Cassia fistula Leaves; UHPLC-QTOF-MS/MS Based Metabolite Profiling and Molecular Docking Insights to Explore Bioactives Role towards Inhibition of Pancreatic Lipase

The present work was aimed at investigating hydroethanolic leaf extracts of Cassia fistula for their antioxidant and pancreatic lipase (PL) enzyme inhibitory properties. The most active extract was selected to profile the phytoconstituents by UHPLC-QTOF-MS/MS technique. Among the tested extracts, the 80% hydroethanolic extract exhibited the maximum levels of total phenolic and flavonoid contents (TPC and TFC) with a contribution of 201.3 ± 2.6 mg of gallic acid equivalent per gram of extract (GAE/g extract), and 116.3 ± 2.4 mg of rutin equivalent per gram of extract (RE/g extract), respectively. The same extract also showed promising 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and PL inhibitory activity with an IC50 (half maximal inhibitory concentration) of 30.5 ± 2.8 µg/mL and 17.31 ± 1.18 μg/mL, respectively. The phytochemical profiling of 80% hydroethanolic extract confirmed the presence of 23 metabolites of immense medicinal significance. Docking studies were conducted to investigate the potential interactions of compounds identified in the study. The docking study-based binding energy data and the interaction scheme both revealed the possible role of the identified compounds towards PL inhibitor. Moreover, energies of frontier molecular orbitals (FMOs), ionization potentials (IP), electron affinities (EA) and molecular electrostatic potentials (MEP) were also explored. The findings of the current work suggest that C. fistula is a promising natural source of antioxidant and antiobesity agents, which may be exploited to add pharmacological functionalities to food.


Introduction
Obesity is one of the leading global health problems that enhances risks of chronic diseases such as diabetes, heart diseases, metabolic syndrome, strokes, hypertension, mental disorders and cancer [1,2]. The prevalence of obesity is very widespread, and about 500 million people of the adult population are reported as obese worldwide [3]. Obesity generally results from an imbalance between high caloric diet intake and expenditure of energy. The extra energy is stored in adipose tissue or adipose mass. PL is important inant compounds present in the most potent C. fistula leaf extract was performed via a sensitive and reliable technique i.e., UHPLC-QTOF-MS/MS, followed by molecular docking and computational studies of identified bioactive compounds, to provide the scientific basis regarding antioxidant and antiobesity activity of C. fistula.

Extract Yield
Extraction is the separation of bioactive constituents from medicinal plants using specific solvents and adopting specific protocols. The aim of the extraction process is to take out plant metabolites from the complex plant matrix. The high demand for antioxidants stimulated ways to explore new methodological alternatives (cleaner and environment friendly) that could improve the yield of extracts and decrease the process cost [25,26].
The percentage yields of different hydroethanolic (20 to 80%) and pure ethanolic C. fistula leaf extracts are displayed in Figure 1. The extraction yields of these samples were in the range from 15.23 ± 0.09% to 26.21 ± 0.06%. The results revealed that the 80% hydroethanolic extract had the maximum extraction yield while the minimum extract yield was obtained by pure ethanolic extract. The solubility of phenolic compounds depends mostly upon the nature of solvents employed and their polarities. In the current work, results of ANOVA revealed that the influence of solvent polarity on extract yield was significant (p < 0.05). Mixtures of water and ethanol in different proportions can extract bioactive constituents from different plant parts. Although water has high ability to extract polar compounds due to its higher polarity compared to other solvents, it is unable to extract high carbon bond compounds, so solvents with suitable extraction coefficients may be prepared by mixing ethanol and water.

Extract Yield
Extraction is the separation of bioactive constituents from medicinal plan cific solvents and adopting specific protocols. The aim of the extraction proce out plant metabolites from the complex plant matrix. The high demand for a stimulated ways to explore new methodological alternatives (cleaner and e friendly) that could improve the yield of extracts and decrease the process co The percentage yields of different hydroethanolic (20 to 80%) and pure fistula leaf extracts are displayed in Figure 1. The extraction yields of these sa in the range from 15.23 ± 0.09% to 26.21 ± 0.06%. The results revealed that droethanolic extract had the maximum extraction yield while the minimum e was obtained by pure ethanolic extract. The solubility of phenolic compoun mostly upon the nature of solvents employed and their polarities. In the cu results of ANOVA revealed that the influence of solvent polarity on extract yi nificant (p < 0.05). Mixtures of water and ethanol in different proportions can active constituents from different plant parts. Although water has high abili polar compounds due to its higher polarity compared to other solvents, it extract high carbon bond compounds, so solvents with suitable extraction coef be prepared by mixing ethanol and water.
Previously reported extract yields of ethanol: water (9:1) and methanolic from C. fistula were 7% and 20.2%, respectively [27,28]. The values of extracti the current work were different compared to those reported in literature. The ation could be attributable to the difference in polarities of the solvents used play an imperative role in increasing the solubility of phytocompounds [29].

