EFFECT OF ELECTROSTATIC INTERACTIONS ON THE STRUCTURE AND DYNAMICS OFA MODEL POLYELECTROLYTE - I - DIFFUSION

The dynamics of a 20 base pair oligonucleotide is studied by dynamic l ight scattering-photon correlation spectroscopy and depolarized Fabry- Perot interferometry. The 20 base pair oligonucleotide is a well-defin ed, albeit short, rigid rod molecule that serves as a model for polyel ectrolyte solution dynamics. The effects of added salt on the solution rotational and translational dynamics are examined in detail as funct ions of the 20-mer concentration. Coupled mode theory together with co unterion condensation theory gives good predictions for the effects of salt on the translational diffusion of the 20-mer at the relatively l ow oligonucleotide concentrations studied. Comparison of the experimen tal results with these theories shows that the effective charge densit y of the polyion in solution is approximately equal to the reciprocal of the product of the Bjerrum length and the counterion charge, nu(eff )congruent to 1/N lambda(B) Calculation shows that the numerical solut ion of the coupled mode theory matrix gives a better fit of our measur ed polyion diffusion coefficients than the approximate equation derive d by Lin, Lee, and Schurr. Simple approximations for the effective rod length, L-eff=L+kappa(-1) and effective rod diameter, d(eff)=d+kappa( -1), are used to model the thermodynamic-hydrodynamic interactions for charged rodlike molecules and to make predictions for the diffusion s econd virial coefficient as a function of added salt concentration. Th is alternative to the coupled mode theory also gives good agreement wi th experiment. The rotational diffusion constants of the oligonucleoti de measured by depolarized Fabry-Perot interferometry show a slowing d own of the rotation at low added salt concentrations as the oligonucle otide concentration is increased. (C) 1998 American Institute of Physi cs. [S0021-9606(98)50241-3].