Inhibitory activity evaluation and mechanistic studies of tetracyclic oxindole derivatives as α-glucosidase inhibitors

A Review on the Development of Novel Heterocycles as α-Glucosidase Inhibitors for the Treatment of Type-2 Diabetes Mellitus

One of the most effective therapeutic decencies in the treatment of Type 2 Diabetes Mellitus is the inhibition of α-glucosidase enzyme, which is present at the brush border of the intestine and plays an important role in carbohydrate digestion to form mono-, di-, and polysaccharides. Acarbose, Voglibose, Miglitol, and Erniglitate have been well-known α-glucosidase inhibitors in science since 1990. However, the long synthetic route and side effects of these inhibitors forced the researchers to move their focus to innovate simple and small heterocyclic scaffolds that work as excellent α-glucosidase inhibitors. Moreover, they are also effective against the postprandial hyperglycemic condition in Type 2 Diabetes Mellitus. In this aspect, this review summarizes recent progress in the discovery and development of heterocyclic molecules that have been appraised to show outstanding inhibition of α-glucosidase to yield positive effects against diabetes.

Synthetic strategies for the construction of C3-fluorinated oxindoles

C3-fluorinated oxindoles are important scaffolds known to demonstrate various biological properties. As bio-isosteres of oxindoles, these compounds have shown tremendous potential in drug research discovery programs. Besides, they also serve as starting materials for synthesizing other fluorine-containing new architectures, thus launching research for developing new methods for their synthesis. Consequently, various approaches have been developed over the years to synthesize C3-fluorinated oxindoles. This review highlights the strategies developed to date to access C3-difluoro and monofluorooxindoles via intermolecular and intramolecular approaches. The key findings of the strategies developed are discussed along with the prevailing mechanism.

Design, synthesis and biological evaluation of novel oxindole analogs as antitubercular agents

Aim: To design, synthesize and evaluate oxindole derivatives for antitubercular activity. Methodology: We synthesized the derivatives, confirmed their structures by 1H/13C NMR and mass spectrometry, and evaluated them for antitubercular activity against Mycobacterium tuberculosis H37Rv strain using the microplate alamarBlue™ assay. Results: Among all the synthesized derivatives, OXN-1, -3 and -7 exhibited excellent antitubercular activity (minimum inhibitory concentration [MIC]: 0.78 μg/ml). Compounds with a MIC ≤1.56 were tested for cytotoxicity against human embryonic kidney cells and were found to be relatively nontoxic. Molecular docking analysis of OXN-1, -3 and -7 was performed to determine their binding patterns at the active site of DNA topoisomerase II (PDB-5BS8). In drug combination studies, OXN-1, 3 and 7 showed synergism with isoniazid. Conclusion: The obtained results reveal that oxindole derivatives exhibit potent antitubercular activity.

Screening and characterisation of potential antioxidant, hypoglycemic and hypolipidemic components revealed in Portulaca oleracea via multi-target affinity ultrafiltration LC-MS and molecular docking

INTRODUCTION

Portulaca oleracea is a commonly used nutritional vegetable and traditional herbal medicine with plenty of nutrients and manifold pharmacological activities. However, the potential active ingredients for its remarkable antioxidant, hypoglycemic and hypolipidemic activities remain unexplored.

OBJECTIVES

The present study aims to systematically evaluate the antioxidant activities of different extracts of P. oleracea and screen bioactive ligands that can interact with α-glucosidase, pancreatic lipase, and superoxide dismutase (SOD).

METHODS

In this research, the antioxidant activities of different parts of P. oleracea and their corresponding total phenolic content (TPC) and total flavonoid content (TFC) were systematically determined. Subsequently, a multi-target affinity ultrafiltration method was developed using affinity ultrafiltration with SOD, α-glucosidase, and pancreatic lipase coupled to liquid chromatography-mass spectrometry (UF-LC-MS). Later, molecular docking was used to further investigate the possible interaction mechanism between these ligands and target enzymes.

