We study semidilute and concentrated solutions of mononucleosomal DNA at three different NaCl concentrations by static and dynamic light scattering, viscosity, and electron cryomicroscopy. At low enough DNA concentrations the second virial coefficient behaves in the usual manner and can be interpreted by a charged rod model. It is possible to understand the concentration and scattering vector dependence of the scattering with the help of recent theoretical analyses of semidilute solutions of charged rods. Moreover, the mutual friction coefficient is in accord with the theory of hydrodynamic screening. At a certain critical concentration which increases with added salt, the intensity of the equilibrium static scattering increases several 100-fold, indicating the DNA aggregates. The dynamic scattering is in line with the static scattering; a very long decay time seems to be associated with the DNA aggregates. Freeze electron micrographs definitely bear out the existence of DNA globules which appear to form loose aggregates. Precautions have been taken to ensure there are no spurious contaminants to the best of our knowledge. Long-range attractive forces between polyions have been postulated recently within the framework of a semiquantitative theory; these ideas are tested by analyzing the ionic-strength dependence of the third virial coefficient and the onset of aggregation. IntroductionFor some time, double-stranded DNA has been suspected of aggregating under conditions where this is unlikely to occur from a balance between electrostatic and dispersion forces. Mandelkern et a1.l concluded from their rotational relaxation times in solutions of sonicated DNA at low salt that DNA may aggregate into bundles containing seven DNA rods although they neglected to account for ionic friction. In the same year Fulmer et a1.2 measured strong slow decays in the dynamic light scattering by DNA suspensions below M simple salt. Sonicated calf-thymus DNA actually gels without liquid-crystalline order at nondilute concentrations, as has been established rheometrially.^!^ By monitoring the steady-state fluorescence polarization, Hard and Kearns5 deduced that monodisperse DNA must start to aggregate at a DNA concentration of 5 g/L in 1 M NaC1. Nicolai and MandeF thought DNA might be aggregating at low salt and nondilute DNA concentrations because the static scattering started to increase once the DNA solution was allowed to remain quiescent after filtration. Contrastingly, the scattering intensity of a flowing suspension was constant: presumably, the aggregates break up

Introduction. From a physical point of view, semiflexible polymer chains are interesting because the persistence segments have a large aspect ratio. For this reason, the interaction between two segments may be sensitive to molecular detail and the orientational and translational degrees of freedom are strongly coupled especially at high concentrations. Double-stranded DNA is an important prototype. In addition, one hopes that physicochemical insight into the behavior of concentrated solutions of DNA will lead, in the long run, to some understanding of complex biophysical phenomena.1We had several straightforward reasons for starting a comprehensive light scattering investigation of concentrated aqueous solutions of mononucleosomal DNA (i.e., concentrations of about 50 g/L up to the onset of the isotropic-to-cholesteric transition). Several earlier studies simply ended at lower concentrations2-4 although other techniques have been employed in the isotropic concentrated regime.5'6 Next, we wanted to test theoretical predictions for the statics and dynamics of charged rods7'8 and to uncover the import of pretransitional phenomena, if any. A final reason was to discover whether attractive forces may be discernible at high ionic strengths, as is the case for xanthan solutions.9 However, in the course of our studies, the scattering curves turned out to deviate so spectacularly from what we expected that we switched our focus to a possible aggregation phenomenon not discussed before. This paper highlights several preliminary investigations of this effect.

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