Total Phenolic and Flavonoid Contents
Medicinal plants/herbs have gained considerable importance, being ric therapeutic phytoconstituents that can lead to the development and formulat Previously reported extract yields of ethanol: water (9:1) and methanolic leaf extracts from C. fistula were 7% and 20.2%, respectively [27,28]. The values of extraction yields in the current work were different compared to those reported in literature. The slight variation could be attributable to the difference in polarities of the solvents used which also play an imperative role in increasing the solubility of phytocompounds [29].

Total Phenolic and Flavonoid Contents
Medicinal plants/herbs have gained considerable importance, being rich sources of therapeutic phytoconstituents that can lead to the development and formulation of novel functional foods/nutraceuticals or phytopharmaceuticals. Most of the plant-based bioactives, such as phenolics and flavonoids, have beneficial health effects. In medicinal plants, high contents of flavonoids and phenolics are associated with antioxidant activity, thus playing a significant role in prevention/treatment of several chronic disorders related to oxidative stress. Due to the beneficial phytocompounds in medicinal plants and trends in natural product consumption in nutraceuticals and the food processing industry, the exploration of medicinal plants is becoming increasingly important [25,30,31].
The amounts of TPCs and TFCs in the leaves of C. fistula are given in Table 1. The results show the difference in levels of TPCs and TFCs in all extracts. The TPCs varied from 138.5 ± 2.3 to 201.3 ± 2.6 mg GAE/g and the maximum amount was obtained with 80% hydroethanolic extract (201.3 ± 2.6 mg GAE/g), followed by 60% hydroethanolic extract (190.6 ± 3.0 mg GAE/g), 40% hydroethanolic extract (181.2 ± 1.9 mg GAE/g) and 20% hydroethanolic extract (152.19 ± 1.4 mg GAE/g). The lowest contents of phenolic compounds found in the pure ethanolic leaf extract might be explained by the lower solubility of polyphenols in this solvent system. A statistically significant difference of means (p < 0.05) was observed between the 80% hydroethanolic extract and other extracts. Likewise, the maximum amount of TFCs was seen in the 80% hydroethanolic extract with a value of 116.3 ± 2.4 mg RE/g, and it was different statistically, with p < 0.05, when compared with those of 60, 40 and 20% hydroethanolic and pure ethanolic extracts. In a previous study, the amount of TPCs and TFCs in aqueous extract of C. fistula were 43.57 ± 1.16 mg GAE/g extract and 4.99 ± 0.10 mg QE/g extract, respectively, which were less than the TPC and TFC values depicted in the current investigation [27]. The slight variation in amounts of TPCs and TFCs in the present work and those already reported might be due to various phytogeographic regions and plant nutrition [32].

DPPH Radical Scavenging Activity In Vitro
Oxidative stress-induced hypertension has promoted an increasing interest in therapeutic potential of antioxidants. The antioxidants could be utilized as supplements to manage different ailments resulting from the oxidative stress, including obesity. However, most of the studies have focused usage of synthetic antioxidants to reduce the harmful impacts of oxidative stress that are often not considered successful due to their myriad side effects [33]. Emerging evidence has suggested that natural products can attenuate the risk of oxidative stress and improve the immune system. Therefore, the exploration of natural products or medicinal herbs as promising therapeutic sources for the management of obesity has been promoted due to their minimal adverse effects [34]. In the current work, the antioxidant effect of C. fistula leaves was evaluated using DPPH radical scavenging assay.
The DPPH radical scavenging assay is used frequently to determine antioxidant effects of plant extracts [11]. Figure 2 present the results (expressed in terms of IC 50 values) of antioxidant activities obtained for the studied extracts and the positive control (BHA). Evaluation of DPPH radical scavenging activity of various tested extracts indicat that the 80% hydroethanolic extract exhibited maximum activity with an IC50 of 30.5 ± 2 µg/mL, while minimum antioxidant activity was seen in the pure ethanolic extract wi an IC50 of 61.3 ± 1.9 µg/mL. As revealed statistically, antioxidant potential of 80% h droethanolic extract was maximum and different significantly (p < 0.05) compared to oth extracts, but slightly less than that of BHA (IC50 of 16.55 ± 1.90 µg/mL). Results reveal that the 80% hydroethanolic leaf extract had significant antioxidant activity and might able to protect the body from the damaging effects produced by oxidative stress. The a tioxidant activity exhibited by a particular plant extract mainly depends upon the natu of the phytochemicals extracted from plant parts. However, their yields are affected the solvent system used for extraction [11]. The difference in antioxidant effect of the cu rent work and reported data could be linked to different level of polyphenolic compoun in the extracts due to solvent polarities and methods of extraction.

