3.1 Computationalstudies
3.1.1Optimization
The geometrical optimized parameters of bambuterol hydrochloride computed by RB3LYP level with 6–31 + G (d) basis are listed inTable (1). The corresponding structure together with the labeling of atoms is shown in Figure (2).the overall geometry has a non-planar structure. The geometry of the benzene ring is seen to be perturbed due to different substituents on the benzene ring.The symmetry of the ring is distorted yielding ring angles greater than 120 at the points of substitutions.
Figure (2)
Table (1)
3.1.2 Mulliken analysis
Charge distributions of the molecule have been computed by performing Mulliken analysis[12]. The theoretical calculations of atomic charges play an important role in the application of quantum mechanical calculations to molecular systems. The calculated results reveal that the biggest values of negative charge are noticed for O5, Cl57, and C9. The carbon atoms of the methyl groups are positively charged. Almost very similar values of positive charges are noticed for all hydrogen atoms forming CH3 groups. The highest value of the positive charge is located on H56connected to N6 (N6 –H56).
3.1.3 Frontier molecular orbitals (FMOs) and density of states (DOS)
The highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital(LUMO) of the title compound are computed with the same level of DFT theory and are shown pictorially in Figure (3) The compositions of both HOMO and LUMO were calculated by Becke method via the Multiwfn program. From Figure (3) it could be seen that the HOMO result from thelone pair present on the chloride atom contributes to the HOMO by 96.7%.The LUMO result mainly from the aromatic system (benzene ring), with a contribution of 78%. The chemical reactivity of the title compound would be assessed based on the global reactivity descriptors. The energy of the HOMO and LUMO is directly related to ionization potential (IP) and electron affinity (EA).
Figure (3)
The value of IP and EA are given according to Koopman’s theorem[13].
IP= -HOMO
EA = -LUMO
The values of IP and EA can be used to deduce the global reactivity descriptors including chemical potential (µ), chemical hardness (η), chemical softness(S), electronegativity (X), and electrophilic index (ω) according to the following equations.
µ = - [(IP + EA)/2]
η = (IP-EA)/2
S = 1/2 η
X = (IP + EA)/2
ω = µ2/ 2 η
According to the values in Table 2, (E)/ (Nmax) is -1.707eV, and the maximal charge acceptance Nmax is 1.203eV. These values reveal the drug's intramolecular charge transfer as well as its ability to interact with and bind to β1-adrenergic receptors.
Chemical hardness (η) and electron transfer energy (E) are 2.836 and − 2.055 eV, respectively. These findings suggest that the charge transfer process in the drug and bioactivity formation of intermolecular interaction with 1-adrenergic receptors and blocking is permissible[14].
Table (2)
The Multiwfn program was used to plot each total, partial, and overlap density state (TDOS, PDOS, and OPDOS). Figures (4) depicts these plots. The curve maps of broadened partial DOS (PDOS) and overlap population DOS (OPDOS) for the title molecule is very useful for visualizing atomic orbitals of different fragments and have significant contributions to the corresponding MOs and chemical bonding. Figure 4 depicts the fragments of the title molecule and their PDOS and OPDOS only in the valence MOs range. The left axis represents TDOS and PDOS, while the right axis represents OPDOS, and the vertical dashed line indicates the position of the HOMO. Red, blue, magenta, brown, and yellow have been observed to have the highest contribution to valence MOs, with comparable amounts of contribution. The green curve represents the OPDOS between fragments 1, and 2, and its positive part indicates that MOs in the corresponding energy range exhibit bonding properties between two fragments (e.g. the one at -0.231 a.u which corresponds to MO 108).
