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)­(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–, SO42–, Cl–, Br–, I–, OAC–, PO43–, ClO4–, HCO3–, H2PO4–, HPO42–, CO32–). 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.