Effect of temperature on DNA secondary structure in the absence and presence of 0.5 M tetramethylammonium chloride.

Changes in the average secondary structures of three different linear DNAs over the premelting region from 5 to 60 degrees C were investigated by measuring their CD spectra and also their torsion elastic constants (<alpha>) by time-resolved fluorescence polarization anisotropy. For one of these DNAs, the Haell fragment of pBR322, the apparent diffusion coefficients [Dapp(k)] at small and large scattering vectors (k) were also measured by dynamic light scattering. With increasing temperature, all three DNAs exhibited typical premelting changes in their CD spectra, and these were accompanied by 1.4- to 1.7-fold decreases in <alpha>. Also for the 1876 base pair fragment, Dapp(k) at large scattering vectors, which is sensitive to the dynamic bending rigidity, decreased by 17%, even though there was no change at small scattering vectors, where Dapp(k) = D0 is the translational diffusion coefficient of the center-of-mass. These observations demonstrate conclusively that the premelting CD changes of these DNAs are associated with a significant change in average secondary structure and mechanical properties, though not in persistence length. In the presence of 0.5 M tetramethylammonium chloride (TMA-Cl) the premelting change in CD is largely suppressed, and the corresponding changes in <alpha> and Dapp(k) at large scattering vectors are substantially diminished. These observations suggest that TMA-Cl, which binds preferentially to A.T-rich regions and stabilizes those regions (relative to G.C-rich regions) against melting, effectively stabilizes the prevailing low-temperature secondary structure sufficiently that the DNA is effectively trapped in that state over the temperature range observed.