PARTIAL STRUCTURE FUNCTIONS OF DNA FRAGMENT SOLUTIONS - CONCENTRATIONEFFECTS

The partial and charge structure functions of 163 base-pairs DNA fragm ent solutions are experimentally determined by small-angle neutron sca ttering at a relatively low concentration of 0.05 M nucleotides/L. In the momentum transfer range q > 0.05 Angstrom(-1), the data agree with theoretical model calculations according to the cell model together w ith a distribution for the counterions around the rodlike DNA fragment obtained from the Poisson-Boltzmann equation. For q > 0.075 Angstrom( -1), the scattered intensities agree with previous results on similar, but higher concentrated (i.e., 0.1 M nucleotides/L) solutions [van de r Maarel; et al. J. Phys. II (France) 1992, 2, 109]. Unlike the previo us data, the present results can be compared with the theoretical curv es up to sufficiently low q values to check the theoretically expected low q upward curvature of the monomer-counterion and the counterion-c ounterion structure functions. A shell-like step radial counterion dis tribution as well as a model in which the condensed counterions are un iformly distributed within a cylindrical volume including the DNA mole cule are checked and found to be in disagreement with the experimental data.