Spectroscopic studies of the interaction between pirimicarb and calf thymus DNA

The interaction between pirimicarb and calf thymus DNA in physiological buffer (pH 7.4) was investigated with the use of Neutral Red (NR) dye as a spectral probe by UV–vis absorption, fluorescence and circular dichroism (CD) spectroscopy, as well as viscosity measurements and DNA melting techniques. The results revealed that an intercalation binding should be the interaction mode of pirimicarb to DNA. CD spectra indicated that pirimicarb induced conformational changes of DNA. The binding constants of pirimicarb with DNA were obtained by the fluorescence quenching method. The thermodynamic parameters, enthalpy change (ΔHθ) and entropy change (ΔSθ) were calculated to be −52.13 ± 2.04 kJ mol−1 and −108.8 ± 6.72 J mol−1 K−1 according to the van’t Hoff equation, which suggested that hydrogen bonds and van der Waals forces might play a major role in the binding of pirimicarb to DNA. Further, the alternative least squares (ALS) method was applied to resolve a complex two-way array of the absorption spectra data, which provided simultaneously the concentration information for the three reaction components, pirimicarb, NR and DNA–NR. This ALS analysis indicated that the intercalation of pirimicarb into the DNA by substituting for NR in the DNA–NR complex.