synopsisConformation of two-stranded DNA in HzO-methanol, H20-ethanol, HzO-isopropanol, and HzO-dioxane solutions a t different concentrations of alkaline ions has been studied with the aid of circular dichroism. The following conclusions are drawn:The conformation of DNA in HzO and HZO-methanol belongs to a family of B forms (B, C, T forms are the representatives of the family). The magnitude of the winding angle between adjacent base pairs (0) is determined by the concentration and type of the cations. In HZ0 the cation action is nonspecific and leads to an increase in 8 value. In 80% methanol the ions act specifically, Cs+ being to stabilize a form with a greater e value, and Lif being with a lesser one. The total e change is likely within the limits At high content of ethanol, isopropanol, or dioxane (-SO%), but not with methanol, and in low ionic strength the conformation of DNA belongs to a family of A forms (A form is one of the members of the family) and is specified by the concentration and type of cation involved. The two-stranded regions of RNA in HzO are also of A type and winds with the rise of cation concentration. The range of e variation is not narrower than30" 6 e 6 33".The conformational transitions within the families (induced by ions) are of noncooperative pattern, wheras the transitions between the families (induced by nonpolar component) are of cooperative pattern. The effect of cations, when specific, is discussed on the basis of steric correspondence between the width of DNA narrow groove and the size of a hydrated cation.of330 < e 6 45".

No abstract

The circular dichroism (CD) spectra of flow-oriented complexes of DNA with proflavine (PF), ethidium (ET) and distamycine (DS) have been studied in the ultraviolet region. The CD spectra with light propagating in parallel to the flow direction were measured by the method of Chung and Holzwarth [1]. Delta epsilon and delta epsilon values have been obtained by this method. It was shown that all the complexes studied exhibit a strong CD anisotropy so that "isotropic" CD spectra measured with a conventional procedure can be attributed to the mutual compensation of the two components of opposite signs.

No abstract

Alterations in CD spectra are found in G-containing oligoribonucleotides after modification with kethoxal (beta-ethoxy--alpha-ketobutyraldehyde). Stacking interactions in kethoxalated oligomers are followed by temperature dependence of their CD amplitudes. It is shown that for oligomers with nucleosides in anti-conformation adduct formation destroys the stacking interaction with 3'-neighbour but not with a 5'-neighbour. For nucleosides in non-standard conformation (i.e. syn-conformation of guanine in GpGpCp) the physical alteractions may be seen in those cases, when the substituting group affects the initial conformation or the interplane base contacts via, for instance, blocking NH(2)-group of guanine in GpUp. The results demonstrated that even a single monomer modification in a polymer chain could not be considered as a local event having no influence on the three-dimensional structure. The degree of conformational disorders depends both on the conformation of single nucleotides in the stack and on the nature of the nearest neighbours of the modified base.