[Conformational transition within the A-form of complementary nucleic acids in solution]

Circular dichroic spectra of A-DNA in 78% ethanol and of tRNA in water and ethanol solutions have been studied at different concentrations of NaCl. An increase in the Na+ concentration from 0.5.10(-4) M to 5.10(-4) M results in a shift of the positive CD band at 264 nm of the A-DNA to a longer wavelength, 272 nm. Simultaneously, the magnitude of the 210 nm band decreases. By contrast in the case of tRNA in water solution an increase in NaCl content results in straight opposite shifts of the CD spectra. This opposite behaviour is shown to the due to a difference in ions effects in water and water-ethanol solutions, since tRNA in the ethanol solution behaves in the same way as A-DNA does in 78% ethanol. We suppose that in aqueous solution in increase in the cation concentration would stabilize the helical conformations with progressively decreasing narrow groove, i. e. more wound. At a high concentration of ethanol (60--80%) the formation of specific complex between the hydrated cations and the double-stranded regions should be taken into consideration. Thus, the hydrated cations may insert into the deep groove exerting the opposite effect of unwinding.