review, we focus our attention on the recovery of antioxidant compounds from rice byproducts by using innovative green techniques that can overcome the limitations of traditional extraction processes, such as their environmental and economic impact. In addition, traditional assays and more innovative methodologies to evaluate the antioxidant activity are discussed. Finally, the possible molecular mechanisms of action of the rice byproduct antioxidant compounds (phenolic acids, flavonoids, γ-oryzanol, and vitamin E) are discussed as well. In the future, it is expected that rice byproduct antioxidants will be important food ingredients that reduce the risk of the development of several human disorders involving oxidative stress, such as metabolic diseases, inflammatory disorders, and cancer.

Genkwanin: An emerging natural compound with multifaceted pharmacological effects

Plant bioactive molecules could play key preventive and therapeutic roles in chronological aging and the pathogenesis of many chronic diseases, often accompanied by increased oxidative stress and low-grade inflammation. Dietary antioxidants, including genkwanin, could decrease oxidative stress and the expression of pro-inflammatory cytokines or pathways. The present study is the first comprehensive review of genkwanin, a methoxyflavone found in several plant species. Indeed, natural sources, and pharmacokinetics of genkwanin, the biological properties were discussed and highlighted in detail. This review analyzed and considered all original studies related to identification, isolation, quantification, investigation of the biological and pharmacological properties of genkwanin. We consulted all published papers in peer-reviewed journals in the English language from the inception of each database to 12 May 2023. Different phytochemical demonstrated that genkwanin is a non-glycosylated flavone found and isolated from several medicinal plants such as Genkwa Flos, Rosmarinus officinalis, Salvia officinalis, and Leonurus sibiricus. In vitro and in vivo biological and pharmacological investigations showed that Genkwanin exhibits remarkable antioxidant and anti-inflammatory activities, genkwanin, via activation of glucokinase, has shown antihyperglycemic activity with a potential role against metabolic syndrome and diabetes. Additionally, it revealed cardioprotective and neuroprotective properties, thus reducing the risk of cardiovascular diseases and assisting against neurodegenerative diseases. Furthermore, genkwanin showed other biological properties like antitumor capability, antibacterial, antiviral, and dermato-protective effects. The involved mechanisms include sub-cellular, cellular and molecular actions at different levels such as inducing apoptosis and inhibiting the growth and proliferation of cancer cells. Despite the findings from preclinical studies that have demonstrated the effects of genkwanin and its diverse mechanisms of action, additional research is required to comprehensively explore its therapeutic potential. Primarily, extensive studies should be carried out to enhance our understanding of the molecule's pharmacodynamic actions and pharmacokinetic pathways. Moreover, toxicological and clinical investigations should be undertaken to assess the safety and clinical efficacy of genkwanin. These forthcoming studies are of utmost importance in fully unlocking the potential of this molecule in the realm of therapeutic applications.

Potential of Tamarind Shell Extract against Oxidative Stress In Vivo and In Vitro

Tamarind shell is rich in flavonoids and exhibits good biological activities. In this study, we aimed to analyze the chemical composition of tamarind shell extract (TSE), and to investigate antioxidant capacity of TSE in vitro and in vivo. The tamarind shells were extracted with 95% ethanol refluxing extraction, and chemical constituents were determined by ultra-performance chromatography-electrospray tandem mass spectrometry (UPLC-MS/MS). The free radical scavenging activity of TSE in vitro was evaluated using the oxygen radical absorbance capacity (ORAC) method. The antioxidative effects of TSE were further assessed in 2,2-azobis (2-amidinopropane) dihydrochloride (AAPH)-stimulated ADTC5 cells and tert-butyl hydroperoxide (t-BHP)-exposed zebrafish. A total of eight flavonoids were detected in TSE, including (+)-catechin, taxifolin, myricetin, eriodictyol, luteolin, morin, apigenin, and naringenin, with the contents of 5.287, 8.419, 4.042, 6.583, 3.421, 4.651, 0.2027, and 0.6234 mg/g, respectively. The ORAC assay revealed TSE and these flavonoids had strong free radical scavenging activity in vitro. In addition, TSE significantly decreased the ROS and MDA levels but restored the SOD activity in AAPH-treated ATDC5 cells and t-BHP-exposed zebrafish. The flavonoids also showed excellent antioxidative activities against oxidative damage in ATDC5 cells and zebrafish. Overall, the study suggests the free radical scavenging capacity and antioxidant potential of TSE and its primary flavonoids in vitro and in vivo and will provide a theoretical basis for the development and utilization of tamarind shell.

