The interaction of DNA restriction fragments with strong electric fields will be investigated using the technique of transient electric birefringence. Reversing field experiments will demonstrate whether the DNA molecules undergo a progressive change in conformation with increasing field strength. Studies of the Kerr constant as a function of counterion concentration and type will provide a test of the Manning theory of counterion condensation, as well as the Mandel and O'Konski theories of electric polarizability. Studies of the Kerr constant of very small fragments will show whether the recently observed L2 dependence of the electric polarizability (and hence the induced dipole) on length extends to fragments smaller than 100 base pairs. Birefringence saturation curves will be measured as a function of counterion concentration and type. The results will be correlated with the birefringence decay curves in order to interpret the apparent linear dependence of the birefringence on electric field strength in intermediate field strength regions. Intercalation of dye molecules into the DNA helix will be investigated with monodisperse fragments, in order to show whether or not the DNA helix is actually elongated by intercalation. Finally, the effect of electric fields on the thermal denaturation of DNA will be studied. The results of all these experiments will clarify the behavior of DNA molecules in strong electric fields, and thus can identify the electrical forces acting on the DNA molecule in the cell; the electrical potential across a typical cell membrane is of the same order of magnitude as the fields used in electric birefringence experiments. The long range goals of this research are to understand the physical properties of DNA and to determine the electrical forces acting on polyelectrolytes.
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