Supercoiled DNA is interwound in liquid crystalline solutions.

Abstract

Two structures have been proposed for supercoiled DNA: it is idealized either as a toroidal ring or as a rod of two interwound duplex chains. The latter model is the most widely depicted but the evidence remains controversial. We have worked with monomers and dimers of two plasmids, pUC8 and pKS414, of similar size and natural superhelical density. pKS414 contains a bend promoting sequence whereas pUC8 does not. In concentrated solutions these plasmids form a partially ordered liquid crystalline phase which is found, using neutron diffraction, to consist of a hexagonally packed assembly of parallel rod-like particles. This shape strongly suggests an interwound conformation for which some structural parameters are deduced. The mass/unit length obtained by combining the area of the hexagonal lattice and the concentration is -3.6 times that of linear DNA. This implies a shallow superhelical pitch angle -36°which, when combined with the known number of supercoil turns, yields the pitch -360 A and radius -80 A for the supercoil. Oriented X-ray fibre diffraction patterns at 92% relative humidity indicate a B type duplex structure. Nicked circular plasmids also form liquid crystals but their behaviour, as a function of concentration, differs from that of the superhelical plasmids.

DNA in a liquid-crystalline environment: Tight bends, rings, supercoils

“…85 Moreover, there is some evidence that the superhelical diameter is more or less constant in certain regimes. 17,82 As seen in Sec. III D, a formal analysis of the entropy is complicated, but it will be dealt with approximately, after an introduction of the case without undulations.…”

“…The result is that the nematic order increases fairly weakly with the DNA volume fraction, and never becomes very high ͑␣Շ15͒. Torbet and Di Capua 82 have found that nicked circular pUC8 plasmids form a liquid-crystalline state at DNA volume fractions vӶ1. The rings definitely become more elongated, the higher the DNA concentration.…”

Crystallization of the Long Biological Macro-Molecules

“…The theory predicts crucial role of super-helicity in DNA condensation, which has been found experimentally [19,24,28]. It was found also that the condensation conditions of closed (with σ ≃ 3 · 10 −2 ) and of open DNA are different [19].…”

Polymer induced condensation of DNA supercoils

“…13 In contrast to the large number of studies on liquidcrystallinity of linear DNA, 14 only a few experimental papers have addressed the arrangement of supercoiled DNA in liquid crystalline condensed states. [15][16][17] These studies do suggest that the structure of individual plectonemic supercoils changes upon condensation. However, with the scattering methods used in these papers, it is difficult to separate scattering due to intersupercoil and intrasupercoil interferences.…”