Quantum-mechanical characteristics of apigenin: Antiradical, metal chelation and inhibitory properties in physiologically relevant media

Density functional theory was used to examine the antioxidant activity of apigenin. All protonated species that are present in a non-negligible population at physiological pH were considered in the study. The ability to scavenge the hydroperoxide radical was evaluated in lipid and aqueous environments. The capacity to halt the Fenton reaction by chelating Fe(III) and Cu(II) ions was also investigated, as was the ability to inhibit xanthine oxidase. The results indicate that these activities may be particularly important in describing the beneficial effects of apigenin, especially because of its lower anti-•OOH potential than Trolox or vitamin C. The findings underscore the significant role of dianion in the antiradical and chelating properties, despite its presence in much lower molar fractions than other ions.

Nitrogen Fertilization Influences the Quantity, Composition, and Tissue Association of Foliar Phenolics in Strawberries

Unlike quantitative changes, the compositional changes of plant phenolics and changes in their tissue association as influenced by the nutrient supply are less well understood. We evaluated the quantity, composition, and tissue association of phenolics in leaves of two Fragaria ananassa cultivars in response to different levels of nitrogen (N) fertilization using global metabolomic approaches. Influence of N supply on phenolic content in both cultivars was similar, but the magnitude of this response was compound specific. Ellagitannins, the most abundant class of phenolic oligomers, were less responsive to the applied N treatments, whereas proanthocyanidins, the less abundant class of phenolic oligomers, exhibited higher fold change. Within mono-phenolics, the hydroxycinnamates were more abundant but showed lower fold change than the hydroxybenzoates. Among flavonoids, the hydroxylated flavonols showed higher abundances than the flavones, with a preferential accumulation of dihydroxylated flavonol at lower N levels. Furthermore, glycosylated flavonols were higher than the acylated forms. The extractable fraction of phenolics was more influenced by the N treatment than the fiber-bound fraction. The extensive compositional modification of phenolics and a greater response of non-bound fractions in response to N rates highlight the potential to use precise management of N supply as an effective strategy to enhance the bioactive compounds in crops.

A topical formulation containing quercetin-loaded microcapsules protects against oxidative and inflammatory skin alterations triggered by UVB irradiation: enhancement of activity by microencapsulation

Ultraviolet B (UVB) irradiation causes free radical production, increase inflammation and oxidative stress, thus, supporting the use of antioxidants by topical administration as therapeutic approaches. Quercetin (QC) is a flavonoid with antioxidant activity, however, high liposolubility makes it difficult to remain in the viable skin layer. Thus, this study evaluated whether microencapsulation of QC would enhance its activity in comparison with the same dose of free QC (non-active dose) and unloaded-microcapsules added in formulation for topical administration in a mouse model of UVB irradiation targeting the skin. Topical formulation containing Quercetin-loaded microcapsules (TFcQCMC) presents physico-chemical (colour, consistence, phase separation and pH) and functional antioxidant stability at 4 °C, room temperature and 40 °C for 6 months. TFcQCMC inhibited the UVB-triggered depletion of antioxidants observed by GSH (reduced glutathione), ability to reduce iron, ability to scavenge 2,2'-azinobis radical and catalase activity. TFcQCMC also inhibited markers of oxidation (lipid hydroperoxides and superoxide anion production). Concerning inflammation, TFcQCMC reduced the production of inflammatory cytokines, matrix metalloproteinase-9 activity, skin edoema, collagen fibre damage, myeloperoxidase activity/neutrophil recruitment, mast cell and sunburn cell counts. The pharmacological activity of TFcQCMC was not shared by the same pharmaceutical form containing the same dose of free QC or unloaded control microcapsules.

