The structure of psi DNA

In concentrated solutions of neutral or anionic polymers (and an adequate cation concentration), DNA condenses into a compact state, which is of interest for its possible relevance to chromosome structure and the packing of DNA in viruses. The X-ray scattering of DNA condensed in this way has been examined with respect to the secondary structure of the helix and the tertiary structure of the compact state. Measurements have also been made with dense aqueous gels of DNA in the absence of poly(ethylene oxide) at ordinary salt concentrations and in 6·0 M-LiCl. All preparations exhibit a well-defined interhelical spacing, implying substantial parallelism, but no lattice spacings higher than first order are observed. Comparison with calculated scattering curves for disoriented helical segments indicates that a structure very close to the B fiber structure prevails in all preparations. No significant contribution from the A or C fiber structures can be detected in either the condensed preparations or the LiCl solutions. Thus there is no basis for attributing the origin of the strongly anomalous circular dichroism spectra to a secondary structure significantly different from that in dilute solution. The decrease in interhelix spacing with increasing polymer concentration is in reasonable accord with expectation on the basis of excluded volume interactions. There is improved short-range order in the polymer-induced compact state as compared with the simple solution having the same interhelix spacing.The results are in reasonably good agreement with expectation for a folded chain structure of the compact state, similar to the usual mode of crystallization of simple linear polymers. There is no evidence for supercoiling.