Antiobesity Activity of C. fistula
Obesity is a major public health concern worldwide and related to several chron disorders. Several synthetic drugs are in use to cure obesity with variable results and a verse effects [6]. Inhibition of dietary absorption by inhibiting PL is one of the effecti approaches for the management of obesity. Several medicinal plants have been report as sources of PL inhibitors with limited adverse effects [2].
PL inhibitory effects were examined to determine the ability of C. fistula leaf extrac as antiobesity agents. Outcomes are presented in Figure 3. Evaluation of DPPH radical scavenging activity of various tested extracts indicated that the 80% hydroethanolic extract exhibited maximum activity with an IC 50 of 30.5 ± 2.8 µg/mL, while minimum antioxidant activity was seen in the pure ethanolic extract with an IC 50 of 61.3 ± 1.9 µg/mL. As revealed statistically, antioxidant potential of 80% hydroethanolic extract was maximum and different significantly (p < 0.05) compared to other extracts, but slightly less than that of BHA (IC 50 of 16.55 ± 1.90 µg/mL). Results revealed that the 80% hydroethanolic leaf extract had significant antioxidant activity and might be able to protect the body from the damaging effects produced by oxidative stress. The antioxidant activity exhibited by a particular plant extract mainly depends upon the nature of the phytochemicals extracted from plant parts. However, their yields are affected by the solvent system used for extraction [11]. The difference in antioxidant effect of the current work and reported data could be linked to different level of polyphenolic compounds in the extracts due to solvent polarities and methods of extraction.

Antiobesity Activity of C. fistula
Obesity is a major public health concern worldwide and related to several chronic disorders. Several synthetic drugs are in use to cure obesity with variable results and adverse effects [6]. Inhibition of dietary absorption by inhibiting PL is one of the effective approaches for the management of obesity. Several medicinal plants have been reported as sources of PL inhibitors with limited adverse effects [2].
PL inhibitory effects were examined to determine the ability of C. fistula leaf extracts as antiobesity agents. Outcomes are presented in Figure 3.
All the extracts exhibited strong inhibition against PL, but inhibitory activity of the 80% hydroethanolic extract was maximum with an IC 50 value of 17.31 ± 1.18 µg/mL. This value was quite impressive when compared with the standard PL inhibitor, orlistat, for which the IC 50 value was 12.34 ± 0.02 µg/mL. These values were followed by 60% hydroethanolic (IC 50 = 26.53 ± 0.99 µg/mL), 40% hydroethanolic (IC 50 = 29.21 ± 2.03 µg/mL) and 20% hydroethanolic (IC 50 = 39.11 ± 2.30 µg/mL) extracts. However, it was noticed that the IC 50 values of both the 40% hydroethanolic and 60% hydroethanolic extracts were found statistically nonsignificant. Comparative analysis revealed that pure ethanolic extract exhibited the lowest enzyme inhibitory action among all extracts (IC 50 of 51.3 ± 1.5 µg/mL). Findings were of great interest and revealed considerable inhibitory effect of C. fistula leaves towards PL and its potential for antiobesity functionality. A recent study linked the in vitro PL lipase inhibition by hydroethanolic extracts of Taraxacum officinale (T. officinale) with the blood biochemistry of obese mice treated with same extract. The T. officinale extract showed PL inhibition with an IC 50 value 146.49 ± 4.24 µg/mL and its application on high fat diet-fed obese mice significantly improved the lipid profile by modulating their total cholesterol, high density lipoproteins and low density lipoproteins [35,36]. All the extracts exhibited strong inhibition against PL, but inhibitory activity of the 80% hydroethanolic extract was maximum with an IC50 value of 17.31 ± 1.18 µg/mL. This value was quite impressive when compared with the standard PL inhibitor, orlistat, for which the IC50 value was 12.34 ± 0.02 µg/mL. These values were followed by 60% hydroethanolic (IC50 = 26.53 ± 0.99 µg/mL), 40% hydroethanolic (IC50 = 29.21 ± 2.03 µg/mL) and 20% hydroethanolic (IC50 = 39.11 ± 2.30 µg/mL) extracts. However, it was noticed that the IC50 values of both the 40% hydroethanolic and 60% hydroethanolic extracts were found statistically nonsignificant. Comparative analysis revealed that pure ethanolic extract exhibited the lowest enzyme inhibitory action among all extracts (IC50 of 51.3 ± 1.5 µg/mL). Findings were of great interest and revealed considerable inhibitory effect of C. fistula leaves towards PL and its potential for antiobesity functionality. A recent study linked the in vitro PL lipase inhibition by hydroethanolic extracts of Taraxacum officinale (T. officinale) with the blood biochemistry of obese mice treated with same extract. The T. officinale extract showed PL inhibition with an IC50 value 146.49 ± 4.24 µg/mL and its application on high fat diet-fed obese mice significantly improved the lipid profile by modulating their total cholesterol, high density lipoproteins and low density lipoproteins [35,36].

