Physical mechanisms of interaction of actinocin derivatives with DNA. 6. Spectrophotometric analysis of DNA-actinocin complexes for actinocin derivatives with different length of methylene chaines

Abstract

Complexation of actinomycin derivatives with calf thymus DNA has been studied as a function of the number of methyl groups (n= 2 - 5) in the side chains by UV-visible spectroscopy. It has been shown that two types of complexes are being formed in DNA-antibiotic solution and the number of the binding sites in the first type of complex formation increases with an increase of both n and ionic strength of the solution. Binding isotherms in solutions containing 2^.10^-2 M NaCl and 0.16 M NaCI have been determined for all the ligands studied in the mixture with DNA. The number of the binding sites and the equilibrium constants of the formation of the first type of complexes have been obtained from the analysis of the binding isotherms. Using the program of optimization of spectrophotometric concentration dependences DALSMOD, binding constants of formation of the first and second types of complexes have been calculated and the number of binding sites for the second type of complexes has been determined, which comprise the solutions of calf thymus DNA and the considered ligands 5-6 or 7-8 base pairs for two above-cited ionic strengths, respectively. Dimerization of the investigated antibiotics was taken into account in the analysis of the binding isotherms and in the calculation of the complexation parameters. It is found that the melting temperature (T_m) of DNA in the mixture with actinocin derivatives increases with decreasing of the length of the side chains in the phenoxazone chromophore.