Marginal Nature of DNA Solutions

We adapt a scanning fluorescence correlation spectroscopy technique to measure the structure factor of complex fluid systems and present the first measurements of the structure of semidilute solutions of long DNA polymers. The measured structure factors exhibit screening effects which, as expected for semidilute polymer solutions, grow stronger with increasing DNA concentration c. The measured concentration dependence of the screening length xi proportional to c{0.53+/-0.02} is unusual, but can be understood within the framework of a marginal solutions theory for semiflexible polymers.