Abstract

The interaction of ethidium bromide with intraphage (T 4) DNA and isolated phage (T 4) DNA has been studied. The partial polarizations of fluorescence of the dye-complexes have been measured at thermal equilibrium at various temperatures and by fast cooling to a constant lower temperature. A close comparison of the results at these two conditions and an additional analysis of them from Perrin's theorem prove that a natural source of depolarization is exhibitant in DNA-dye complexes at ordinary temperatures. This depolarization effect may be attributed to some internal motions or rotations in DNA. Alternatively, the effect may be due to conformational changes within the framework of the DNA double helix, which provide a different environment for the dye. The above depolarization effect is most effective in the temperature range 37–64°.

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