Quasielastic light scattering: effect of ionic strength on the internal dynamics of DNA.

Abstract

AbstractQuasielastic light scattering is used to study the effect of ionic strength on the dynamic behaviour of DNA. In a first approach the spectrum of scattered light is analyzed in terms of a single relaxation process. The large difference between the observed behaviour and that expected according to a pure diffusional process reflects the contribution associated with internal modes, which increases with decreasing ionic strength. Such behaviour is better analyzed in terms of a double relaxation process by using two relaxation times, the reciprocals of which are equal to DK2 and DK2 + τi−1 (K), respectively, where τi (K) is an average value describing the set of modes observed at a given K value. Relative intensity and relaxation times, which are the more accurate parameters, were used to interpret the results. The observed increase of the relative contribution of internal modes with decreasing ionic strength is actually a relative decrease of the diffusional contribution induced by a corresponding increase of the radius of gyration RG. On the other hand, the reciprocal τi−1 (K) of the relaxation time is a linear function of K2 in the analyzed KRG range and is insensitive to ionic strength between 10−2M and 1M. These results, when discussed according to Rouse's model, lead to define for each value of τi−1 (K) a corresponding mean‐squared equilibrium length 〈μ〉 which is found to be a linear function of K−2.