Light-scattering study of DNA condensation: competition between collapse and aggregation.

Light-scattering studies were done to investigate the DNA collapse transition, a large and discontinuous reduction in the radius of gyration. Of particular concern was differentiating the compaction of a single DNA molecule from aggregation. Solutions of RK2 plasmid DNA (Mr = 37 X lo6) or bacteriophage T7 DNA (Mr = 25 X lo6) were titrated with the condensing reagents spermidine in aqueous solvent or magnesium ion in ethanol-water solvent. The transition was followed by the change in scattering at a single angle or by the change in the angular dependence of scattering. At concentrations below 1 Fg/mL, only aggregation could be detected by observation a t a single angle; therefore, to study the collapse transition, it was necessary to measure the angular dependence of scattering. The intensities measured between the angles 30' and 60" were fit to known scattering functions. At low Concentrations of the condensing reagent, the data were consistent with the scattering function of a random coil. On the other hand, during the transition at higher reagent concentrations, the curve that fit the data required two components-the scattering function for a random coil with a large radius of gyration, plus that for a sphere with a radius about one-fifth of that of the coil. The fractional concentration of the sphere increased with increasing condensing-reagent concentration. This two-component behavior is in apparent contrast to the situation with a more flexible polymer such as polystyrene, in accord with theoretical predictions. At still higher reagent concentrations, aggregation was apparent. Condensation to a collapsed state was reversible without hysteresis, while dissolution of the aggregated state nearly always occurred with hysteresis. Qualitative agreement between the observed DNA collapse transition and the theoretical phase diagram presented in the preceding paper was found, although the light-scattering results did not show quantitative agreement with the simple theoretical model.