SALT EFFECTS ON THE STRUCTURE AND INTERNAL DYNAMICS OF SUPERHELICAL DNAS STUDIED BY LIGHT-SCATTERING AND BROWNIAN DYNAMICS

Using laser light scattering, we have measured the static and dynamic structure factor of two different superhelical DNAs, p1868 (1868 bp) a nd simian virus 40 (SV40) (5243 bp), in dilute aqueous solution at sal t concentrations between 1 mM and 3 M NaCl. For both DNA molecules, Br ownian dynamics (ED) simulations were also performed, using a previous ly described model. A Fourier mode decomposition procedure was used to compute theoretical light scattering autocorrelation functions (ACFs) from the ED trajectories. Both measured and computed autocorrelation functions were then subjected to the same multiexponential decompositi on procedure, Simulated and measured relaxation times as a function of scattering angle were in very good agreement. Similarly, computed and measured static structure factors and radii of gyration agreed within experimental error. One main result of this study is that the amplitu des of the fast-relaxing component in the ACF show a peak at 1 M salt concentration. This nonmonotonic behavior might be caused by an initia l increase in the amplitudes of internal motions due to diminishing lo ng-range electrostatic repulsions, followed by a decrease at higher sa lt concentration due to a compaction of the structure.