[Features of melting of DNA complexes with mitoxantrone at low concentrations]

The melting of DNA complexes with the anticancer mitoxantrone preparation at the mu = 0.11 and mu = 0.011 NaCl ionic strengths was investigated by the methods of microcalorimetry and spectrophotometry. It was shown, that at the 0.011 M NaCl the dependence of the melting temperature (Tm) upon the mitoxantrone concentration passed at minimum. The decrease of Tm was observed in the region, where one mitoxantrone molecule fell approximately on 100 base pairs DNA. There was observed a deeper minimum for DNA sarcoma 45. In the region of the mitoxantrone concentration the enthalpy of melting of the complexes increases linearly with the increase of the mitoxantrone concentration. The mentioned regularities were not observed at the mu = 0.11 M NaCl. The observed phenomenon is explained qualitatively by the entropy increase of the coiled state of DNA ligand complex on account of additional freedom of ligand's revolving.