In this work, multispectroscopic atomic force microscopy and molecular modeling [ONIOM 2(B3LYP/6- 31 + + G(d,p): Universal Force Field (UFF)) level] techniques were used to study the interaction between Calf- Thymus-DNA (CT-DNA) and pyriproxyfen (PYR) insecticide. The binding constant of PYR with double-strand deoxyribonucleic acid (ds-DNA) was obtained by ultraviolet–visible absorbance spectroscopy as 2.8 · 10 4 at 20C. Thermodynamic parameters, that is, DH, DS, and DG, were - 53.82 kJ mol - 1 , 96.11 J mol - 1 , and - 82.46 KJ mol - 1 , respectively. Thermal denaturation study of DNA with PYR revealed the DT m of 3.0 and 6.0C at r i = 0.5 and 1.0, respectively. The Fourier transform infrared study showed a major interaction of PYR with G-C and A-T base pairs and a minor perturbation of the backbone PO 2 group. Further, PYR induces detectable changes in the circular dichroism spectrum of CT-DNA. In fluorimetric studies, the dynamic enhancement constants (k D ) and bimolecular enhancement constant (k B ) were calculated, which showed that the fluorescence enhancement was initiated by a static process in the ground state. The hybrid of quantum mechanical/molecular mechanics theoretical calculations revealed that the interaction is base sequence dependent, and PYR interacts more with DNA via the AT base sequence. From the data we concluded that PYR may interact with ds-DNA via two modes: intercalating and outside groove binding.