RESULTS

Among them, the ethyl acetate (EA) fraction showed the highest antioxidant activity along with the highest TPC and TFC, and four compounds in the EA fraction were quickly retrieved as potential SOD, α-glucosidase, and pancreatic lipase ligands, respectively. Molecular docking revealed that these potential ligands exhibited strong binding ability and inhibitory activities on SOD, α-glucosidase, and pancreatic lipase.

CONCLUSION

The present study revealed that P. oleracea can be used as a functional food with excellent antioxidant, hypoglycemic and hypolipidemic effects. Meanwhile, the integrated strategy based on multi-target UF-LC-MS and molecular docking also provided a powerful tool and a multidimensional perspective for further exploration of active ingredients in P. oleracea responsible for the antioxidant, hypoglycemic and hypolipidemic activities.

Synthesis and biological evaluation of isatin derivatives containing 1,3,4-thiadiazole as potent a-glucosidase inhibitors

A series of (Z)-3-(2-(1,3,4-thiadiazol-2-yl)hydrazono)-1-substituted indolin-2-ones derivatives (3a-3m) were designed and synthesized. All newly synthesized compounds were evaluated for their a-glucosidase inhibitory activity with resveratrol as positive control in vitro. Except for 3i and 3j, all of the compounds showed a potent inhibitory activity against a-glucosidase with IC₅₀ values in the range of 3.12 ± 1.25 to 45.95 ± 1.26 μM and the purity of these compounds was greater than 95%. The IC₅₀ values were being compared to the standard resveratrol (IC₅₀ = 22.00 ± 1.15 μM) and it was found that compounds 3b, 3d-3h were found to be more active than resveratrol. Specifically, (Z)-3-(2-(1,3,4-thiadiazol-2-yl)hydrazono)-1-(4-chlorobenzyl)indolin-2-one (3d) exhibited the most potent a-glucosidase inhibitory activity with IC₅₀ value of 3.12 ± 1.25 μM. The kinetic analysis revealed that compound (3d) is noncompetitive inhibitor. Structure activity relationship has been established for all compounds. Furthermore, the binding interactions of compound 3d with the active site of a-glucosidase were confirmed through molecular docking. This study has identified a new class of potent a-glucosidase inhibitors for further investigation.

Hypoglycemic and hypolipidemic dual activities of extracts and flavonoids from Desmodium caudatum and an efficient synthesis of the most potent 8-prenylquercetin

Since glucolipid metabolism disorders is often the mono-target therapy fails in managing blood glucose and lipid levels and the other complications, it is urgent and necessary to seek for the new potential drugs or functional food acting on multi-targets. The hypoglycemic and hypolipidemic dual activities of the root, stems and leaves of Desmodium caudatum, which is used for traditional Chinese medicine, was evaluated. Twelve extracts with different extraction conditions were prepared and extract 9 was find to exhibit potential inhibitory activities of fructose-1, 6-bisphosphatase (FBPase), α-glucosidase, and pancrelipase, as well as promote cellular glucose consumption and reduce cellular content of lipid. Five flavonoids were isolated and identified from extract 9, among which 8-prenylquercetin exhibited potent α-glucosidase (IC₅₀ = 4.38 μM) and FBPase (IC₅₀ = 3.62 μM) dual inhibitory activity, which were 75-fold higher than acarbose (IC₅₀ = 330.10 μM) and comparable with AMP (IC₅₀ = 2.92 μM). In addition, 8-prenylquercetin was able to promote glucose consumption and reduce lipid content. Besides, an efficient synthesis of the most potent 8-prenylquercetin was developed from inexpensive and commercially available rutin in 21% overall yield by 6 steps, which lay the foundation of preparation sufficient amount for follow-up study.

Secoiridoid dimers and their biogenetic precursors from the fruits of Cornus officinalis with potential therapeutic effects on type 2 diabetes

Cornusdiridoid A-F (1-6), six unusual cornuside-morroniside secoiridoid dimers, and their possible new biogenetic precursor, 3″,5″-dehydroxycornuside (7), together with four known secoiridoids (8-11), were obtained from the fruits of Cornus officinalis. Their structures were elucidated on the basis of various spectroscopic and chemical methods. A plausible biosynthetic pathway of compounds 1-11 was proposed. The α-glucosidase inhibitory, antioxidant and anti-inflammatory activities of these isolates were evaluated. Some of them emerged out as potent antidiabetic, anti-inflammatory and free radical scavenging agents. Molecular docking was also carried out for antidiabetic target α-glucosidase to investigate the possible binding modes of the most potent α-glucosidase inhibitor, vincosamide (9). These results revealed that the secoiridoids from C. officinalis fruits may be served as new potential antidiabetic agents to prevent and treat type 2 diabetes.

