[DNA compact form. VI. Changes of DNA secondary structure under conditions preceding its compaction in a solution].

Optical and thermochemical properties of E. coli DNA molecules are compared in solutions containing poly(ethyleneglycol) (PEG) in concentrations at which compactization is not yet observed. It is shown that under conditions preceding DNA compactization (CPEG less than 60 mg/ml) changes in CD spectra occur which suggest that the secondary structure of some DNA fragments is altered. These changes of the secondary structure result from dehydration of DNA molecules in PEG-containing solutions. Electron micrographs of DNA molecules obtained under conditions preceding compactization suggest that under these conditions linear DNA molecules may form "four-stranded" fragments as well as double-stranded "loops".