Changes in binding affinity between ofloxacin and calf thymus DNA in the presence of histone H1: Spectroscopic and molecular modeling investigations

The interaction between ofloxacin (Oflx) and calf thymus DNA (ct DNA) in the presence and absence of histone H1 was investigated in Tris buffer (pH=6.8) using UV-vis absorption and fluorescence spectroscopy, as well as resonance light scattering (RLS), transition temperature, circular dichroism (CD), viscosity and molecular modeling techniques. The results indicated that H1 induced changes in the binding affinity between Oflx to ct DNA and that the major binding mode was intercalative binding. The RLS results implied changes in particle size and induced structural changes of the ct DNA-Oflx complex in the presence of H1 as a ternary system as opposed to as a binary system. The binding constant, Ksv, and the number of binding sites, n, of ct DNA-Oflx in the presence of H1 were obtained by fluorescence quenching that showed a static quenching behavior of the complex formation in the ternary system. The Ksv values of H1-Oflx, ct DNA-Oflx and (ct DNA-H1) Oflx were (9.86 ±0.6) × 107 M-1, (9.08 ± 0.4) × 107 M-1 and (4.13 ± 0.3) × 107 M-1 respectively. The Ksv values showedkey role of H1 in binding affinity of Oflx to ct DNA that caused the different of interaction behaviorbetween ct DNA and Oflx in the presence of H1. The thermodynamic parameters were calculated from the fluorescence data measured at three different temperatures, and showed that the binding of Oflx to the ct DNA-H1 complex was driven mainly by hydrogen bonding and van der Waals forces. The CD technique demonstrated that, in the presence of H1, the ellipticity values of the ct DNA-Oflx complex changed, which gave rise to a different chirality of ct DNA-Oflx induced by H1. Viscosity measurements revealed that the specific viscosity of ct DNA increased with the Oflx concentration in the absence and presence of H1. Different curves confirmed that Oflx bound to ct DNA with varying affinities in the absence and presence of H1. Molecular modeling confirmed the experimental results regarding the binding of Oflx to ct DNA in the absence and presence of H1.