An Overview of Computer-aided Drug Design Tools and Recent Applications in Designing of Anti-diabetic Agents

BACKGROUND

In this fast-growing era, high throughput data is now being easily accessed by getting transformed into datasets which store the information. Such information is valuable to optimize the hypothesis and drug design via computer-aided drug design (CADD). Nowadays, we can explore the role of CADD in various disciplines like Nanotechnology, Biochemistry, Medical Sciences, Molecular Biology, etc. Methods: We searched the valuable literature using a pertinent database with given keywords like computer-aided drug design, anti-diabetic, drug design, etc. We retrieved all valuable articles which are recent and discussing the role of computation in the designing of anti-diabetic agents.

RESULTS

To facilitate the drug discovery process, the computational approach has set landmarks in the whole pipeline for drug discovery from target identification and mechanism of action to the identification of leads and drug candidates. Along with this, there is a determined endeavor to describe the significance of in-silico studies in predicting the absorption, distribution, metabolism, excretion, and toxicity profile. Thus, globally, CADD is accepted with a variety of tools for studying QSAR, virtual screening, protein structure prediction, quantum chemistry, material design, physical and biological property prediction.

CONCLUSION

Computer-assisted tools are used as the drug discovery tool in the area of different diseases, and here we reviewed the collaborative aspects of information technologies and chemoinformatic tools in the discovery of anti-diabetic agents, keeping in view the growing importance for treating diabetes.

Oxindole and its derivatives: A review on recent progress in biological activities

Oxindole has been shown to be a pharmacologically advantageous scaffold having many biological properties that are relevant to medicinal chemistry. The simplicity and widespread occurrence of this scaffold in plant-based alkaloids have further reinforced oxindole's merit in the domain of novel drug discovery. First extracted from Uncaria tomentosa, commonly the known as cat claw's plant which was found abundantly in the Amazon rainforest, molecules with the oxindole moiety have been shown to be common in a wide variety of compounds extracted from plant sources. The role of oxindole as a chemical scaffold for fabricating and designing biological drugs agents can be ascribed to its ability to be modified by a number of chemical groups to generate novel biological functions. This review is aimed at providing a description of the general chemistry based on existing corresponding structure-activity relationships (SARs) and compile all recent developmentary studies on oxindole-derived compounds as a successful pharmaceutical agent. A substantial group of oxindole derivatives are chiefly being tested as anticancer agents, however, a several oxindole derivatives have been shown to possesses antimicrobial, α-glucosidase inhibitory, antiviral, antileishmanial, antitubercular, antioxidative, tyrosinase inhibitory, PAK4 inhibitory, antirheumatoid arthritis and intraocular pressure reducing activities, to name a few. In this review we show the potential value of developing newer oxindole derivatives with an improved range of pharmacological implications as well as identifying drugs possessing oxindole core, that are showing and serving increased efficacy in clinical practice.

Utilization of the common functional groups in bioactive molecules: Exploring dual inhibitory potential and computational analysis of keto esters against α-glucosidase and carbonic anhydrase-II enzymes

