Can reliable torsion elastic constants be determined from FPA data on 24 and 27 base-pair DNAs?

Structural ensemble and microscopic elasticity of freely diffusing DNA by direct measurement of fluctuations

Precisely measuring the ensemble of conformers that a macromolecule populates in solution is highly challenging. Thus, it has been difficult to confirm or falsify the predictions of nanometer-scale dynamical modeling. Here, we apply an X-ray interferometry technique to probe the solution structure and fluctuations of B-form DNA on a length scale comparable to a protein-binding site. We determine an extensive set of intrahelix distance distributions between pairs of probes placed at distinct points on the surface of the DNA duplex. The distributions of measured distances reveal the nature and extent of the thermally driven mechanical deformations of the helix. We describe these deformations in terms of elastic constants, as is common for DNA and other polymers. The average solution structure and microscopic elasticity measured by X-ray interferometry are in striking agreement with values derived from DNA-protein crystal structures and measured by force spectroscopy, with one exception. The observed microscopic torsional rigidity of DNA is much lower than is measured by single-molecule twisting experiments, suggesting that torsional rigidity increases when DNA is stretched. Looking forward, molecular-level interferometry can provide a general tool for characterizing solution-phase structural ensembles.

Effects of ethylene glycol on the torsion elastic constant and hydrodynamic radius of p30δ DNA

Upon increasing the concentration of ethylene glycol (EG) at 37 degrees C, the twist energy parameter, E(T), which governs the supercoiling free energy, was recently found to undergo a decreasing (or reverse) sigmoidal transition with a midpoint near 20 w/v % EG. In this study, the effects of adding 20 w/v % EG on the torsion elastic constant (alpha) of linear p30delta DNA and on the hydrodynamic radius (R(H)) of a synthetic 24 bp duplex DNA were examined at both 40 and 20 degrees C. The time-resolved fluorescence intensity and fluorescence polarization anisotropy (FPA) of intercalated ethidium were measured in order to assess the effects of 20 w/v % EG on: (1) alpha; (2) R(H); (3) the lifetimes of intercalated and non-intercalated dye; (4) the amplitude of dye wobble in its binding site; and (5) the binding constant for intercalation. The effects of 20 w/v % EG on the circular dichroism (CD) spectrum of the DNA and on the emission spectrum of the free dye were also measured. At 40 degrees C, addition of 20 w/v % EG caused a substantial (1.27- to 1.35-fold) increase in alpha, a significant change in the CD spectrum, and a very small, marginally significant increase in R(H), but little or no change in the amplitude of dye wobble in its binding site or the lifetime of intercalated dye. Together with previously reported measurements of E(T), these results imply that the bending elastic constant of DNA is significantly decreased by 20 w/v % EG at 40 degrees C. At 20 degrees C, addition of 20 w/v % EG caused a marginally significant decrease in alpha and very little change in any other measured properties. Also at 20 degrees C, addition of 30 w/v % betaine caused a marginally significant increase in alpha and significant but modest change in the CD spectrum, but very little change in any other properties.