[Effective diameter of DNA].

Different methods of determination of effective diameter of DNA double helix under different ionic conditions in solution are reviewed. This value is a characteristic of excluded volume effects in DNA and allows for electrostatic repulsion between segments of the molecule. The values of the effective diameter found by completely different methods are in excellent agreement. This allows to conclude that in wide range of ionic and DNA concentrations a model of unpermeable cylinders provides adequate description of DNA conformational properties. The effective double helix diameter grows rapidly with decreasing salt concentration and can be several times higher than the geometric one. Even in solutions of physiological ionic strength corresponding to 200 mM NaCl the value of the effective diameter is 5 nm. Such a sharp dependence of DNA effective diameter on salt concentration should be taken into account when analyzing conformational properties of superhelical DNA.