Circular dichroism of double-stranded RNA in the presence of salt and ethanol

We have investigated CD spectra of double‐stranded RNA (dsRNA) in a variety of ethanolic and salt solutions. Solution conditions were varied by the addition of salts and ethanol, alone, or in combinations. (1) On addition of either salt or ethanol alone to solutions of dsRNA, we found that the positive CD band at 264 nm increased by up to 7% (in salt) or 12% (in ethanol) and underwent a long‐wavelength shift of 3 nm. Concomitantly, the negative CD band at 296 nm decreased. These CD changes may be indicative of an A‐ to A′‐RNA secondary‐conformation change on dehydration. (2) On addition of both salt and ethanol, in various combinations that caused precipitation of dsRNA, three types of CD spectral changes were found, which we take to indicate at least two different types of intermolecular arrays of RNA molecules. Type I precipitate was flocculent and its CD spectrum had a differential scattering component at long wavelengths. Type II condensate was finely dispersed and was characterized by a doubling of the CD magnitude at 264 nm and simultaneous disappearance of the 210 nm band, to give a CD spectrum close to that previously reported for RNA condensed in the presence of polyethylene glycol and salt [Ψ+ RNA; Evdokimov, Y.M., Pyatigorskaya, T.L., Kadikov, V.A., Polyvtsev, O.F., Doskocil, J., Koudelka, J. & Varshavsky, Y.M., (1976) Nucleic Acids Res. 3, 1533–1547]. Type III precipitate was flocculent and had CD spectra that included features that were present for Types I and II condensates; thus, the RNA in this precipitate may not have been in a unique intermolecular array.