Abstract
The sonicated and fractionated poly(rG)·poly(rC) (I) and poly(dG)·poly(dC) (II) samples mostly with Na+ counterions were studied, on a comparative basis, to determine the spatial arrangement of the constituent G and C bases with respect to the double-stranded helical axis in ionic aqueous solutions. The reduced electric linear dichroism, Δe/e, was measured at 7 °C in 1.5−24 kV/cm field strength and 300−190 nm wavelength regions. By analyzing the field strength dependence of observed Δe/e values with theoretical orientation functions, the saturated reduced dichroism, (Δe/e)s, at 260 nm was evaluated to be −0.96 ± 0.02 for I and −0.93 ± 0.01 for II. The electric linear dichroism (ELD) spectra of these duplexes are very close to each other, being undulatory, but not constant, in the whole wavelength region. The (Δe/e)s values are never as high as −1.5, indicating that the optical transition moments of the nucleic acid bases responsible for the UV absorption bands are not normal to the helix axis. By analyzin...
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