SECONDARY STRUCTURE OF CONDENSED DNA - WIDE ANGLE, SMALL-ANGLE X-RAY-SCATTERING, AND CIRCULAR-DICHROISM STUDIES

Abstract

Ethanol precipitated DNA shows a circular dichroism (CD) spectrum of the +.psi.-type which is similar to that of DNA in the A-form, DNA condensed with cetyl-trimethylammonium-bromide (CTAB) shows, depending on the condensation velocity, a CD spectrum of the -.psi.-type, or a CD spectrum only slightly modified from that of DNA in solution. The 1st spectrum is similar to that of DNA in the C-form, and the 2nd to that of DNA in the B-form. Comparison of the large-angle X-ray scattering of the 3 DNA condensates with the scattering curves calculated from the atom coordinates for the A-, B-, and C-form of DNA shows that the secondary structure of the DNA belongs in all 3 cases to the B-family. The secondary structure of DNA alone therefore does not determine the type of CD spectrum. The CD spectrum of condensed DNA is essentially determined by the supramolecular structures of the partially crystalline DNA condensates. These supramolecular structures can be demonstrated by the small-angle X-ray diagrams. The condensation of DNA by ethanol and CTAB proceeds in the form of a partial crystallization of the DNA.