Spectroscopic elucidation (FT-IR, FT-Raman and UV-visible) with NBO, NLO, ELF, LOL, drug likeness and molecular docking analysis on 1-(2-ethylsulfonylethyl)-2-methyl-5-nitro-imidazole: An antiprotozoal agent

1-(2-ethylsulfonylethyl)-2-methyl-5-nitro-imidazole (1EMI) C₈H₁₃N₃O₄S also known as Tinidazole, selected for its antiprotozoal property is extensively used for spectroscopic elucidations and computational aspects using density functional methods. Along with spectral conclusions, further investigations on fundamental reactive properties such as electrical, optical, nonlinear combined with DFT simulations were performed. Molecular docking procedure supports the results of chosen appropriate antiprotozoal agent based on ligand-protein interactions. Experimental and simulated (B3LYP/6-311++G (d,p)) IR and Raman spectra showed concurrence. NLO analysis through first order hyperpolarizability parameter helps in finding the potential of 1EMI as a good NLO candidate. Charge delocalization and the stability of the compound were discussed using natural bond orbital (NBO) analysis. Furthermore, Electron localization function (ELF), local orbital locator (LOL), and Frontier molecular orbitals (FMO) were studied. Besides, Mulliken population analysis on atomic charges, Energy gap, chemical potential, global hardness, softness, ionization potential, electronegativity, electrophilicity index along thermodynamic parameters (enthalpy, entropy and heat capacity) have been calculated. Drug likeness parameters and molecular docking approach enabled to check pharmaceutical potential and biological activity of 1EMI. The biological activity of 1EMI through ligand and protein interactions have been confirmed theoretically for the treatment of Malaria, Invasive aspergillosis and Mycobacterium tuberculosis with respect to chosen proteins. Three different activity targets and protein interactions are quite successful revealing the bond distances, intermolecular energy, binding energy and inhibition constant. 2D interaction profile image of the two maximum interacted proteins and also Ramachandran plot used to show stereochemistry of selected protein. The activities of 1EMI were studied in accordance with literature survey and the results were presented.

Flavones' and Flavonols' Antiradical Structure-Activity Relationship-A Quantum Chemical Study

Flavonoids are known for their antiradical capacity, and this ability is strongly structure-dependent. In this research, the activity of flavones and flavonols in a water solvent was studied with the density functional theory methods. These included examination of flavonoids’ molecular and radical structures with natural bonding orbitals analysis, spin density analysis and frontier molecular orbitals theory. Calculations of determinants were performed: specific, for the three possible mechanisms of action—hydrogen atom transfer (HAT), electron transfer–proton transfer (ETPT) and sequential proton loss electron transfer (SPLET); and the unspecific—reorganization enthalpy (RE) and hydrogen abstraction enthalpy (HAE). Intramolecular hydrogen bonding, catechol moiety activity and the probability of electron density swap between rings were all established. Hydrogen bonding seems to be much more important than the conjugation effect, because some structures tends to form more intramolecular hydrogen bonds instead of being completely planar. The very first hydrogen abstraction mechanism in a water solvent is SPLET, and the most privileged abstraction site, indicated by HAE, can be associated with the C3 hydroxyl group of flavonols and C4’ hydroxyl group of flavones. For the catechol moiety, an intramolecular reorganization to an o-benzoquinone-like structure occurs, and the ETPT is favored as the second abstraction mechanism.

Exploring the detailed spectroscopic characteristics, chemical and biological activity of two cyanopyrazine-2-carboxamide derivatives using experimental and theoretical tools

This article represents the spectroscopic and computational studies of two new pyrazine compounds. In order to establish the structure and functional nature of the compounds, we have employed Fourier transformed infrared (FT-IR) and Raman spectra, nuclear magnetic resonance (NMR) spectra, and ultraviolet (UV) absorptions and have compared them with the simulated computational spectra and found that they are in the agreeable range. Simulated hyperpolarisability values are used to obtain the nonlinear optic (NLO) activity of the compound, to be used in organic electronic materials. The charge transfer and related properties was investigated by the simulation of electronic spectrum with time dependent density functional theory (TD-DFT). Natural transition orbitals (NTO) provides information about which region of the molecules are more involved in the electronic transitions and the charge transfer properties for the lowest energy excitation have been analyzed on the basis of electron density variation. Molecular dynamics simulations provide information about the behavior of the molecule in solutions. Frontier orbital analysis and study of various reactivity descriptors like ALIE and Fukui provided deep knowledge on the reactivity side. Molecular docking has been also performed to investigate the interaction between title molecules and exhibits inhibitory activity against Pseudomonas aeruginosa Enoyl-Acyl carrier protein reductase (Fabl).

