1H nuclear magnetic resonance investigation of flexibility in DNA.

In this paper we report successful observations of proton NMR spectra of native double helical salmon sperm and calf thymus DNA of various lengths. Measurements of the linewidths arising from proton--proton dipolar interactions are used to obtain information about the dynamic behavior of DNA helices in solution. Depending upon which protons are used to monitor the local internal motions of the DNA, different results are obtained. The lowfield resonances from hydrogen-bonded imino protons in the base pairs indicate that the correlation time for reorientation of base pairs is less than 3 x 10(-7) sec, whereas correlation times for motion of neighboring sugar protons relative to the aromatic protons are less than 5 x 10(-8) sec for DNA that is over 200 base pairs long. These observations indicate that there is considerably more internal flexibility in the DNA molecues, especially in the backbone, than is indicated by classic hydrodynamic studies of DNA.