The synthesis, characterization, crystal structure and HSA-binding of a mixed ligand Cuisothiosemicarbazonato complex with adenine nucleobase as an auxiliary ligand are reported. Xray crystallography indicates that the cationic complex has a binuclear centrosymmetric structure in which the isothiosemicarbazone molecule coordinates to copper as a tridentate NNO ligand. Adenine participates in the structure in the neutral 7-H tautomer form in which N9 as its most basic nitrogen atom completes the square planar geometry. The planar complex entities are anti-parallel stacked and joined by means of long Cu–O bonds involving oxygen atoms from the other isothiosemicarbazone molecule, giving a [4+1] square pyramidal coordination around the copper ion. Adenine molecule plays a pivotal role in establishing intermolecular hydrogen bonding in the supramolecular structure. HSA interactions of the complex are investigated thorough spectroscopic and theoretical methods. Emission studies indicate that the complex quenches fluorescence emission of HSA by a static mechanism. They also show that hydrophobic forces are primary interactions between complex and protein. Circular dichroism studies illustrate that the complex induces unfolding of HSA through converting part of the α-helical content to β-structures and random coils. Finally, docking studies indicate that the complex places in the upper part of interdomain region of HSA.

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