UHPLC-QTOF-MS/MS Based Metabolite Profiling
Profiling of metabolites in the 80% hydroethanolic leaf extract (most potent) from C. fistula was conducted by employing UHPLC-QTOF-MS/MS. The base peak chromatogram of the tested sample is presented in Figure 4. Analytical information of all identified phytochemicals (including precursor ions ([M-H] − ), predicted formula, mass error and MS/MS fragment ion peaks) is provided in Table 2, whereas most probable fragmentation patterns of all identified phytocompounds are displayed in Figure 5.

UHPLC-QTOF-MS/MS Based Metabolite Profiling
Profiling of metabolites in the 80% hydroethanolic leaf extract (most potent) from C. fistula was conducted by employing UHPLC-QTOF-MS/MS. The base peak chromatogram of the tested sample is presented in Figure 4. Analytical information of all identified phytochemicals (including precursor ions ([M-H] − ), predicted formula, mass error and MS/MS fragment ion peaks) is provided in Table 2, whereas most probable fragmentation patterns of all identified phytocompounds are displayed in Figure 5.       , C6H12O6 and C11H20O7 groups, respectively, from the precursor ion, which agrees with previous literature. Compound (10) was identified tentatively as 2″-O-Rhamno-sylicariside [47]. Compound (11), with a predicted molecular formula of C30H22O9 and observed mass of 526.12 mDa, was designated as dendrocandin H [48].   (1) was identified probably as patuletin [37,38]. Compound (2) was identified as erigoster A, with predicted mass of 558.14 (C 27  moieties, respectively, from the precursor ion. The observed mass of the compound was 538.09 mDa with a mass error of 1.9 mDa. Based on the mass fragmentation pattern and reported literature, compound (6) might be robustaflavone [43,44]. The mass spectrum of metabolite (7) presented an [M-H] − ion peak at m/z 611 with the molecular formula C 27 H 32 O 16 , and was in agreement with data reported in the literature [45]. , respectively, from the precursor ion, which agrees with previous literature. Compound (10) was identified tentatively as 2 -O-Rhamno-sylicariside [47]. Compound (11), with a predicted molecular formula of C 30 H 22 O 9 and observed mass of 526.12 mDa, was designated as dendrocandin H [48]. Hence, compound (13) was tentatively identified as benzyl alcohol xylopyranosyl (1-6) glucopyranoside [48,50]. The characteristic fragment ions for compound (14) (16) was identified tentatively as epicatechin and was in accordance with literature data [54,55]. Compound (17) was identified as sesamol, with a molecular formula of C 7 H 6 O 3 . A daughter ion produced at 121 suggested the elimination of an -OH group, in accordance with the literature data [56]. Compound (18) corresponds to the predicted molecular formula of C 34 H 58 O 6 with an observed mass of 562.42 mDa [56,57]. In its mass spectrum, the fragments were at 543

