Synthesis of Benzimidazole Derivatives and their Biological Investigation

Abstract

The study was designed to synthesize substituted benzimidazole derivatives from reactions between o-phenylenediamine and various aldehyde derivatives using water as reaction medium employing several surface active agents as catalyst at room temperature. At the same time to assume the biological activity of the synthesized molecules through molecular docking on Acetylcholine esterase enzyme model. The current synthetic procedure was preceded by initial optimization of reaction condition for the synthesis of substituted benzimidazole derivatives. For this purpose we utilized o-phenylenediamine as starting and 4-Cl-benzaldehyde as aldehyde prototype, varying the equivalent mmol ratios such as 1:1 and 1:2 keeping different types of surfactants like such as benzalkonium chloride (BKC), sodium dodecyl sulfate (SDS), Tetra-n-butylammonium bromide (TBAB) and Tetra-n-butylammonium iodide (TBAI) in a fixed mmol equivalent proportion. All reactions are carried out at room temperature using water as reaction medium. With optimized reaction condition then several reactions were carried out between o-phenylenediamine and various aldehyde derivatives. Furthermore synthesized substituted benzimidazole derivatives were screened through molecular docking on Acetylcholine esterase enzyme to recognize the binding site and affinities for better inhibition with reference to the standard inhibitor pralidoxime. All surfactants were found to be effective catalyst at room temperature providing good to moderate yields at varying ratios of o-phenylenediamine and 4-Cl benzaldehyde. We observed that at 1:1 ratio of reactants 2-substituted benzimidazole (2-(4-chlorophenyl)-1H-benzo[d]imidazole) is prominent product obtained. But at 1:2 ratios reaction condition favors 1,2-disubstituted benzimidazole (1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-benzo[d]imidazole). Among the four surfactants, Benzalkonium chloride shows excellent affinity to produce 2-substituted benzimidazole at both 1:1 and 1:2 ratios. Total of six benzimidazole derivative namely-2-(4-chlorophenyl)-1H-benzo[d]imidazole (Code: MS4ClB) and 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-benzo[d]imidazole (Code: DS4ClB); 4-(1H-benzo[d]imidazol-2-yl) phenol (Code: MSB4OH) and 4-(1-(4-hydroxybenzyl)-1H-benzo[d]imidazol-2-yl) phenol (Code: DSB4OH); 2-phenyl-1H-benzo[d]imidazole (Code: MSBA) and 1-benzyl-2-phenyl-1H-benzo[d]imidazole (Code: DSBA) were obtained with good to moderate yield. All the products are confirmed by TLC checking with in house reference product and 1H-NMR.

Binding affinity of 2-(4-chlorophenyl)-1H-benzo[d]imidazole to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -9 compared to -5.7 of pralidoxime. The product MS4ClB tends to interact with five amino acid residues present at the binding site namely val294, Tyr341, Tyr337, Tyr124 and Trp86. This interaction is due to pi to pi bond interaction with tryptophan and tyrosine residues, alkyl and alkyl to pi interaction with Valine residue at the binding site. This interaction is not sufficiently strong for prominent inhibitory effect on the enzyme. This binding interaction is also affected by the molecular orientation at the binding site. Docking study reveals that the molecule is oriented to the edge of the binding site where the aromatic system of the interacting amino acids faces in slightly different angle rendering the interaction much weaker. The binding site of the enzyme contains both H-bond donor and acceptor groups. The chloride substitution on the side chain renders the molecule to interact with the H-bonding accepting group at binding site. But bond distances are much longer to create a strong bonding. The molecules also possess bulkier chlorophenyl side chain which can interact with hydrophobic interaction at hydrophobic region of the binding site. Binding affinity of 2-(4-chlorophenyl)-1H-benzo[d]imidazole to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -8.9 compared to -5.7 of pralidoxime. Docking of this molecule at binding site reveals the possible pi-anion interaction with Glu 292 residue, pi-pi stacked interaction with Tyr341 and Trp286 through benzyl side chain of the substitution. Docking study reveals that the benzyl side chain substituted at 1-possition is oriented to amino acid residue face as well as to H-bonding accepting site which enables this molecule to exhibit greater affinity than its mono substituted congener. But bond distance is much longer to create a strong bonding. The molecule also possesses bulkier chlorophenyl side chain which can interact with hydrophobic interaction at hydrophobic region of the binding site. Binding affinity of 4-(1H-benzo[d]imidazol-2-yl) phenol to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -8.3 compared to -5.7 of pralidoxime. Molecular Docking reveals that molecule MSB4OH is oriented at the edge surfaces of most of the interacting amino acid residues and is oriented far from the H-bond accepting site of the enzyme. The molecule interacts with pi-donor H bond with Tyr 124 and tyr337. The molecule possesses hydroxyl group at side chain rendering weak interaction with hydrophobic region of the binding site. Binding affinity of 4-(1-(4-hydroxybenzyl)-1H-benzo[d]imidazol-2-yl) phenol to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -8.3 compared to -5.7 of pralidoxime. Molecular Docking reveals that molecule DSB4OH is oriented at the edge surfaces of most of the interacting amino acid residues and is oriented to the H-bond accepting site of the enzyme which promotes conventional H-bonding with Glu313 residue at binding pocket. The molecule possesses hydroxyl groups at side chain rendering weak interaction with hydrophobic region of the binding site. Binding affinity of 2-phenyl-1H-benzo[d]imidazole to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -10.8 compared to -5.7 of pralidoxime. Molecular Docking reveals that this smaller molecule MSBA is oriented at the face surfaces of most of the interacting amino acid residues and is oriented to the H-bond donor site of the enzyme which promotes conventional H-bonding with Arg296 residue at binding pocket. Binding affinity of 1-benzyl-2-phenyl-1H-benzo[d]imidazole to acetylcholine esterase is much stronger than that of pralidoxime with maximum binding affinity of -9 compared to -5.7 of pralidoxime. Molecular Docking reveals that this smaller molecule DSBA is oriented at the edge surfaces of most of the interacting amino acid residues and is not oriented to the H-bond donor site of the enzyme. Hence this molecule forms pi-pi weak interaction with amino acid residue like Trp86 and Tyr124 and Tyr337.this molecule possesses weak interpolated charge and much oriented to hydrophobic interaction. Thus benzalkonium chloride is effective and environmentally benign alternative for synthetic purpose with easy workup procedure. Molecular docking reveals good binding affinities for the candidates synthesized in this study. There is lacking of formation of strong classical H-bonding among the candidates with the amino acid residues at the binding site of Acetylcholine esterase enzyme. Hydrogen bonding as well as hydrophobicity can be improved by substituting different groups on the benzene ring system on both benzimidazole heterocycles and on the side chain to render the candidates as good irreversible Acetylcholine esterase enzyme inhibitor of clinical importance.