Understanding DNA conformational dynamics: answering questions and questioning answers.

Phosphorus-31 and especially Carbon-13 NMR measurements have recently become primary input to the understanding of DNA solution dynamics. While the 31P measurements are inherently easier, the quality of 31P dynamics information is suspect and therefore 13C measurements are preferred. In fact, it is necessary to obtain several kinds of 13C data (T1s, NOE's, linewidths, integrated peak intensities) over a wide range of magnetic fields (13C NMR frequencies) in order to identify major features of DNA internal motions. Further information comes from variation of temperature and DNA fragment length and/or concentration. Most of our 13C measurements have been performed at 37.7-90.6 MHz on fully double stranded monomer size (147 base pair) DNA at concentrations in phosphate buffer of approximately less than 10 to approximately greater than 200 mg ml-1; temperatures studied range from 6 to 55 degrees C. Other measurements have been performed on monomer-size single-stranded DNA at 85 and 92 degrees. The large data set we have acquired appears to answer some important questions about the nature and extent of DNA overall and internal motional dynamics. However, the picture remains incomplete and a number of questions arise from these results: 1. Overall motion of the double stranded DNA fragments follows expected hydrodynamic behavior; 2. Restricted but rapid internal motion along the DNA structure is well represented by a spaghetti-like wobbling-in-a-cone model; 3. DNA-DNA Interactions and solvent ordering, present at relatively low DNA concentrations, partially quench the internal motion, consistent with hinge-model structural changes (and the spaghetti model above) but not as compatible with in-plane torsional motion models; 4. The deoxyribose C-2' sites undergo additional motion which is partially uncoupled from the internal wobbling motions: 5. At high DNA concentrations, a phase transition occurs, resulting in ordered structures which drastically affect DNA internal dynamics; 6. DNA interacting with ethidium does not greatly change its conformational mobility; 7. DNA interacting with Hg2+ ions shows less than anticipated change in internal DNA dynamics. The remaining challenge is to interpret our current results in terms of specific conformational processes and to understand why the conformational mobility of double stranded DNA is relatively unhindered by major structural perturbants such as intercalating ethidium and mercury ion.