SMALL-ANGLE X-RAY-SCATTERING OF SEMIDILUTE RODLIKE DNA SOLUTIONS - POLYELECTROLYTE BEHAVIOR

Semidilute solutions of persistence length, mononucleosomal DNA (160 +/- 5 base pairs, M(r) = 1.06 x 10(5)) with and without added salt have been studied by small-angle X-ray scattering (SAXS). The DNA concentration ranged from 30 to 182 mg/mL. The scattering intensity of salt-free solutions displays a broad maximum at scattering vector q(m), indicative of solution structure caused by the electrostatic and excluded-volume interactions between charged DNA fragments. The peak vanishes when enough salt is added that the screened coulomb potential becomes shorter range than the interparticle distance. q(m) scales with the DNA concentrations as C1/2. This accords with scaling theory and similar predictions and agrees with SAXS and small-angle neutron scattering results obtained by others on semidilute solutions of semiflexible synthetic polyelectrolytes. When account is taken of the expanded effective DNA diameter in no-salt or low-salt solutions, these solutions have number densities-rho above the limit (pi-d(eff)L2/4)rho > 4.48 predicted by Onsager to lead to a transition to an anisotropic, liquid crystalline phase. The peak spacing under these conditions is consistent with hexagonally aligned arrays. However, we observe no evidence of anisotropy. This may be confirmation of the suggestion by Stroobants et al. (1986) that twisting forces between charged rods (for which the crossed orientation has a lower energy than the parallel) raise the concentration required for the transition.