Computational evaluation of the reactivity and pharmaceutical potential of an organic amine: A DFT, molecular dynamics simulations and molecular docking approach

2-[N-(carboxymethyl)anilino] acetic acid (PIDAA) molecule has been spectroscopically characterized and computationally investigated for its fundamental reactive properties by a combination of density functional theory (DFT) calculations, molecular dynamics (MD) simulations and molecular docking procedure. A comparison drawn between the simulated and experimentally attained spectra by FT-Raman and FT-IR showed concurrence. The natural bond orbital (NBO) analysis enabled in comprehending the stability and charge delocalization in the title molecule. The first hyperpolarizability which is an important parameter for future studies of nonlinear optics (NLO) was calculated to check the potential of the molecule to be an NLO material. Besides, frontier molecular orbitals (FMO), electron localization function (ELF) and localized orbital locator (LOL) analysis were performed. Energy gap (ΔE), electronegativity (χ), chemical potential (μ), global hardness (η), softness (S), Mulliken population analysis on atomic charges and thermodynamic properties of the title compound at different temperatures have been calculated. The local reactive properties of PIDAA have been addressed by MEP and ALIE surfaces, together with bond dissociation energy for hydrogen abstraction (H-BDE). MD simulations have been used in order to identify atoms with pronounced interactions with water molecules. The pharmaceutical potential of PIDAA has been considered by the analysis of drug likeness parameters and molecular docking procedure. The biological activity of the molecule in terms of molecular docking has been analyzed theoretically for the treatment of SARS and minimum binding energy calculated. The Ramachandran plot was used to check the stereochemistry of the protein structure. In addition, a comparison of the physiochemical parameters of PIDAA and commercially available drugs (Yu et al., 2004; Tan et al., 2004; Elshabrawy et al., 2014; Chu et al., 2004; Gopal Samy and Xavier, 2015) were carried out.

Caffeoylquinic Acids: Separation Method, Antiradical Properties and Cytotoxicity

Twelve chlorogenic acid derivatives and two flavones were isolated from Moquiniastrum floribundum (Asteraceae, other name: Gochnatia floribunda). Compounds were evaluated in relation to their cytotoxicity and antiradical properties. Cytotoxicity was not observed for compounds, however, chlorogenic acid derivatives showed antiradical activity and were more active than the Trolox standard. Quinic acid esterified with caffeoyl group at C-4 position showed higher antiradical activity compared to acylation at C-3 or C-5 positions. Additional caffeoyl groups esterified in quinic acid increase the antiradical activity observed for 4-caffeoylquinic acid. Excepted to 3,4-dicaffeoylquinic acid methyl ester, methyl ester derivatives show higher capacity of trapping radicals than their respective acids. Consequently, the presence of caffeoyl group at C-4 position of quinic acid is suggested as fundamental to obtain the highest antiradical activity.

Theoretical study of the antioxidant capacity of the flavonoids present in the Annona muricata (Soursop) leaves

A theoretical approach was used to evaluate the antioxidant capacity of 20 flavonoids reported in Annona muricata leaves. The theoretical study was at the GGA level using the wB97XD functional and the cc-pvtz basis set. The calculations were performed in gas phase and implicit solvent phase. The flavonol robinetin (03c) and the flavanol gallocatechin (01c) are species that exhibited the best antioxidant capacity in the HAT, SEPT, and SPLET mechanisms. On the other hand, in the SET I mechanism, flavonol quercetin (03b) was the best, and in the SET II mechanism, the most favored species is the flavanol catechin (01a). However, these species do not achieve to overcome the antioxidant capacity presented by the Trolox.

A DFT-based study of the hydrogen-bonding interactions between myricetin and ethanol/water

Flavonoids are vital constituents of propolis that are responsible for its medicinal activity. Flavonoid extraction commonly employs ethanol and water as solvents. In the extraction reaction, hydrogen-bonding interactions play a crucial role. In this study, hydrogen-bonding interactions between myricetin-an abundant flavonoid in propolis-and ethanol or water were studied theoretically using density functional theory (DFT) methods. The molecular geometry and charge properties of the myricetin monomer were analyzed first. After careful optimization, nine stable myricetin-CH₃CH₂OH/H₂O complex geometries were obtained. Hydrogen bonds were confirmed to exist in these optimized structures. The most stable structures were found to be those with hydrogen bonds involving the hydrogen atoms of hydroxyl groups and the oxygen atom of the keto group of myricetin. The characteristics of the hydrogen-bonding interactions in the optimized structures were carefully analyzed. The hydrogen bonds in the optimized geometries were shown to be closed-shell-type interactions. H5' in ring B of myricetin presented the strongest interaction. The hydrogen bonds were found to be Coulombic interactions. Those between the hydrogen atoms of the hydroxyl groups in myricetin and the oxygen atoms in CH₃CH₂OH and H₂O were of moderate strength and had some covalent character, while the others were weak and were dominantly electrostatic in character.

