[Irreversible secondary structure changes in concentrated aqueous solutions of DNA]

"Aqueous" and "electrolytic" solutions of DNA from chicken erythrocytes different concentrations of NH4Cl have been investigated. Solutions of the first type were prepared from concentrated aqueous solutions of DNA and the second type solutions by solving a solid DNA in a supporting electrolyte. Studies of the melting curves showed that even the concentrated aqueous solutions of DNA were not completely native. The changes of the secondary structure of DNA caused by solving in deionized water are irreversible. This is confirmed by the abnormal dependence of melting temperatures of DNA on the ionic strength of aqueous solutions that is not characteristic for electrolytic solutions, that is not characteristic for electrolytic solutions. It has been shown that the type of dependence of the melting temperatures on the ionic strength is a more precise test of the secondary structure of DNA than a molar extinction or hyperchromic koefficient.