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The binding interactions between bovine serum albumin (BSA) and three pyridine derivatives, i.e., 2-(5-bromopyridin-3-yl) acetic acid ( L1 ), 3-bromo-5-nitropyridine ( L2 ) and 2-chloro-4-nitropyridine ( L3 ), have been carried out using UV-Vis and fluorescence spectroscopic methods. Fluorescence intensity quenching is observed by adding L2 and L3 to the BSA solution. The quenched fluorescence emission is due to the static nature. An isothermal titration calorimetry (ITC) experiment shows the binding ability of L1 with BSA. The binding constants are found to be 7.23 ± 0.32 × 10 5 M −1 for L1 . The thermodynamic parameters were calculated from ITC measurements (i.e., ∆H = −2.78 ± 0.08 kcal/mol, ∆G = −5.65 ± 0.25 kcal/mol, and -T∆S = −2.87 ± 0.11 kcal/mol), which indicated that the protein-ligand complex formation between L1 and BSA is mainly due to the hydrogen bonds and van der Waals interactions. Cyclic voltammetry (CV) and structure activity and relationship (SAR) studies have been carried out to establish the relationship between ligands and proteins. Additionally, we conducted an antibacterial assay with gram-positive Staphylococcus aureus , Enterococcus faecalis , and negative bacterial strains Acinetobacter baumannii and Escherichia coli against L1 , L2 , and L3 , aiming to address the challenges posed by the co-existence of multidrug-resistant bacteria. Finally, drosophila is used to test the cytotoxicity of ligands L1 , L2 , and L3 ’s in vitro.

Two pyrrolo-based compounds, 1H-pyrrolo[3,2-b]pyridine-3-carboxylic acid ( L1 ) and 1H-pyrrolo[3,2-c]pyridine-4-carboxylic acid ( L2 ), were employed for the detection of bovine serum albumin (BSA) by UV-Vis and fluorescence spectroscopic methods in phosphate buffer solution (pH = 7). In the presence of L1 and L2 , the fluorescence emission of BSA at 340 nm was quenched and concomitantly a red-shifted emission band appeared at 420 nm ( L1 )/450 nm ( L2 ). The fluorescence spectral changes indicate the protein-ligand complex formation between BSA and L1/L2 . An isothermal titration calorimetry (ITC) experiment was conducted to determine the binding ability between BSA and L1/L2 . The binding constants are found to be 4.45 ± 0.22 × 10 4 M −1 for L1 and 2.29 ± 0.11 × 10 4 M −1 for L2 , respectively. The thermodynamic parameters were calculated from ITC measurements (i.e. ∆rH = −40 ± 2 kcal/mol, ∆rG = −4.57 ± 0.22 kcal/mol and −T∆rS = 35.4 ± 1.77 kcal/mol), which indicated that the protein-ligand complex formation between L1/L2 with BSA is mainly due to the electrostatic interactions. The protein-ligand interactions were studied by performing molecular docking. Further, the antibacterial assay of L1 and L2 was conducted against gram-positive and gram-negative bacterial strains in an effort to address the difficulties caused by the co-occurrence of antimicrobial and multidrug-resistant bacteria. E. coli and S. aureus were significantly inhibited by L1 and L2 . The L1 exhibits 13, 12 and 15 mm, whereas L2 exhibits a 2, 3 and 5 mm zone of inhibition against S. aureus , S. pyogenes  and E. coli , respectively. In silico molecular docking of L1 and L2 was performed with bacterial DNA gyrase to establish the intermolecular interactions. Finally, the in vitro cytotoxicity activities of the ligands L1 and L2 have been carried out using drosophila. Graphical abstract