Docking Studies
The affinity of identified phytochemicals with PL was predicted by the docking studies. Bonding energies of the phytochemicals with lipase (PDB No. 1LPB), calculated by the MOE are given in Table 3. Binding energies calculated for compounds ranged from −15.0210 to −10.0161 KJ/mol. The lowest energy was exhibited by Undulatoside A. All the phytochemicals identified in the C. fistula extract showed binding energies lower than orlistat, which was found to be −9.1309 kJ/mol for PL. The binding profiles of the phytochemicals are presented in the Figure 6. The binding profile of orlistat reveals that it interacts with TYR114 and PHE77 residues by hydrogen bonds, while it can interact with the ILE78, TYR114, SER152, PRO180, PHE215, ARG256, ALA259, ALA260, LEU264 residues through other nonbonding interactions. The catalytic tirade of 1LPB consists of SER152, HIS263 and ASP176, but SER152 is mainly responsible for the activity of lipase. So, the compounds showing interactions with these amino acid residues can alter the lipase activity. However, other amino acid residues like PHE77 and TYR114 are also key residues of the active pocket of lipase (PDB No. 1LPB). The compounds, showing the lowest binding energies, exhibited interactions with the key residues of lipase. The binding profile of undulatoside A shows that it can interact with SER152, TYR114 through a hydrogen bond, and shows pi-sigma interaction with ILE78 and ARG256, while it interacts with ALA260, LEU264 and TRP252 by pi-alkyl interaction. Conventional hydrogen bonding with SER152 was also depicted by the bindings profiles of erigoster A, menisdaurin, 3-

Computational Studies
Plants are considered as essential source of phytocompounds. Many therapeutic agents have been identified in medicinal plants. However, some of these medicinal plants might contain a wide variety of chemical compounds, often with undetermined biological properties [62]. The antioxidant activities of polyphenolic compounds depend upon their ability to resist harmful effects of free radical species. There exists a close relationship between structural properties and antioxidant activities. Among various structural features, the effect of substituent is the most significant factor that influences antioxidant activities of polyphenolic compounds [63]. A quantum computational approach has proven useful in determining the structure, reaction and the electronic structures of the molecules [64]. It is a cost-effective and predictive method. It provides valuable information about the antioxidant character of polyphenolic phytocompounds [65]. The density functional descriptors for structure-activity relationships are pivotal in the examination of interactional aspects, active sites, and possible biological functions of phytoconstituents. It is noteworthy to shed light on FMOs, IP, EA and MEP to explore the unique and important biological and pharmacological properties of compounds. The results acquired from the computational study of the identified phytoconstituents, in addition to molecular docking studies, might be very helpful in exploring their possible phytochemical role. Hence, these findings may be helpful for future studies related to isolation and characterization of potent metabolites for the pharmacological/nutraceutical applications.
The current work involved the computation of active sites for the most relevant and practical information. The electronic environments of functional groups of compounds present in a particular extract, and interaction with active sites of enzymes, provides a great deal of information about medicinal potential. The information on electronic properties and MEP exploration is significant in elaborating the role of active sites in compounds displaying fascinating properties. The charge density/spatial distribution of FMOs, i.e., the highest occupied molecular orbitals (HOMOs) as well as lowest unoccupied molecular orbitals (LUMOs) is illustrated as Figure 7.

Computational Studies
Plants are considered as essential source of phytocompounds. Many therapeutic agents have been identified in medicinal plants. However, some of these medicinal plants might contain a wide variety of chemical compounds, often with undetermined biological properties [62]. The antioxidant activities of polyphenolic compounds depend upon their ability to resist harmful effects of free radical species. There exists a close relationship between structural properties and antioxidant activities. Among various structural features, the effect of substituent is the most significant factor that influences antioxidant activities of polyphenolic compounds [63]. A quantum computational approach has proven useful in determining the structure, reaction and the electronic structures of the molecules [64]. It is a cost-effective and predictive method. It provides valuable information about the antioxidant character of polyphenolic phytocompounds [65]. The density functional descriptors for structure-activity relationships are pivotal in the examination of interactional aspects, active sites, and possible biological functions of phytoconstituents. It is noteworthy to shed light on FMOs, IP, EA and MEP to explore the unique and important biological and pharmacological properties of compounds. The results acquired from the computational study of the identified phytoconstituents, in addition to molecular docking studies, might be very helpful in exploring their possible phytochemical role. Hence, these findings may be helpful for future studies related to isolation and characterization of potent metabolites for the pharmacological/nutraceutical applications.
The current work involved the computation of active sites for the most relevant and practical information. The electronic environments of functional groups of compounds present in a particular extract, and interaction with active sites of enzymes, provides a great deal of information about medicinal potential. The information on electronic properties and MEP exploration is significant in elaborating the role of active sites in compounds displaying fascinating properties. The charge density/spatial distribution of FMOs, i.e., the highest occupied molecular orbitals (HOMOs) as well as lowest unoccupied molecular orbitals (LUMOs) is illustrated as Figure 7.

