The circular dichroism and X-ray diffraction of DNA condensed from ethanolic solutions.

It is known that DNA in aqueous-ethanol solutions undergoes a B to A conformational change between 60% and 80% (w/w) ethanol. We have found that precipitates formed by adding salt to DNA in 60% and 80% ethanolic solutions can be very different. DNA precipitated from 60% ethanol forms a fine condensate that only slowly settles out of suspension and shows a characteristic differential scattering of circularly polarized light at long wavelengths. DNA precipitated from 80% ethanol forms a flocculent aggregate that exhibits the CD spectral features of the A conformation. Data from circular dichroism spectra of natural and synthetic nucleic acids and from X-ray diffraction patterns of the precipitates show that DNA molecules precipitated from 60% and 80% ethanol are, respectively, in the B and A conformation. Therefore, the different secondary conformations of DNA in ethanolic solutions are maintained during precipitation under these conditions. These results are of general importance for the preparation and study of condensed forms of DNA, since a relatively small change in the extent of dehydration can change the secondary conformation of DNA and markedly affect the character of a subsequent precipitate.