Organoruthenium(II) complexes appended with racemic pyrazoline ligands: integrated experimental and theoretical insights revealing their apoptotic efficacy.

Herein, we report four new cationic ruthenium(II)-(p-cymene) racemic complexes (C1-C4) containing N,S-coordinated pyrazoline ligands. The contemplated organoruthenium compounds from dimeric Ru(II)-(η6p-cymene) chlorido moieties and pyrazoline-based ligands were synthesized and characterized using standard analytical, spectral and ESI-HRMS data. The pseudo-octahedral geometry, neutral bidentate N and S chelation of pyrazoline thiosemicarbazones, and cationic nature of all four complexes were confirmed through single-crystal X-ray diffraction (SC-XRD) study. The geometry of individual enantiomers of the respective enantiomeric pairs was optimized using density functional theory. Furthermore, frequency calculations, single-point calculations, frontier molecular orbital analysis and TD-DFT were also carried out. Biomolecular interactions of the complexes were investigated using UV-visible and fluorescence spectroscopy techniques with calf-thymus DNA (CT-DNA) and bovine serum albumin (BSA). Anticancer activity of the racemic complexes C1-C4 against human colorectal adenocarcinoma (HT-29) and human lung cancer (A549) cell lines assessed using MTT assay indicated the IC50 concentrations ranging from 15.39 to 21.86 µM and 17.29 to 24.29 µM for HT-29 and A549 cells, respectively, and the values are comparable to that of the positive control (cisplatin) and also the same assay was performed on human epithelial kidney cells (HEK-293) to determine the selectivity index of the complexes. As these four complexes possess similar molecular structures, based on the better selectivity index and promising in vitro performance, complex C2 was selected for more detailed anticancer assays. DAPI, DCFDA, and JC-I cellular staining assays, along with flow cytometry, were performed on C2-treated A549 cells. The complex C2 induced apoptosis through DNA damage, ROS generation, and mitochondrial depolarization. Molecular docking studies on the DFT-optimized R and S conformers of the complexes were undertaken towards a spectrum of cancer-related protein targets to validate their binding interactions. The complexes demonstrated favorable binding interactions with B-DNA, BSA, cyclooxygenase-2 (COX-2) and B-cell lymphoma-2 (Bcl-2), with high binding affinity values of -8.12, -6.6, -6.25 and -7.53 kcal mol-1, respectively. Simultaneously, binding interactions of the individual enantiomers (R and S) of the respective pairs of each complex towards the target proteins were also examined, but they did not vary greatly among the complexes.