Molecular properties and In silico bioactivity evaluation of (4-fluorophenyl)[5)-3-phen-(4-nitrophenyl yl-4,5-dihydro-1H-pyrazol-1-yl]methanone derivatives: DFT and molecular docking approaches

Objectives Molecular structures, spectroscopic properties, charge distributions, frontier orbital energies, nonlinear optical (NLO) properties and molecular docking simulations were analyzed to examine the bio-usefulness of a series of (4-fluorophenyl)[5-(4-nitrophenyl)-3-phenyl-4,5-dihydro-1H-pyrazol-1-yl]methanone derivatives. Methods The compounds were studied through computational methods. Equilibrium optimization of the compounds was performed at the B3LYP/6-31G(d,p) level of theory, and geometric parameters, frequency vibration, UV–vis spectroscopy and reactivity properties were predicted on the basis of density functional theory (DFT) calculations. Results The energy gap (ΔEg), electron donating/accepting power (ω−/ω+) and electron density response toward electrophiles/nucleophiles calculated for M1 and M2 revealed the importance of substituent positioning on compound chemical behavior. In addition, ω−/ω+ and ΔEn/ΔEe indicated that M6 is more electrophilic because of the presence of two NO2 groups, which enhanced its NLO properties. The hyperpolarizability (β0) of the compounds ranged from 5.21 × 10−30 to 7.26 × 10−30 esu and was greater than that of urea; thus, M1–M6 were considered possible candidates for NLO applications. Docking simulation was also performed on the studied compounds and targets (PDB ID: 5ADH and 1RO6), and the calculated binding affinity and non-bonding interactions are reported. Conclusion The calculated ω− and ω+ indicated the electrophilic nature of the compounds; M6, a compound with two NO2 groups, showed enhanced effects. Molecular electrostatic potential (MEP) analysis indicated that amide and nitro groups on the compounds were centers of electrophilic attacks. The magnitude of the molecular hyperpolarizability suggested that the entire compound had good NLO properties and therefore could be explored as a candidate NLO material. The docking results indicated that these compounds have excellent antioxidant and anti-inflammatory properties.

Methods: The compounds were studied through computational methods. Equilibrium optimization of the compounds was performed at the B3LYP/6-31G(d,p) level of theory, and geometric parameters, frequency vibration, UVevis spectroscopy and reactivity properties were predicted on the basis of density functional theory (DFT) calculations.
Results: The energy gap (DEg), electron donating/ accepting power (uÀ/uþ) and electron density response toward electrophiles/nucleophiles calculated for M1 and M2 revealed the importance of substituent positioning on compound chemical behavior. In addition, uÀ/uþ and DEn/DEe indicated that M6 is more electrophilic because of the presence of two NO 2 groups, which enhanced its NLO properties. The hyperpolarizability (b 0 ) of the compounds ranged from 5.21 Â 10 À30 to 7.26 Â 10 À30 esu and was greater than that of urea; thus, M1eM6 were considered possible candidates for NLO applications. Docking simulation was also performed on the studied compounds and targets (PDB ID: 5ADH and 1RO6), and the calculated binding affinity and non-bonding interactions are reported.
Conclusion: The calculated u À and u þ indicated the electrophilic nature of the compounds; M6, a compound with two NO 2 groups, showed enhanced effects. Molecular electrostatic potential (MEP) analysis indicated that amide and nitro groups on the compounds were centers of electrophilic attacks. The magnitude of the molecular hyperpolarizability suggested that the entire compound had good NLO properties and therefore could be explored as a candidate NLO material. The docking results indicated that these compounds have excellent antioxidant and anti-inflammatory properties.