Compound 21
Compound 22 Compound 19 Compound 20   Spatial distribution exposed detailed intramolecular charge transfer from HOMOs to LUMOs. The antioxidant and free radical scavenging activity of bioactive molecules and entities are interlinked with the charge density distribution of FMOs, which illuminate the possible active sites. The spatial distribution of FMOs overlapping molecules highlighted that these phytomolecules could be of great therapeutic use regarding their antioxidant and antiobesity properties. Energies of FMOs and HOMO-LUMO energy gaps (Egaps) are significant parameters to understand the electronic nature of biologically active substances. Energies of HOMOs (EHOMOs, EHOMOs-1), LUMOs (ELUMOs, ELUMOs+1) and Egaps of different phytocompounds at B3LYP/6-31G** level are shown in Table 4.  Spatial distribution exposed detailed intramolecular charge transfer from HOMOs to LUMOs. The antioxidant and free radical scavenging activity of bioactive molecules and entities are interlinked with the charge density distribution of FMOs, which illuminate the possible active sites. The spatial distribution of FMOs overlapping molecules highlighted that these phytomolecules could be of great therapeutic use regarding their antioxidant and antiobesity properties. Energies of FMOs and HOMO-LUMO energy gaps (E gaps ) are significant parameters to understand the electronic nature of biologically active substances. Energies of HOMOs (E HOMOs , E HOMOs-1 ), LUMOs (E LUMOs , E LUMOs+1 ) and E gaps of different phytocompounds at B3LYP/6-31G** level are shown in Table 4. Studies have shown that diffraction approaches can be very helpful to determine MEP on practical grounds [66]. The MEP illustrates a broad range of nuclear and electronic charge distributions, and helps in understanding and predicting the reactivity of different entities [67]. In Figure 8, the MEP mapped for identified phytocompounds is shown in color schemes. Higher negative potential regions (suitable for electrophilic attack) are indicated by the red color, and higher positive potential regions (favoring nucleophilic attack) are specified by the blue color. The declining order of MEP is blue > green > yellow > orange > red. Here, the red color indicates the strongest repulsion while the blue color indicates the phase of substantial attraction. Figure 8 revealed that oxygen atoms of the methoxy, hydroxy, keto or carboxylic group, as well as -CN, would have negative electrostatic potential. On the other hand, hydrogen atoms present in hydroxyl and methyl groups have positive electrostatic potential. These results clearly indicate that in the case of electrophilic or nucleophilic attack, reasonable repulsion would be at oxygen, CN and -COOH/-H, while significant attraction would be at -H/oxygen atoms, CN, and -COOH groups, respectively. Studies have shown that diffraction approaches can be very helpful to determine MEP on practical grounds [66]. The MEP illustrates a broad range of nuclear and electronic charge distributions, and helps in understanding and predicting the reactivity of different entities [67]. In Figure 8, the MEP mapped for identified phytocompounds is shown in color schemes. Higher negative potential regions (suitable for electrophilic attack) are indicated by the red color, and higher positive potential regions (favoring nucleophilic attack) are specified by the blue color. The declining order of MEP is blue > green > yellow > orange > red. Here, the red color indicates the strongest repulsion while the blue color indicates the phase of substantial attraction. Figure 8 revealed that oxygen atoms of the methoxy, hydroxy, keto or carboxylic group, as well as -CN, would have negative electrostatic potential. On the other hand, hydrogen atoms present in hydroxyl and methyl groups have positive electrostatic potential. These results clearly indicate that in the case of electrophilic or nucleophilic attack, reasonable repulsion would be at oxygen, CN and -COOH/-H, while significant attraction would be at -H/oxygen atoms, CN, and -COOH groups, respectively.  Antioxidants have the ability to donate electrons to free radicals and stabilize the radical. The antioxidant ability of a compound can be determined by IP in a single electron Antioxidants have the ability to donate electrons to free radicals and stabilize the radical. The antioxidant ability of a compound can be determined by IP in a single electron transfer mechanism. The IP is a vital parameter that illuminates electron transfer range (computed as IP = -E HOMO ). Compounds having low IP values can reflect higher antioxidant activity due to ease of electron transfer. In the current study, phenol was used as standard, and the findings revealed that the understudy phytochemicals exhibited low IP values when compared to phenol i.e., 8.33 eV [68], which would be a supporting factor for antioxidant potential. The data of IP predicted that the isolated compounds would have good antioxidant activities. It can be interpreted from IP values that the compounds may show a better electron donating effect in comparison to phenol with maximum E HOMO and minimum IP values. The MEP mapping of active sites, spatial distribution of FMOs and IP indicated that these phytochemicals would be reactive with good antioxidant and antiobesity capability.
The in vitro antioxidant activity and PL inhibition of C. fistula extracts were supported by the sound evidence of computational data. The secondary metabolites identified in hydroethanolic extract of C. fistula were mainly responsible for the antioxidant and PL inhibitory action. The experimental outcomes suggest that the C. fistula has medicinal potential related to oxidative stress and obesity. Therefore the C. fistula may be a potential candidate for food fortification to add antiobesity functionalities in food systems.

