ORIGINAL ARTICLE Fluorescence, DNA Interaction and Cytotoxicity Studies of 4,5-Dihydro-1H-Pyrazol-1-Yl Moiety Based Os(IV) Compounds: Synthesis, Characterization and Biological Evaluation Bharat H. Pursuwani 1 & Bhupesh S. Bhatt 1 & Foram U. Vaidya 2 & Chandramani Pathak 2 & Mohan N. Patel 1 Received: 28 May 2020 /Accepted: 3 December 2020 # The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021 Abstract Osmium(IV) pyrazole compounds and ligands were synthesized and well characterised. Ligands were characterized by heteronuclear NMR spectroscopy ( 1 H& 13 C), elemental analysis, IR spectroscopy and liquid crystal mass spectroscopy. Os(IV) complexes were characterized by ESI-MS, ICP-OES, IR spectroscopy, conductance measurements, magnetic measure- ments and electronic spectroscopy. Binding of compounds with HS-DNA were evaluated using viscosity measurements, ab- sorption titration, fluorescence quenching, and molecular docking, which show effective intercalation mode exhibited by com- pounds. Binding constant of Os(IV) complexes are found to be 8.1 to 9.2 × 10 4 M -1 . Bacteriostatic and cytotoxic activities were carried out to evaluate MIC, LC 50 , and IC 50 . The compounds have been undergone bacteriostatic screening using three sets of Gram +ve and two sets of Gram -ve bacteria. MIC of complexes are found to be 72.5–100 μM, whereas that of ligands fall at about 122.5–150 μM.. LC 50 count of ligands fall in the range of 16.22–17.28 μg/mL whereas that of complexes of Os(IV) fall in the range of 4.87–5.87 μg/mL. IC 50 of osmium compounds were evaluated using HCT-116 cell line. All the Os(IV) compounds show moderate IC 50 . Keywords Binding . Absorption titration . Molecular docking . HCT-116 Introduction The field of medicinal inorganic chemistry were explored widely [1]. Metal complexes with biologically active het- erocyclic ligands enhance its applications [2]. Design and synthesis of such compounds having less side effects and more potentiality has been a primary step towards explor- ing its application in biological medicinal chemistry [3–5]. Transition metals have been studied widely since they often alter and bind with the DNA nucleic bases [6, 7]. The key area of interest for the chemist have been developing drugs which provide resistance to diseases, more effective and less toxic or having less side effects [8, 9]. Design of heterocyclic ligands which fit with metal ions geometrically is equally important [10]. Substitution on heterocyclic rings enhance biological potentiality of ligand as well as its metal ion based complex [11]. The substituents having ability to enhance liphophilicity, sta- bilize the oxidation state of metal ion based complex and stabilize the complex making it kinetically inert and ther- modynamically stable in important aspect of designing such compounds concluded by structural and activity re- lationship [12]. DNA has been a primary target of biomolecules to cure diseases genetically by interaction with base pairs via electro- static, Vander Waal and hydrophobic forces of attractions. Complexes with octahedral geometry have been found to be kinetically more inert since releasing of bioactive ligand on specific DNA site is of prime importance. Quinoline based heterocycles have been proved to have good binding affinity than ciprofloxacin drug [13]. Binding affinity of quinoline heterocycles are highly stabilized by hydrogen bonding inter- actions concluded by docking studies [14, 15]. Quinoline class of heterocycles have been proved to possess greater cytotox- icity activities when explored to study on wide cancer cell lines [16, 17]. * Mohan N. Patel jeenen@gmail.com 1 Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India 2 Cell Biology Laboratory, Indian Institute of Advanced Research, Koba, Gandhinagar, Gujarat 382421, India https://doi.org/10.1007/s10895-020-02657-1 / Published online: 3 January 2021 Journal of Fluorescence (2021) 31:349–362