Biophysical and Thermodynamic Investigations on the Differentiation of Fluorescence Response towards Interaction of...

The Fe(III) complex [Fe(L)(NO₃)(H₂O)]⁺ (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 (NO₃^-, SO₄^2-, Cl^-, Br^-, I^-, OAC^-, PO₄^3-, ClO₄^-, HCO₃^-, H₂PO₄^-, HPO₄^2-, CO₃^2-). 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.