A study of conformation of nucleic acids in solution by means of circular dichroism

Abstract

The circular dichroism of various ribo- and deoxyribonucleic acids was measured in the spectral region of 340 to 230 m μ ; all ribonucleic acids (transfer RNA, ribosomal RNA, tobacco mosaic virus RNA and Reovirus RNA) exhibit similar curves composed of a positive band centred at 265 m μ . The intensity of this band gradually decreases in conditions favouring helix-coil transitions, at higher temperatures or at increasing concentrations of ethanol, parallel to the increase in absorbance. This identity of the curves for circular dichroism of various RNA's indicates that within all native RNA's an important fraction of the molecule must have a comparable secondary structure. The circular dichroic spectra of DNA from calf thymus, fowl erythrocytes and Escherichia coli also lead to identical results. The curve is composed of two parts: one positive, with a maximum at 273 m μ ; and one negative, with a minimum at 243 m μ . At high temperature a decrease of intensity of circular dichroism was observed, but at intermediate temperatures (about 40 to 55°C) the intensity of the positive band increases without any observable changes in absorbancy. In 80% alcohol at low salt concentration, the circular dichroic curve of DNA is essentially composed of a positive band, the rotational strength of which is about three times greater than that of the native form. The spectra of DNA in alcohol compared to native DNA in aqueous solutions indicate an increased absorption in the region of 275 m μ . It is suggested that DNA in 80% alcohol exists in an intermediate form which is different from its native and random coil conformations. In contrast to polyriboadenylic acid, the intensity of the positive circular dichroic bands of nucleic acids, DNA and RNA, are relatively low; this probably results from certain geometrical features of their helical structures.