Conformation changes in the DNA molecule caused by γ-irradiation of its water ethanol solutions of great ionic strength

Application of spectral methods for studying DNA damage induced by gamma-radiation

Spectral methods can provide a variety of possibilities to determine several types of radiation-induced DNA damage, such as nucleobase destruction and local denaturation. DNA UV absorption and CD spectra measured at room temperature undergo noticeable alteration under the action of γ-radiation. We have applied the Spirin method of total nucleobases determination, and have measured the molar extinction coefficient of DNA and DNA CD spectra for solutions with different NaCl concentrations (3mM-3.2M) and containing MgCl₂, exposed to γ-radiation with the doses of 0-10³Gy. The melting temperatures of DNA in irradiated solutions at the doses of 0-50Gy were obtained with the help of spectrophotometric melting. It was found that the amount of destructed nucleobases and radiation-induced loss of DNA helicity significantly decreases with the rise of the ionic strength of the irradiated solution. Substitution of a portion of Na⁺ ions on Mg²⁺ while keeping the total ionic strength constant (μ=5mM) does not affect the considered radiation effects. The role of the structure and composition of the DNA secondary hydration layer in the radiation-induced damages is discussed.

Unequal effect of ethanol-water on the stability of ct-DNA, poly[(dA-dT)]₂ and poly(rA)·poly(rU). Thermophysical properties

Ethanol affects unequally the thermal stability of DNA and RNA. It stabilizes RNA, while destabilizing DNA. The variation of the relative viscosity (η/η0) of [poly(dA-dT)]2 with temperature unveils transitions close to the respective denaturation temperature, calculated spectrophotometrically and calorimetrically. From the raw data densities and speeds of sound, the volumetric observables were calculated. In all cases studied, a change in sign from low to high ethanol content occurred for both partial molar volume (ϕV) and partial molar adiabatic compressibility (ϕK(S)). The minima, close to 10%, should correspond to the highest solvation and the maxima, close to 30%, to the lowest solvation. For 40-50% ethanol, the solvation increases again. The complex structure of ethanol-water, for which changes are observed in regions close to such critical concentrations, justifies the observed behaviour. The variation of ϕV and ϕK(S) was sharper for RNA compared with respect to DNA, indicating that the solvation sequence is poly(rA)·poly(rU) < ct-DNA < [poly(dA-dT)]2.