Figure (4)
3.1.4 Molecular electrostatic potential (MEP) map
To forecast reactivity, find places for nucleophilic and electrophilic attacks on the molecule, and lastly assess the biological recognition process and hydrogen bonding interactions, one can use the concept of molecular electrostatic potential (MEP), which is connected to electron density[15].Drug-receptor interactions and the electrostaticpotential (ESP) V(r) have both been extensively studied[16, 17]. Figures (5) depicts the 3D ESP map of the investigated molecule to determine the different electrostatic potential values at both electrophilic and nucleophilic sites. The most electrostatically positive, most negatively charged, and zero electrostatic potential regions are represented by the colors blue, red, and white on the MEP surface, respectively. The figure shows 5 surface maxima for positive potential sites surrounding the hydrogen atoms and 4 surface minima for the negative potential sites on chlorine and oxygen atoms. The global minimum on the surface (-49.9 kcal/mol) is located on Cl57. While the location of the surface's global maxima (+ 46.8 kcal/mol) is located on H27. We note that the strongest nucleophilic and electrophilic sites, as well as the global maximum and minimum, are found in the vicinity of hydrogen chloride. According to the MEP map, the region that contains the hydrogen chloride molecule has a significant biological activity, plays a more significant role than any other location in the recognition of the drug in biological systems, and can establish halogen bonds.
Figure (5)
3.1.5 Reduced Density Gradient
Equation(1) gives the reduced density gradient (RDG), which is a fundamental dimensionless quantity derived from the density and its first derivative[18].
$${\text{RDG(}}r{\text{)}}=\frac{1}{{2{{\left( {3{\pi ^2}} \right)}^{\frac{1}{3}}}}}\frac{{\nabla \rho (r)}}{{\rho {{(r)}^{\frac{4}{3}}}}}$$
(1)
Large negative values of sign (λ2) in RDG tails indicate attractive interactions (such as dipole-dipole or hydrogen bonding); if the sign (λ2 ) is large and positive, the interaction is non-bonding (steric effect).
Near-zero values indicate very weak van der Waals interactions. Multiwfn was used to generate the RDG of the BB.HCl structures,whichwerethen plotted using the VMD program[19]. Figures (6) depicts these results.
Figure (6)
Green colors represent van der Waals interactions, while the red color represents strong repulsion. Inside the ring, repulsive interactions were observed, while van der Waals interactions were observed between the hydrogen atoms.The blue colors in BB.HCl indicates hydrogen bonding and halogen bonding interactions[20].
3.1.6 Natural bond orbital (NBO) analysis
Natural bond orbital analysis of bambuterol hydrochloride molecule is performed using NBO 3.1 module as implemented in Gaussian 09 program to confirm the charge transfer and conjugation interaction within the molecule. A Second order perturbation approach Fock matrix gives an examination of the energetic importance of electron transfer from the donor (Lewis-type NBOs) to accepter (non-Lewis NBOs). The stabilization energy,E(2), represents the degree of electron transfer from donor to acceptor known as the degree of electron delocalization[21].
E (2) = △Ei j= q i j =\(\frac{F {(i, j )}^{2}}{Ej-Ei}\)
Where q ij is donor orbital occupancy, F (i, j) is the off–diagonal Fock matrix element, Ej and Eiare the diagonal element (orbital energies).E (2) value can represent (reflect) the intensity of electron donor and electron acceptor and the degree of conjugation of the structure. The occupancy and energies of (i) and (j) with △Eijof the most significant lone pair (LP) and boning to anti-bonding \({BD}^{*}(\)σ*⁄\({\pi }^{*}\)) are given in table (3). It can be seen from table (3) that the majority of the contributions to the stability of the drug comes from lone pair interaction donor (NBO (i)) with anti-bonding BD* orbitals acceptor (NBOj)significant contributions of lP(1) N7⟶\({\sigma }^{*}\)(O4 – C21) and lP(1) N8⟶\({\sigma }^{*}\)(O5 – C22).
Table (3)
The other significant interactions gave a stronger stabilization energy value of 220.31and 231.09 kJ/mol to the structure are the interaction between the antibonding of C16–C19 and C18 –C20 respectively. Significant contribution to the stabilization energy could be added by LP (4) Cl57⟶LP*H27 of 102.55 value of E2 (i).