Introduction
Pyrazole derivatives are heterocyclic compounds recognized to possess a wide range of biological and pharmacological activities. 1,2 The unique properties of pyrazoles have been attributed to the electrophilic substitution reactions that occur specially at position 4, and nucleophilic attacks that occur at positions 3 and 5, thus leading to diverse pyrazole structures with broad potential applications in areas such as medicine, agriculture and technology. 3e5 In addition, compounds containing a pyrazole nucleus have been reported to be anti-inflammatory, antiviral, anticancer, antiparasitic, antibacterial, antirheumatoid, antidepressant, analgesic, antinociceptive, antihypertensive, antipyretic and antifungal agents. 6e12 Beyond these biological activities, this class of compounds displays substantial nonlinear optical (NLO) properties, 13e17 electroluminescent properties due to photo-inducedelectron transfer, 18 and light amplification properties due to stimulated emission or lasing/random lasing action. 14,15 Several pyrazole derivatives, such as 3-(1,1dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole, have been investigated for their NLO properties. The size of the nano-crystals of the compound has been suggested to play a major role in the excitation or emission efficiency. 19,20 Thus, these compounds can be used for ultrafast optics. 21 The optical nonlinearity of a series of N-substituted-5-phenyl-1H-pyrazole-4-ethyl carboxylates of compounds in chloroform solution has been assessed, and these compounds have been found to be good candidates for NLO applications. 22 In addition, a series of (Z)-2-(4-nitrophenyl)-3-(1-phenyl-4,5dihydro-1H-pyrazol-3-yl)acrylonitrile and (E)-3-(4nitrostyryl)-1-phenyl-4,5-dihydro-1H-pyrazole compounds have been found to have several electron accepting groups attached and to show high NLO responses dependent on functionalization of the pyrazoline derivatives. 23 The compounds 1-N-phenyl-3(3,4-dichlorophenyl)-5-phenyl-2pyrazoline, 24 diethyl-1H-pyrazole-3,5-dicarboxylate and 4-(4-bromophenyl)-1-tert-butyl-3-methyl-1H-pyrazol-5amine 25 have been studied with experimental and density functional theory (DFT) methods and found to have promising NLO properties.

Theoretical details
Before DFT calculations, an equilibrium conformer search was performed on all compounds with a semiempirical AM1 method to identify the lowest conformer for each compound, which was used for further DFT calculations. 29 All calculations were performed on these compounds with Becke's three parameter hybrid functional DFT, with Lee, Yang and Parr correlation, 30 and optimized at B3LYP/6-31G(d,p) level of theory in gas.
Frequency calculation was also performed by using the same basis set to confirm that the optimized molecules were minima, as characterized by positive harmonic frequencies 31,32 in Spartan 14. 33 The DFT hybrid B3LYP functional has been reported to overestimate the fundamental modes; however, this overestimation can be addressed by calculating harmonic frequencies with a scaling factor of 0.9619 to yield frequencies consistent with experimental data. 34 The molecular descriptors calculated from conceptual DFT were the ionization potential (I ¼ ÀHOMO), electron affinity (A ¼ ÀLUMO), chemical hardness (h), chemical potential (m), global electrophilicity (u), electron donating power ðu À Þ, electron accepting power (u þ Þ, nucleofugality (DE n ) and electrofugality (DE e ) (equations (1)e(7)). 35e39 Results and discussion

Geometry parameters
The geometry parameters extracted from the equilibrium structures optimized at the B3LYP/6-31G(d,p) level for the six compounds M1eM6 are displayed in Tables 2 and 3. The results were compared with the geometries of 4-(3-(2-amino-3,5-dibromophenyl)-1-(4-nitrobenzoyl)-4,5-dihydro-1H-pyrazol-5-yl)benzonitrile predicted at the same level of theory. 28 The calculated C3eN7 bonds were 1.299, 1.298, 1.300, 1.299, 1.299 and 1.298 Å for M1eM6, respectively, in agreement with the 1.299 Å calculated for Ma. This type of bond has been experimentally observed to be 1.275 Å and calculated to be 1.287 Å for bis-spiropipridinon/pyrazole derivatives. 40    These two bond angles indicated that the pyrazole ring was distorted from planarity by the aryl and benzoyl rings, as reflected in the dihedral angles (Table 3), thus revealing that aryl and benzoyl groups have more profound effects on the pyrazole ring than the extended R1 and R2 substituents.

