Biophysical and Thermodynamic Investigations on the Differentiation of Fluorescence Response towards Interaction of DNA: A Pyrene-Based Receptor versus Its Fe(III) Complex.

The Fe(III) complex [Fe(L)(NO 3 )(H 2 O)] + ( 1 ) was prepared using a structurally characterized Schiff base ligand, 1-((pyren-1-ylimino) methyl) naphthalen-2-ol ( HL ), to develop an optical probe for fluorimetric recognition of DNA. An electrospray ionization-mass spectrometry (ESI-MS) study was carried out to ascertain the composition of 1 and the geometry of 1 was optimized by density functional theory (DFT) calculations. Compared to the strong intrinsic fluorescence of the ligand ( HL ), 1 was only weakly fluorescent. The interaction of 1 with DNA was investigated through different biophysical techniques. The fluorescence emission of 1 appeared to increase progressively in the presence of calf thymus (CT) DNA and this was utilized for the fluorimetric recognition of DNA. In comparison with 1 , in the presence of DNA, the ligand HL showed quenching in its emission. The selectivity of 1 towards DNA was also confirmed in the presence of a large number of environmentally pertinent anions (NO 3 - , SO 4 2- , Cl - , Br - , I - , OAC - , PO 4 3- , ClO 4 - , HCO 3 - , H 2 PO 4 - , HPO 4 2- , CO 3 2- ). Comprehensive DNA-binding experiments also showed that complex ( 1 ) and HL interacted with CT-DNA with different efficacies; the affinity of 1 was about six times higher than that of HL . Calorimetric studies showed that the 1 -DNA association progressed with large positive entropy changes. In contrast, the association of HL with DNA was an enthalpy-driven process. Molecular docking results confirmed that the binding of 1 with CT-DNA progressed by intercalation and other noncovalent interactions.