3.1.7 The electron localization function (ELF)
ELF provides important information on the electron environment, chemical bonding, and atomic shell on the surface of the molecule. The colored map of ELF drawn by Multiwfn software is shown in Figure (7) higher values of ELF plotted between the range 0.85 − 1.0 Bohr indicate the strong localization of electrons and formation of covalent bonds, whereas lower values lie between 0.0 and 0.4 Bohr denote the strong delocalization of electrons. From the ELF color map, we observe highly localized areas surrounding hydrogen atoms indicated by red color. Delocalized electrons predicated around oxygen and carbon atoms are shown in blue. The high value of electron localization occurs between the carbon atoms in the ring with the hydrogen atoms due to the overlap of the SP orbitals of carbon with the S orbital of hydrogen. Deformation of the ELF distribution around the oxygen is due to hydrogen and halogen bond interactions.
Figure (7)
The 3D plot of ELF for bambuterol hydrochloride is shown in Figure (8).the monosynaptic basin is associated with the lone pair whereas the disynaptic basin belongs to covalent interaction. The monosynaptic lone pair regions of oxygen, nitrogen, and chloride occupy more space. While the disynaptic associated with OH, NH, and CH occupy less space due to halogen and hydrogen bond interactions.
Figure (8)
3.1.8 Vibration analysis.
The molecule contains 57 atoms and hence 165 modes of vibrations. These 165 normal modes of vibrations are distributed among the symmetry specie as follow, considering C2 point symmetry.
┌vibration =111\({A}^{{\prime }}\)+ 54\({A}^{"}\)
\({A}^{{\prime }}\) and \({A}^{"}\) represent the vibrations which lie within in plane and out of plane, respectively. In C2 group, the symmetry of the molecule is a non-planar structure and has the 165 modes in irreducible representation. The calculated vibrations wavenumbers are higher than experimental values for the majority of the normal modes due to the environment of performing vibrations (gas phase for theoretical and solid state for experimental), and the fact that the experimental values are inharmonic wave numbers while the calculated values are harmonic ones. Therefore, the computed wavenumbers are scaled down using a scaling factor of 0.9608 to discard the anharmonicity present in the real system. The methyl group makes a significant contribution to the vibration spectrum of bambuterol hydrochloride since the molecule contains 7 methyl groups. Therefore, we will discuss the assignment of methyl group vibrations in detail. Nine fundamental modes of vibrations can be associated with each methyl group: two asymmetric stretching, one symmetric stretching, two asymmetric deformations, one symmetric deformation, two rocking vibrations, and one torsion mode of vibration. The computed (scaled) vibrations of the methyl group together with the experimentally observed frequencies and the assignments ate present in table (4).
Table (4)
3.1.9 AIM calculations.
Atoms in molecules (AIM) theory is a convenient method to analyze the hydrogen bonding and other interactions in various molecular systems and has been extensively used to classify and understand bonding interactions in terms of quantum mechanical parameters and their derivatives as electron density (𝞺). The theory of AIM efficiently describes H-bonding and it's a concept without borders. One of the advantages of this theory is that one can obtain information on the change in electron density distribution as the result of their bond formation or complex formation[22]. The molecular graph of the molecule using AIM theory is shown in Figure (9) the topological parameters of non-covalent interactions are grouped in a table (5). AIM results show that bambuterol hydrochloride is characterized by 4BCPs of non-covalent character. Two describing hydrogen bonding, and two characters halogen bonding interactions. According to the values of the parameters reported in table (5). We can classify the non-covalent interactions as weak hydrogen and halogen bonding interaction except for N6-H27……Cl57 strong bonding interactions [23].
Figure (9)
Table (5)
3.1.10 Nonlinear optical effects and first hyper polarizability.
NLO is at the cutting edge of current research because it provides the critical functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for emerging technologies such as telecommunications, signal processing, and optical interconnections[24, 25]. The prediction of non-linear optical (NLO) properties of a molecule by quantum chemistry plays an important role in the design of materials in modern communication technology[25]. Organic molecules, in particular, are being studied due to their higher NLO susceptibilities caused by electron cloud movement
from donor to acceptor, rapid NLO response times, high laser damage thresholds, and low dielectric constants. Table (6) shows the dipole moment, polarizability, and first hyper polarizability components of the title compound which invariant are calculated with a numerical derivative of the dipole moment using RB3LYP/6–31 + G(d) level of DFT. The total static dipole moment, average linear polarizability, anisotropy of polarizability, and first hyper polarizability can be calculated using the equations below [25].