Plant Collection and Identification
Mature leaves of C. fistula were collected from Azad Jammu & Kashmir, Pakistan and identified at the Department of Botany University of Gujrat, Gujrat Pakistan. For identification, a voucher specimen # UOGCHEM46/2018 was also deposited.

Extract Preparation
The freshly collected leaves were cleaned, dried, and quenched in liquid nitrogen immediately. The purpose of quenching was to preserve secondary metabolites for longer periods. After that, lyophilization was carried out at −68 • C for 48 h. The lyophilized leafy material was converted into a fine powder (60-mesh size) and kept at extremely low temperature (−80 • C) for further analysis. Dried powder (10 g) was then extracted with 100 mL of hydroethanolic solvent systems of various concentrations (i.e., 20-80% hydroethanol (v/v), and pure ethanol) at 35 ± 0.2 • C and a humidity of 25 ± 5% for a period of two days. Sample extracts under study were then vortexed for 2 h utilizing Wise-Mix SHO1D, DAIHAN Scientific, Korea, ultrasonicated for 30 min at 20 KHz. After centrifugation for 10 min at 13,000 rpm, filtration was carried out by applying a filtration assembly connected with a vacuum pump, the filtrate evaporated with the help of rotary vacuum evaporator to avoid the loss of phytoconstituents and freeze dried again at −68 • C for further use.

Total Phenolic Contents
The TPCs of the extracts were determined by Folin-Ciocalteu's (FC) method with slight modifications [69]. Briefly, the sample extracts were dissolved in methanol and mixed with FC-reagent. After that, samples were mixed with 20% sodium carbonate solution (4 mL) and incubated for 90 min at room temperature. The absorbance was noted at 750 nm. Gallic acid was employed as a reference and results were presented as milligram of gallic acid equivalent per gram (mg GAE/g).

Total Flavonoid Contents
The TFCs of each extract were investigated by a reported spectrophotometric method with slight changes [70]. Briefly, sample extracts were dissolved in methanol followed by adding 0.10 mL sodium nitrite (0.5 M), 0.15 mL aluminum chloride of (0.3 M) solution and 3.4 mL of 30% methanol. After a short while, 1 mL sodium hydroxide (1 M) was added, and measurement of absorbance were performed at 510 nm. Rutin was employed as a reference and results were reported as milligram of rutin equivalent per gram (mg RE/g).

Antioxidant Activity
The DPPH (2,2-diphenyl-1-picrilhydrazyl) scavenging capability assay was applied to appraise antioxidant effect of each extract. Analysis was conducted as per a previously established scheme with slight modifications [25]. Briefly, the sample extracts were mixed with the solution of DPPH in methanol and after incubation for 20 min at 35 • C absorbance was recorded at 517 nm. The free radical scavenging ability (%) was computed as follows.
Scavenging % = Absorbance of control − Absorbance of blank Absorbance of control Butylatedhydroxyanisole (BHA) was employed as reference standard and outcomes were reported as IC 50 (µg/mL). All the experiments were performed thrice.

