Flexibility and curvature of duplex DNA containing mismatched sites as a function of temperature.

The flexibility and curvature of duplex DNAs containing mismatched sites have been monitored as a function of temperature. The diffusion coefficients are dependent on the flexibility and the curvature of the DNA, and these have been determined by NMR-based methods. The diffusion coefficients, D, depend on a Boltzmann term and the viscosity of the solvent, eta, which is also temperature dependent. To analyze the temperature dependence of the diffusion results, the shape function, S(f) = etaD/T, is used. The shape functions do not have the viscosity and temperature dependence of the diffusion coefficients. The presence of mismatched sites significantly enhances the shape function of duplex DNA at all temperatures examined. The observed increases in the shape functions are attributed to the mismatched sites acting as localized flexible joints. The results on the temperature dependence of the shape functions, the optical absorbance, and the proton chemical shifts indicate that local melting at, and adjacent to, mismatched site occurs at a lower temperature than the overall melting of the duplexes. The localized melting gives rise to a considerable increase in the shape function. The contribution of the curvature of the mismatched sites to the enhanced diffusion has been examined. A DNA with mismatches that are in phase with respect to the helical repeat and a DNA which has the mismatches out of phase with respect to the helical repeat have been examined. The results indicate that mismatched sites have modest curvature.