145 Spectral characteristics of DNA-EtBr and DNA-MB complexes

The mathematical approach has been applied in order to analyze the isosbestic point on the absorption spectra of DNA-ethidium bromide (EtBr) complexes and it was shown that the isosbestic point is registered at λ  = 510 nm wavelength. Using the mentioned model the interval of concentration of DNA/EtBr ratio was determined, at which all spectra pass through the isosbestic point. The interval of concentration was also calculated for that case when all the spectra are deviated from the isosbestic point. Analogically, the same analyses have been carried out for DNA-MB complexes. The obtained results show that a pseudo-isosbestic point is registered on the absorption spectra of DNA-MB complexes (Moodi et al., 2013; Vardevanyan et al., 2015).

The characteristics of the absorption spectra of DNA complexes with EtBr and methylene blue (MB) at 0.02 M ionic strength of the solution were experimentally studied. The wavelength interval for DNA-EtBr complexes was 400 ≤ λ ≤ 600 nm, while for the complexes with MB it was 500 ≤ λ ≤ 750 nm. It was revealed that when EtBr concentration is constant in the solution, by the increase of DNA/EtBr concentration ratio the absorption maximum values decrease and are shifted towards longer wavelengths approximately by 40 nm (Vardevanyan et al., 2005). In the case of MB, at the same conditions, the decrease of maximum absorption is slight and the shift towards longer wavelengths is approximately by 5–7 nm. Moreover, on the absorption spectra of DNA-EtBr complexes the isosbestic points clearly appear at 510 nm, while the presence of it on the spectra obtained for DNA-MB complexes is highly arguable. Obtained results indicate the fact that although EtBr and MB are intercalators, they bind differently to DNA at 0.02 M ionic strength. In the case of EtBr, the complete intercalation into double-stranded DNA takes place, while in the case of MB the semi-intercalation into one of the strands occurs. Besides, in both cases, a relatively weak electrostatic binding to DNA is registered.