Diabetes mellitus, a progressive chronic disease, characterized by the abnormal carbohydrate metabolism is associated with severe health complications including long term dysfunction or failure of several organs, cardiovascular and micro-angiopathic problems (neuropathy, nephropathy, retinopathy). Despite the existence of diverse chemical structural libraries of α-glucosidase inhibitors, the limited diabetic treatment due to the adverse side effects such as abdominal distention, flatulence, diarrhoea, and liver damage associated with these inhibitors encourage the medicinal research community to design and develop new and potent inhibitors of α-glucosidase with better pharmacokinetic properties. In this perspective, we demonstrate the successful integration of common functional groups (ketone & ester) in one combined pharmacophore which is favorable for the formation of hydrogen bonds and other weaker interactions with the target proteins. These keto ester derivatives were screened for their α-glucosidase inhibition potential and the in vitro results revealed compound 3c as the highly active inhibitor with an IC₅₀ value of 12.4 ± 0.16 μM compared to acarbose (IC₅₀ = 942 ± 0.74 μM). This inhibition potency was ~76-fold higher than acarbose. Other potent compounds were 3f (IC₅₀ = 28.0 ± 0.28 μM), 3h (IC₅₀ = 33.9 ± 0.09 μM), 3g (IC₅₀ = 34.1 ± 0.04 μM), and 3d (IC₅₀ = 76.5 ± 2.0 μM). In addition, the emerging use of carbonic anhydrase inhibitors for the treatment of diabetic retinopathy (a leading cause of vision loss) prompted us to screen the keto ester derivatives for the inhibition of carbonic anhydrase-II. Compound 3b was found significantly active against carbonic anhydrase-II with an IC₅₀ of 16.5 ± 0.92 μM (acetazolamide; IC₅₀ = 18.2 ± 1.23 μM). Compound 3a also exhibited comparable potency with an IC₅₀ value of 18.9 ± 1.08 μM. Several structure-activity relationship analyses depicted the influence of the substitution pattern on both the aromatic rings. Molecular docking analysis revealed the formation of several H-bonding interactions through the ester carbonyl and the nitro oxygens of 3c with the side chains of His348, Arg212 and His279 in the active pocket of α-glucosidase whereas 3b interacted with His95, -OH of Thr197, Thr198 and WAT462 in the active site of carbonic anhydrase-II. Furthermore, evaluation of ADME properties suggests the safer pharmacological profile of the tested derivatives.

Rhodium-catalyzed iminoiodane-mediated oxyamidation studies of 5-vinyluracil derivatives using aryl and alkyl sulfamates

The dirhodium tetraacetate-catalyzed iminoiodane-mediated reaction of 1,3-dimethyl-5-vinyluracil with phenyl sulfamate provided a high yield of 5-(1-acetyl-2-phenylsulfamoyl)ethyluracil via regioselective nucleophilic ring opening by acetate anion of the transiently formed 5-(1,2)-N-phenylsulfonylaziridine intermediate. This 1,2-oxyamidation reaction was found to be general for a variety of aryl- and alkylsulfamates as well as for various 1,3-dialkyl-5-vinyluracil derivatives. Addition of an alcohol to the reaction mixture allowed formation of the corresponding 1-alkoxy products. A selection of the substituted uracil derivatives prepared by this procedure was evaluated for cytotoxic activities in HCT-116 and HepG2 cancer cell lines and showed either no or modest activities. Further evaluation for α-glucosidase inhibition revealed that compounds 15ca and 15da were more active than acarbose, the reference inhibitor. This methodology thus allows efficient preparation of highly functionalized uracil derivatives thereby providing a synthetic route to novel compounds with potentially useful biological activities.

Preparation, characterization and in vitro hypoglycemic activity of banana condensed tannin-inulin conjugate

To enhance the hypoglycemic effects of inulin, banana condensed tannins (BCT) were grafted onto inulin via a free radical method to synthesize the novel BCT grafted inulin (BCT-g-inulin) complex. Spectroscopic methods, XRD, TGA, 1H NMR, GPC and morphology analyses were utilized to characterize the structural properties of the BCT-g-inulin complex, and our results confirmed the conjugation of BCT and inulin. The conjugation possibly occurred between the hydroxyl group attached at the C6 position of inulin and the C6/C8 position of flavon-3-ol units of BCT. The grafting ratio and grafting efficiency of the BCT-g-inulin complex were 357.54 ± 2.98 g kg-1 complex and 74.57 ± 1.44%, respectively. The data of the antioxidant assays indicated that the BCT-g-inulin complex showed a significantly higher antioxidant activity than native inulin. Also, the grafting reaction remarkably improved the in vitro anti-diabetic activity of inulin. The glucose adsorption capacity and glucose dialysis retardation index of the BCT-g-inulin complex were remarkably higher than those of inulin, while the BCT-g-inulin complex showed much stronger inhibitory effects against α-amylase and α-glucosidase compared with inulin. Notably, the inhibition of both α-amylase and α-glucosidase by the BCT-g-inulin complex occurred through mixed-competitive mode. On the basis of fluorescence spectroscopy, the fluorescence of α-amylase and α-glucosidase could be quenched by the BCT-g-inulin complex through a static quenching mechanism. Hence, the BCT-g-inulin complex might have the potential to be developed as an effective anti-diabetic agent.