Frontier molecular orbital and UVevis absorption properties
The frontier orbital energies, such as the highest occupied molecular orbital energy (HOMO), lowest unoccupied molecular orbital (LUMO) and band gaps, are critical parameters for kinetic and thermodynamic stability studies, and prediction of reactivity and photochemical properties. 41e46 The frontier orbital molecular overlay revealed that the HOMO overlay was essentially on ring, extending over the two nitrogen atoms of the pyrazole ring, whereas the LUMO overlay was on the phenyl ring for M1, M3, M4 and M5, and was on the benzoyl ring for M1 and M6 and M6 presented DEg than M1; however, M6 showed the lowest DEg because of lowering of the LUMO energy (À2.90 eV) and stabilization of the HOMO, thereby decreasing the p-electron density of the aromatic rings. Thus, M6 was expected to be relatively more reactive toward nucleophiles ( Table 2).
Other calculated reactivity descriptors such as chemical hardness (h), chemical potential (m) and global electrophilicity (u) were, respectively, 1.71, À4.21 and 5.182 eV for M1; would be a good electron acceptor with strong electron pulling effects toward the two NO 2 groups. This finding was in agreement with the effect of the NO 2 group observed on 4-(3-(2-amino-3,5-dibromophenyl)-1-(4-nitrobenzoyl)-4,5dihydro-1H-pyrazol-5-yl)benzonitrile by Santhi and Bharathi. 28 The electron donating power (u À ) and electron accepting power (u þ ) describe the tendency of a molecule to release electrons and to accept electrons, respectively; a smaller u À indicates a better donor of electron density, and a greater u þ indicates better accepting electron capacity.
The values for u À demonstrated that M1, M4 and M5 were good electron donors, whereas u þ indicated the tendency of M2, M3 and M5 to be good electron acceptors, in line with the m and u values ( Table 4).
The absorption peaks, oscillator strength and percentage of the molecular orbitals involved in transitions, calculated for M1eM6 at B3LYP/6-31G (p,d), are displayed in Table 5   OS was characterized as an nep* transition. All the compounds had one or two triplet transitions, thus potentially indicating that they possessed both orbital unpaired and spin unpaired electrons. Consequently, singlet transitions with sufficiently long lifetimes might have led to de-inversion of the spin of some of the electrons, thus generating a triplet.

Molecular electrostatic potential analysis
The static distribution of charge density is associated with the electrostatic potential map (MEP) distribution of charge on a molecule, and is a useful parameter for analyzing and predicting the responsiveness of a molecule toward an incoming electrophile or nucleophile during reaction initiation. 47 This parameter has been successfully used to explain stacking and self-assembly of polymeric molecules and dyes, and the orientation of molecules in three-dimensional crystals. 48 The MEP was simulated according to the optimized geometry obtained from DFT calculation to predict electrophilic and nucleophilic sites of attack. In Figure 2, blue indicates a positive region indicating electrophilic centers/electron-deficient areas; red represents the negative regions (areas with excess electrons) for nucleophilic reactivity/electron-rich centers of a molecule; and green represents regions with essentially zero potential. 49e51 Generally, the MEP ranged from red to orange to yellow to green to blue (Figure 1). The negative (red) regions were located at the carbonyl oxygen and cyano group, thus indicating the most probable sites for electrophilic attack. The regions with excess electrons could result in intermolecular pulling of positive regions of nearby molecules, thus distorting the pep stacking arrangement of these molecules.
However, these bands were calculated for N-methyl-N-(2,4,6-trinitrophenyl) nitramide to be in the region of 1633e1591 cm À1 and 1388e1348 cm À1 , and were assigned to vNO 2 asymmetric and symmetric stretching vibrations, respectively. 54 The vNH 2 group (electron donor) stretching was 1560 cm À1 for M1 and M3; 1559 cm À1 for M2, M4 and M6; and 1556 cm À1 for M5. Generally, aromatic compounds containing fluorine show vCeF stretching in the region 1000e1400 cm À1 . 55 However, compounds M1 and M2 showed these vibrations at 1237 cm À1 and have been reported at 1223 cm À1 . 28