Inhibition of Lipoxygenase and Peroxidase Reaction by Some Flavonols and Flavones: The Structure-Activity Relationship

Some flavonoids were investigated for their effects on lipoxygenase and peroxidase. The strongest inhibitor of lipoxygenase was kaempferol with one hydroxyl group situated at the 4’ position in the B ring, with activity of 21.2±2.03 calculated per μmole of compound. The weakest inhibition was observed for diosmetin with a hydroxyl group at the 3′ position and a methoxyl group at 4′ in the B ring, with activity of 1.17±0.77 per μmole.

Peroxidase was most strongly inhibited by quercetin (22.7±0.05) with two hydroxyl groups in the B ring at 3′ and 4′. The weakest inhibitor of peroxidase was genkwanin (0±0.16) with one hydroxyl group at position 4′ in the B ring and methoxyl at position 7 in the A ring. The correlation coefficient between reduction of Fe3+ by flavonoids and inhibition of lipoxygenase by these compounds was 0.72 and the reduction of Fe3+ and inhibition of peroxidase was 0.24. The results show that inhibition of peroxidase is weakly associated with reducing properties of phenols and inhibition of lipoxygenase may be associated with antioxidant properties of flavonoids.

A density functional theory study on the hydrogen bonding interactions between luteolin and ethanol

Ethanol is one of the most commonly used solvents to extract flavonoids from propolis. Hydrogen bonding interactions play an important role in the properties of liquid system. The main objective of the work is to study the hydrogen bonding interactions between flavonoid and ethanol. Luteolin is a very common flavonoid that has been found in different geographical and botanical propolis. In this work, it was selected as the representative flavonoid to do detailed research. The study was performed from a theoretical perspective using density functional theory (DFT) method. After careful optimization, there exist nine optimized geometries for the luteolin - CH₃CH₂OH complex. The binding distance of X - H···O, and the bond length, vibrational frequency, and electron density changes of X - H all indicate the formation of the hydrogen bond in the optimized geometries. In the optimized geometries, it is found that: (1) except for the H2', H5', and H6', CH₃CH₂OH has formed hydrogen bonds with all the hydrogen and oxygen atoms in luteolin. The hydrogen atoms in the hydroxyl groups of luteolin form the strongest hydrogen bonds with CH₃CH₂OH; (2) all of the hydrogen bonds are closed-shell interactions; (3) the strongest hydrogen bond is the O3' - H3'···O in structure A, while the weakest one is the C3 - H3···O in structure E; (4) the hydrogen bonds of O3' - H3'···O, O - H···O4, O - H···O3' and O - H···O7 are medium strength and covalent dominant in nature. While the other hydrogen bonds are weak strength and possess a dominant character of the electrostatic interactions in nature.

Probing the influence of carboxyalkyl groups on the molecular flexibility and the charge density of apigenin derivatives

Apigenin is an important flavonoids due to its antidiabetic bioactivity. It was reported experimentally that the 7-substituent derivative of apigenin has higher biological activity than 4'- and 5-substituted derivatives while introducing sole carboxyalkyl group -(CH₂)₇COOH into the parent structure. Molecular docking studies indicated that the other two derivatives have lower binding affinities than the 7-substituent derivative (-7.52 kcal mol^-1), which is considered to be a better inhibitor than the parent molecule. Almost all of the carbon atoms and oxygen atoms are coplaner for all three molecules in solution phase, however, all carboxyalkyl groups bend inside into the parent molecules in the active site, and the jagged geometries of the carbon chains are destroyed correspondingly. In addition, most of the electron densities of the chemical bonds for all molecules are decreased, especially the 7-substituent derivative. In contrast, most of the Laplacian values for three molecules are increased in the active site, which suggests that the charge densities at the bond critical point (bcp) are much more depleted than the solution phase. Dipole moments of derivatives are all increased in the active site, suggesting strong intermolecular interactions. After interacting with the S. cerevisiae α-glucosidase, only the 7-substituent derivative has the lowest energy gap ΔE _HOMO-LUMO, which indicates the lowest stability and the highest inhibition activity. Graphical abstract Probing the influence of carboxyalkyl groups on the molecular flexibility and the charge density of apigenin derivatives.