Pancreatic Lipase Inhibitory Assay
The capability of optimized extracts to inhibit the activity of PL was appraised using a previously described method [3] with minor changes. In brief, sample extracts were prepared in Tris-HCl buffer (0.01 M) followed by addition of Arabic gum (10 g) and olive oil (10% w/v in Tris-HCl buffer) mixture utilizing a homogenizer for 20 high pressure homogenization passes. Extracts were resuspended in methanol, allowed to react with a solution of 1mg/mL PL (25 units/mL), and 2 mL of emulsion were used for assay. The sample extracts were then incubated at 4 • C for 30 min followed by adding substrate solution (2 mL) and reincubated at 37 • C for 30 min. After that, the reactions were stopped by adding a mixture acetone and ethanol (1:1) into the reaction mixtures and titrated against a solution of sodium hydroxide (0.02 M) for the neutralization of free fatty acids (pH, 9.4). The PL inhibitory activity (%) was computed as follows.
where, V c = Volume of base utilized for control and vs. = Volume of base utilized for sample

UHPLC-QTOF-MS/MS-Based Metabolite Profiling
UHPLC-QTOF-MS/MS technique was employed for the identification of metabolites in the most potent extract. The methanolic extract was prepared by dissolving dry extract in aqueous CH 3 OH. The obtained extract was filtered using a poly-tetrafluoroethylene filter (0.45 µm size). UHPLC-based separation was performed on a Waters ACQUITY chromatograph with a binary pump system, a degasser and autosampling assembly. An ACQUITY UPLC HSS T3 column (C-18 with dimensions of 1.8 µm × 2.1 mm x 100 mm) was used at 40 • C. Gradient elution was conducted by means of a binary solvent system (water having formic acid (0.1%) as solvent A and acetonitrile as solvent B) with variable compositions. The composition of the mobile phase changed during experiments in a pattern of 1% B for 0 min; 1% B for 0.5 min; 35% B for 16 min, 100% B for 18 min; 1% B for20 min with flow rate of 0.6 mL/min. A sample volume of 1 µL was injected. The chromatograph was equipped with a hybrid mass spectrometer by Waters (Vion IMS QTOF). Ionization was done in both negative and positive electrospray ionization mode. An operational capillary voltage of 1.50 kV was used, and a reference capillary voltage of 3.00 kV was set for experimental runs. A source temperature of 120 • C, desolvation gas flow of 800 L/h and desolvation gas temperature of 550 • C was used. A cone gas flow of 50 L/h was established using Nitrogen (>99.5%). Data were obtained in high definition MS E mode ranging from 50 to 1500 m/z at 0.1 s/scan. Argon gas (99.999%) was utilized as the collision-induced dissociation gas. A mass spectral data base compound identification was utilized.

Molecular Docking Studies
Docking studies of all identified phytocompounds were conducted by Molecular Operating Environment (MOE 2016.08). The 3-D structure of PL was downloaded and phytocompounds were docked into its active sites. Binding energies were computed after the preparation of ligand-enzyme conformations and energy minimization. Both hydrophilic as well as hydrophobic interactions on different amino acid residues were studied. Results of molecular docking were assessed and surfaces were analyzed graphically applying MOE as well as discovery-studio-visualizer.

Computational Studies
Computational approaches provide significant information regarding free radical scavenging activity of antioxidants. The computational protocol known as quantum chemical-based test can be utilized for quantification of antioxidant protection for overall radical scavenging activity and to predict trends [71]. The specialty of computational methods in predicting antioxidant properties of phytocompounds is that ionization energy values can be obtained for compounds under study [72]. Presently, first-principles calculations are of great significance to explain the different characteristics of materials. Numerous investigations have revealed the efficacy and utility of density functional theory (DFT) as a useful analytical tool to determine the electronic environment to correlate with the actual data [73][74][75]. DFT is known for optimization of ground-state geometries of molecules belonging to various classes [76]. B3LYP has been established as a rational approach among different DFT functionalities [77]. The current study involved the optimization of S 0 geometry by B3LYP/6-31G** level in Gaussian16 software.

Statistical Analysis
Results were summarized in the form of mean (±) standard error of experiments carried out in triplicate. Differences among data were executed by one way analysis of variance (ANOVA) utilizing Minitab (version-17) statistical software.

Conclusions
The results confirmed strong antioxidant and antiobesity attributes of C. fistula leaves. The 80% hydroethanolic leaf extract of C. fistula was established as the better fraction to exhibit antioxidant activities and PL enzyme inhibition. Metabolite profiling of same extract explored the presence of 23 bioactive constituents, and many of them were pharmacologically effective. The findings of molecular docking and computational studies explored possible effects of the identified phytocompounds as antioxidant and antiobesity agents. Hence, the 80% hydroethanolic leaf extract of C. fistula with promising antioxidant and lipase inhibitory activity can be used for innovative naturopathic antiobesity approaches. The considerable antioxidant activity, metabolite availability and antiobesity attributes identify C. fistula as a reasonable indigenous candidate for food fortification. The extract may be further processed for development of antiobesity functional foods and phytopharmaceuticals.