Polarizability and hyperpolarizability
The static polarizability (a), hyperpolarizability (b) and electric dipole moment (m) based on the finite field method were calculated for the six compounds at DFT B3LYP/6-31G(d,p). The total static dipole moment (m), mean polarizability (a o ) and mean first hyperpolarizability (b 0 ) were defined by using the x,y,z component, as shown in equations (8)e (14): The presence of electron donating and electron withdrawing groups on p-conjugated molecules changes the ground state charge distribution and enhances asymmetric polarization of the molecules. Consequently, large nonlinear responses are correlated with a rapid response time; therefore, these molecules are desirable candidates for NLO applications. 56,57 Any molecule with a minimum value of 4.187944 Â 1 À30 esu for the first hyperpolarizability is considered a good candidate for NLO applications. 58 Therefore, these molecules' NLO properties, polarizability and hyperpolarizability were assessed. The dipole moment, an essential parameter explaining the intermolecular for Ma (4-[3-(2-amino-3,5-dibromophenyl)-1-(4fluorobenzoyl)-4,5-dihydro-1H-pyrazol-5-yl]benzonitrile) has been reported to be 3.31 Â 10 À23 esu. 28 However, the b 0 for M1eM6 was 6.04, 5.21, 6.15, 6.74, 6.02 and 7.26 3.31 Â 10 À30 esu, respectively, and has been reported to be 8.47 Â 10 À30 esu for Ma. 28 The b 0 values for the studied compounds were lower than that of Ma, but approximately 16 times higher than that of urea (0.372 Â 10 À30 esu). 57 The model compounds had higher b 0 values than those of diphenylmethylidene-5-methyl-1H-pyrazole-3-carbohydrazide (b 0 ¼ 2.08 Â 10 À30 esu) 59 and diethyl-1-H-pyrazole-3,5-dicarboxylate (b 0 ¼ 1.01 Â 10 À30 esu). 24 Interchanging the position of NO 2 and F, as shown in M1 and M2, led to a decrease in b 0 by 0.83 Â 10 À30 esu and an increase in absorption wavelength (l max ) by 75 nm in M2. Thus, the position, nature and point of attachment of substituents on the model compound strongly affect the properties of the compound. 57 In addition, M6 with two NO 2 groups increased b 0 ; therefore, the presence of an electron withdrawing group (nitro) on the phenyl ring contributed to higher hyperpolarizability values, possibly because of an inductive effect of the electron withdrawing group on the electronic density in the molecule. 28 Our results showed that M1eM6 compounds may be suitable for NLO applications, and the magnitude of molecular hyperpolarizability was improved by functional group modification (Table 7).