Synthesis of tetracyclic oxindoles and evaluation of their α-glucosidase inhibitory and glucose consumption-promoting activity

A series of tetracyclic oxindole derivatives was synthesized by asymmetric 1, 3-dipole reaction in 2-4 steps in 57-86% overall yields. These compounds were evaluated for α-glucosidase inhibitory and glucose consumption-promoting activity in vitro. Compound 4l competitively and reversibly inhibited α-glucosidase (IC₅₀ = 3.64 μM) with activity 14-fold higher than that of acarbose. Docking analysis substantiated these findings. In addition, compound 4l exhibited significant glucose consumption promoting activity at 1 μM.

Anti-diabetic drugs recent approaches and advancements

Diabetes is one of the major diseases worldwide and is the third leading cause of death in the United States. Anti-diabetic drugs are used in the treatment of diabetes mellitus to control glucose levels in the blood. Most of the drugs are administered orally, except for a few of them, such as insulin, exenatide, and pramlintide. In this review, we are going to discuss seven major types of anti-diabetic drugs: Peroxisome proliferator-activated receptor (PPAR) agonist, protein tyrosine phosphatase 1B (PTP1B) inhibitors, aldose reductase inhibitors, α-glucosidase inhibitors, dipeptidyl peptidase IV (DPP-4) inhibitors, G protein-coupled receptor (GPCR) agonists and sodium-glucose co-transporter (SGLT) inhibitors. Here, we are also discussing some of the recently reported anti-diabetic agents with its multi-target pharmacological actions. This review summarises recent approaches and advancement in anti-diabetes treatment concerning characteristics, structure-activity relationships, functional mechanisms, expression regulation, and applications in medicine.

Docking and QSAR analysis of tetracyclic oxindole derivatives as α-glucosidase inhibitors

The α-glucosidase inhibitors are considered as important agents in drug discovery against diabetes mellitus. Molecular docking and quantitative structure-activity relationship (QSAR) were performed based on a series of tetracyclic oxindole derivatives to elucidate key structural properties affecting inhibitory activity and support the design of new α-glucosidase inhibitors. The molecular docking results demonstrate that at least two hydrogen bonds between Thr681 and Arg676 residues and the oxygen atoms in amid groups have an important role in the optimum binding of inhibitors. In addition, the sum of polar contacts of Arg699, Arg670, Glu792 and Glu301 residues with the α-glucosidase inhibitors have more than one third of total binding free energy. The docked conformations of the inhibitors with the best binding free energy were used to construct QSAR models. As a primary survey and a graphical comparing tool, the partial least squares-discriminant analysis (PLS-DA) technique was successfully employed to classify active and inactive inhibitors. The validated QSAR analysis were performed through genetic algorithm-partial least squares (GA-PLS) and support vector machine (SVM) techniques. The QSAR model reveals that important features of J3D, Mor26 u and HOMA have a high predictive capability (R²_p = 0.837, Q²_LOO = 0.871, R²_LSO = 0.790 and r²_m  = 0.758) using GA-PLS/SVM strategy. Generally, the suggested QSAR analysis based on classification, docking and GA-PLS/SVM strategy may help suggest chemical scaffold to design novel oxindole derivatives as α-glucosidase inhibitors.