µ = (µ x 2 + µy2 + µz2 )1\2
α = \(\frac{1}{3}\) ( αxx + αyy + αzz)
△α = \(\frac{1}{\surd 2}\) [ (αxx - αyy )2 + (αyy - αzz)2 + ( αzz - αxx )2 + 6αxx2 ]1\2
β =[ ( βxxx + βxyy + βxzz )2 + (βyyy + βxxy + βyzz)2 + ( βzzz + βxxz + βyyz )2 ]1\2
Table (6)
The calculated values of total static dipole moment µ, the average linear polarizability α ,the anisotropy of the polarizability ∆α ,and the first hyper polarizability β using the RB3LYP/6-31+G(d) level of DFTmethodare8.5Debye,297.5 a.u,540.42 a.uand2.676×10-30e.s.u, respectively.
Urea is one of the prototypical molecules used in the study of the NLO properties of molecular systems, and it is frequently used as a threshold value for comparative purposes. The values of µ, α, and β obtained with the RB3LYP/6-31+G(d)method for urea are 1.373 Debye, 3.831 Å3 and 3.729×10-31 cm5 e.s.u.-1, respectively[26].The title compound's first hyper polarizability is 8 times that of urea. The title compound may be a potential candidate in the development of NLO materials based on the magnitude of its first hyper polarizability. As a result, this molecule could serve as a potential building block for nonlinear optical materials.
3.1.11. Molecular docking
Molecular modelling and visualization were performed on Human butyrylcholinesterase using Molecular Operating Environment (MOE) 2019.01. The structure of Human butyrylcholinesterase in complex with thioflavine T obtained from the RCSB Protein Data Bank (PDB ID: 6esy). Bambuterol was prepared with the standard protocol in MOE 2019 and the energy of the docked compound was minimized with gradient RMSE of 0.0001kcal/mol. Then, butyrylcholinesterase structure was prepared by using the MOE QuickPrep protocol. The docking was done using the method of Alpha triangle placement with Amber10: EHT forcefield. Refinement was performed with Forcefield and scored using the Affinity dG scoring system.
3.1.11.1. Results
Docking protocol was validated by re-docking of the co-crystalized thioflavine T at the active site of butyrylcholinesterase (PDB ID: 6esy), Fig. (10). the re-docking rmsd = 1.2363 Å and binding score = -6.81 Kcal.mol-1. All the key interactions accomplished by the co-crystalized ligand with the key amino acids in the binding site is reproducible using the followed docking setup, mentioned in the experimental section. The validated docking setup was then used to investigate the ligand-receptor interactions and binding patterns for bambuterol hydrochloride (score = -7.24 Kcal.mol-1), Figure (11). The amino acid residues involved in interaction at binding site with co-crystallized thioflavine T are Ser53, Ile55, Trp56 and Asn57[27], where the main interactions are H-bonding with amino acid residues through oxygen atom of pyranose ring and attached OH groups. In addition, amidic NH group formed π-H bond with indole ring of Trp56 residue.
Figure (10)
Figure (11)
The docking investigation showed that bambuterol hydrochloride depicted the same interactions as thioflavine T at different poses, but the common residues are Ile55, Trp56 and Asn57 and these amino acid residues as discussed before are essential for inhibition of butyrylcholinesterase enzyme. Beside these amino acids, bambuterol hydrochloride interacts with other amino acids as extra binding interactions Table (7) that are mainly H-bonding which induced and driven by carbamate ester group with Thr59, Lys60, Asn63 residues and H-bonding between primaryNH2 group and Asp54 residue and all that justified the lower binding score and higher affinity of bambuterol than thioflavine T towards butyrylcholinesterase enzyme.
Table (7)