Molecular docking
The antihypertensive and antioxidant properties of the studied pyrazole derivatives (M1eM6) were investigated via molecular docking, and the obtained results were compared with the results for rolipram and taurine. The antihypertensive activity of the compounds was evaluated by considering their inhibitory activity against phosphodiesterases (PDEs). PDEs are enzymes participating in cAMP and cGMP homeostasis by acting on phosphodiester bonds. 60,61 When PDE is inhibited, the cAMP and cGMP levels increase, thereby decreasing calcium levels in cells.
cAMP, another mediator that controls pro-inflammation and anti-inflammation. 61,62 To investigate the PDE inhibitory activity of the compounds, we performed molecular docking of the compounds on PDE4 (PDB ID: 1RO6) downloaded from the Protein Data Bank. The downloaded 1RO6 was complexed with the rolipram drug, a selective PDE4 inhibitor that increases the quantity of cAMP in immune and nerve cells 63,64 and compared the docking results. Antioxidants are common food additives that inhibit cellular damage mainly through their free radical scavenging ability. 64,65 Free radicals are reactive oxygen species produced in the body through various metabolic processes, in phagocytosis, in prostaglandin synthesis and in the cytochrome P-450 system, as a result of exposure to different physiochemical conditions or pathological states. 66 Excessive free radicals in the body lead to a condition known as oxidative stress, which harmfully alters proteins, lipids and DNA, and can initiate the progression of pathologies including immune system deterioration, atherosclerosis and abnormal cell growth leading to nucleofugality cancer. 66 Analysis of pyrazoline derivatives has indicated that they are promising antioxidants. 67e69 Therefore, we examined the model compounds for their antioxidant ability by docking them against a dehydrogenase inhibitor downloaded from the Protein Data Bank (PDB ID: 5ADH). Taurine is an antioxidant involved in protection of hepatic tissue by deactivating reactive oxygen species, thereby removing formation of osmoregulation, calcium homeostasis, lipid peroxidation A recent study has indicated that molecular docking of pyrazole derivatives such as carboxy pyrazole derivatives with various cancer cells (breast, MCF-7; bone marrow, K-562; and cervix, HeLa), 6 aryl pyrazoles with tyrosinase enzyme, 85 pyrazole-phenyl semicarbazone derivatives with a-glucosidase, 8 imidazoleepyrazole conjugates with aglucosidase 10 and 4-aryl-N-(5-methyl-1H-pyrazol-3-yl)benzamides with Acinetobacter baumannii protein11 was in agreement with experimental observations, and has also detailed the nature of the protein-ligand interactions. The use of molecular docking for in silico screening of bioactivity of heterorganic compounds, as well as drug design and discovery, has become frequent and relevant in pharmacology. Therefore, docking serves as a reliable and time saving method for simulation of binding poses of ligand conformations in the active sites of receptors, and calculation of the binding affinity and interactions of protein-ligand complexes. 86 The binding affinity of the stable ligands docked with dehydrogenase inhibitor (PDB ID: 5ADH) ranged from À8.8 to 9.3 kcal/mol: M1 (À9.0 kcal/mol), M2 and M3 (À9.3 kcal/ mol) and M4, M5 and M6 (À8.8 kcal/mol). Similar binding affinities (À8.3 to À9.5 kcal/mol) have been reported for 1benzyl-2-phenyl-1H-benzimidazole derivatives docked with dehydrogenase (PDB ID: 5ADH). 87 The binding affinity calculated for taurine was À3.7 kcal/mol, as shown in Table 8, thus indicating that these compounds may be excellent inhibitors for APO-liver dehydrogenase and thus possess good antioxidant properties. Ligand interactions with the binding pocket of dehydrogenase (PDB ID: 5ADH) revealed that ARG 369 and ARG 202 are involved in hydrogen bond interactions with the NO 2 group of M1;  (Table 8 and Figure 3). Taurine is involved in hydrogen bonding with SER 367, ARG 369 and ARG 47, and also participates in Van der Waals interactions with VAL 203 and CYS 46. Similar binding modes for protein-ligand interactions have been observed for 1-benzyl-2-phenyl-1H-benzimidazole derivatives docked with dehydrogenase (PDB ID: 5ADH), and ILE 269, VAL 203, GLY 202, PRO 295 and ARG 47 amino acid residues have been found to be involved in the interactions. 87 The ligand bound at the active site of the 1RO6 receptor ( Figure 4) (Table 8 and Figure 4). The free energy for binding of rolipram with PDE (PDB ID: 1RO6) in the active pocket was À9.7, À9.2, À9.4, À9.9, À8.5 and À8.9 kcal/mol for M1eM6, respectively, whereas À8.8 kcal/mol was calculated for rolipram. These findings suggest that all model compounds except M5 exhibit PDE inhibitory activity than rolipram drug.
The calculated u þ revealed that the electrophilic nature of the compounds was increased by the presence of electron withdrawing groups, and the effect was particularly pronounced in the compound with two NO 2 groups, M6. MEP analysis showed that amide and nitro groups on the compounds were centers of electrophilic attacks, and the magnitude of the molecular hyperpolarizability suggested that the compounds might have NLO properties.
The binding affinity for the protein-ligand complexes ranged from À8.8 to À9.3 kcal/moldvalues higher than that of taurine, an antioxidant involved in inhibition of dehydrogenase (PDB ID: 5ADH) in hepatic tissue. For PDE (PDB: 1RO6), the binding affinity ranged from À8.5 to À9.9 kcal/mol, whereas that of rolipram, a selective inhibitor of PDE4, was À8.8 kcal/mol, thereby indicating that only M5 (À8.5 kcal/mol) had a lower binding affinity than rolipram. Thus, the docking results showed that these compounds may be excellent antioxidant and anti-inflammatory agents.

Source of funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of interest
The authors have no conflict of interest to declare.

Ethical approval
Not applicable.