Electric Dichroism

Among the methods avallable for the analysis of structures of drug—DNA complexes in solution, the electric dichrolsin is particularly simple, does not require much material, and provtdes information, which cannot be obtained as conveniently by other methods The principle of the electric dichroism is straightforward: electric field pulses are used to align DNA molecules in the direction of the electric field vector, the molecular alignment is analyzed by measurements of the absorbance of polarized light (cf. Fig. 1 ). One of the advantages of the method is the fact that some important information may already be derived without using any complex theory. In addition, more detailed conclusions may be derived on the basis of appropriate theones, which have been developed up to a rather high degree of sophistication. Three different types of information are available:

1.	The direction and the magnitude of the absorbance change induced by the field pulses indicates the orientation of the light-absorbing chromophor with respect to the long axis of the DNA,
2.	The time constant(s) of the molecular rotation process indicate(s) the hydrodynamic dimensions of the complex,
3.	The electric parameters of the complexes are usually not a target of investigations on drug—DNA complexes and, thus, are not discussed in this contribution Among the books (1 –4 ) published on the method, the one of Fredericq and Houssler (1 ) is still the most advisable one for an introduction, although the examples are not up to date.

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