PRECIPITATION OF DNA BY POLYAMINES - A POLYELECTROLYTE BEHAVIOR

Conditions of double-stranded DNA precipitation by the polyamines sper midine and spermine have been determined experimentally and compared t o theoretical predictions. The influence of the concentrations of DNA and added monovalent salt, and of the DNA length has been investigated in a systematic manner. Three regimes of DNA concentrations are obser ved. We clarify the dependence of these regimes on the monovalent salt concentration and on the DNA length. Our observations make possible a rationalization of the experimental results reported in the literatur e. A comparison of the precipitation conditions of different kinds of polyelectrolytes suggests a general process. Our experimental data are compared to the ''ion-bridging'' model based on short-range electrost atic attractions. By starting from the spinodal equation, predicted by this model, and using the limiting form of Manning's fractions of con densed counterions, analytical expressions of the precipitation condit ions have been found in the three regimes. Experimental and theoretica l results are in good agreement.