Structure and charge distribution in DNA and poly(styrenesulfonate) aqueous solutions

DNA and synthetic poly(styrenesulfonate) (PSS) solutions without excess simple salt were investigated with small-angle neutron scattering. For both polyelectrolytes, the transition from the rod to the coil regime was covered by an appropriate choice of molecular weights. The polymer, polymer-counterion, and counterion partial structure functions were obtained using contrast variation. For PSS, the single-chain scattering (form function) was observed from samples with zero-average polyion scattering length density contrast. The PSS polymer structure can be described by a locally rodlike configuration, but the projected monomer repeat distance 0.17 nm is smaller than the value expected for a fully stretched (trans) conformation. The PSS persistence length is of order 10 nm and does not agree with any theoretical analysis based on either the bending rigidity of a wormlike chain or modern variational results. The interpolymer structure was derived and compared with results based on the random-phase approximation. Poor agreement was observed, due to the high linear polyion charge density and, hence, strong electrostatic coupling. For highly charged linear polyelectrolytes, it was shown that from the full set of partial structure functions information on the radial counterion profile can be obtained without resorting to a model describing chain correlations. For PSS and DNA, the data agree with the counterion distribution obtained from the classical Poisson-Boltzmann theory and the cylindrical cell model, if the momentum transfer is far greater than the inverse persistence length.