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

Quasielastic 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 DK 2 and DK 2 + τ i −1 ( K ), respectively, where τ i ( 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 R G . On the other hand, the reciprocal τ i −1 ( K ) of the relaxation time is a linear function of K 2 in the analyzed KR G range and is insensitive to ionic strength between 10 −2 M and 1 M . These results, when discussed according to Rouse's model, lead to define for each value of τ i −1 ( K ) a corresponding mean‐squared equilibrium length 〈μ 〉 which is found to be a linear function of K −2 .