STRUCTURE AND CHARGE-DISTRIBUTION IN DNA AND POLY(STYRENESULFONATE) AQUEOUS-SOLUTIONS

DNA and synthetic poly(styrenesulfonate) (PSS) solutions without exces s simple salt were investigated with small-angle neutron scattering. F or both polyelectrolytes, the transition from the rod to the coil regi me was covered by an appropriate choice of molecular weights. The poly mer, polymer-counterion, and counterion partial structure functions we re obtained using contrast variation. For PSS, the single-chain scatte ring (form function) was observed from samples with zero-average polyi on scattering length density contrast. The PSS polymer structure can b e described by a locally rodlike configuration, but the projected mono mer repeat distance 0.17 nm is smaller than the value expected for a f ully 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 resu lts based on the random-phase approximation. Poor agreement was observ ed, due to the high linear polyion charge density and, hence, strong e lectrostatic coupling. For highly charged linear polyelectrolytes, it was shown that from the full set of partial structure functions inform ation on the radial counterion profile can be obtained without resorti ng to a model describing chain correlations. For PSS and DNA, the data agree with the counterion distribution obtained from the classical Po isson-Boltzmann theory and the cylindrical cell model, if the momentum transfer is far greater than the inverse persistence length.