Biomolecular interactions of 1,3-bis(2-arylimino)isoindolate-based palladium(II) complexes: Substitution kinetics, DNA/protein-binding, and molecular docking approaches

A series of 1,3-bis(2-arylimino)isoindoline Pd(II) complexes core viz.; Chlorido(1,3-bis(2-pyridylimino) isoindoline)palladium(II), Pd1, Chlorido(1,3-bis(4-methyl-2-pyridylimino)isoindoline)palladium(II), Pd2, Chlorido(1,3-bis(2-pyridylimino)benz(f)isoindoline)palladium(II), Pd3 and Chlorido(1,3-Bis(1isoquinolylimino)isoindoline)palladium(II), Pd4 were synthesized, appropriately characterized and the crystal structure of Pd2 elucidated. The kinetics and mechanism of the substitution of the chloride ligand with thiourea ligands, Tu, Dmtu and Tmtu, from the complexes were investigated under pseudo-first-order conditions. The analyses were performed using stopped-flow analyzer or UV-visible spectrophotometer. The reactions proceeded through two consecutive steps for most complexes with exception of Pd4 showing only a single step. The substitution rates followed the order: Pd1>Pd2>Pd3>Pd4 due to varying degrees of steric influences and sigma-/pi-donations caused by the methylation and benzannulation on the cis-/trans-positions of the pyridyl rings of the BPI. The quenching of the fluorescence of CT-DNA/EB by the Pd(II) complexes suggests static quenching mechanism. The simulated docking of the complexes onto CT-DNA suggests they bind mainly in the minor grooves of DNA. The UV-Visible absorption titrations and quenching of tryptophan (Trp) fluorescence of BSA by the complexes depict reasonable interactions which occur mainly in the hydrophobic domains of the former. The order of strength of the interaction of the complexes with DNA or BSA is consistent with the rates of substitution kinetics.