Synthesis & α-glucosidase inhibitory & glucose consumption-promoting activities of flavonoid-coumarin hybrids

AIM

The research of novel and potent antidiabetic agents is urgently needed for the control of the exploding diabetic population. We previously reported the synthesis and antidiabetic activity of natural 8-(6"-umbelliferyl)-apigenin (1), but its antidiabetic targets are not known. Therefore, four series of derivatives were synthesized and evaluated for their antidiabetic activities. Results & methodology: Compounds (5a) and (14a) were identified as new α-glucosidase and α-amylase dual inhibitors. The kinetic analysis of the most potent α-glucosidase inhibitor of each series of compounds revealed that they inhibited α-glucosidase in irreversible modes. In addition, compounds (5a) and (14a) showed potent glucose consumption-promoting activity.

CONCLUSION

Compounds (5a) and (14a) could be regarded as promising starting points for the development of antidiabetic candidates.

Effect of Agave americana L. on the human, and Aspergillus oryzae S2 α-amylase inhibitions

Among phenolic compounds, Agave americana L. extract contained puerarin (38.4%) and p-coumaric acid (12.29%) (pCa). From the Lineweaver-Burk plots, pCa and puerarin demonstrated a competitive and a non competitive inhibitions towards human α-amylase activity, respectively. PCa exhibited a higher human inhibitory activity with an IC₅₀ of 98.8 μM which was about 2.3 times than acarbose. Puerarin (IC₅₀ = 3.87 μM) and pCa (IC₅₀ = 10.16 μM) also showed an excellent inhibition for Aspergillus oryzae S2 α-amylase activity. The inhibitions of the described biocatalysts compounds towards both amylases were significantly decreased when they were pre-incubated with starch. The binding modes of these compounds were evaluated in silico. The binding efficiency order of these molecules in terms of polar contact numbers for both enzymes was in agreement with the in vitro studies. These findings provided a rational reason to establish the isolated compounds capability as therapeutic target for hyperglycaemia modulation and antifungal therapy.

Synthesis, Biological Evaluation, and Molecular Docking Studies of Novel Isatin-Thiazole Derivatives as α-Glucosidase Inhibitors

A series of novel isatin-thiazole derivatives were synthesized and screened for their in vitro α-glucosidase inhibitory activity. These compounds displayed a varying degree of α-glucosidase inhibitory activity with IC₅₀ ranging from 5.36 ± 0.13 to 35.76 ± 0.31 μm as compared to the standard drug acarbose (IC₅₀ = 817.38 ± 6.27 μm). Among the series, compound 6p bearing a hydroxyl group at the 4-position of the right phenyl and 2-fluorobenzyl substituent at the N1-positions of the 5-methylisatin displayed the highest inhibitory activity with an IC₅₀ value of 5.36 ± 0.13 μm. Molecular docking studies revealed the existence of hydrophobic interaction, CH-π interaction, arene-anion interaction, arene-cation interaction, and hydrogen bond between these compounds and α-glucosidase enzyme.

Natural Prenylchalconaringenins and Prenylnaringenins as Antidiabetic Agents: α-Glucosidase and α-Amylase Inhibition and in Vivo Antihyperglycemic and Antihyperlipidemic Effects

Inhibition of α-glucosidase and α-amylase decreases postprandial blood glucose levels and delays glucose absorption, making it a treatment strategy for type 2 diabetes. This study examined in vivo and in vitro antidiabetic activities of natural prenylchalconaringenins 1 and 2 and prenylnaringenins 3 and 4, found in hops and beer. 3'-Geranylchalconaringenin (2) competitively and irreversibly inhibited α-glucosidase (IC₅₀ = 1.08 μM) with activity 50-fold higher than that of acarbose (IC₅₀ = 51.30 μM) and showed moderate inhibitory activity against α-amylase (IC₅₀ = 20.46 μM). Docking analysis substantiated these findings. In addition, compound 2 suppressed the increase in postprandial blood glucose levels and serum levels of total cholesterol and triglycerides in streptozotocin-induced diabetic mice. Taken together, these results suggest that 2 has dual inhibitory activity against α-glucosidase and α-amylase and alleviates diabetic hyperglycemia and hyperlipidemia, making it a potential functional food ingredient and drug candidate for management of type 2 diabetes.

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.