FIGURE 1.

Schematic representation of the phase diagram obtained in a previous study (Raspaud et al., 1988) for the precipitation of DNA fragments by spermine. The precipitation domain, where the dense precipitate separates from the dilute supernatant, is limited by the C_precip and C_redissol curves. In this representation, each experimental point is defined by the DNA concentration (C_{DNA phosphate}) and the spermine salt concentration (C_spermine). Experimental points were located along five lines: (1a) C_DNA = 0.03 mM; (1b) C_DNA = 0.6 mM; (1c) C_DNA = 3 mM; (2) C_DNA = 90 mM; and (3) at different monovalent salt concentrations. Two protocols were used to prepare the samples: 1), starting from spermine-DNA solution, water was added to dilute the sample, thus crossing the C_redissol threshold, as indicated by the top arrow; and 2), starting from Na-DNA solutions, spermine was added to induce the precipitation by crossing the C_precip threshold, as indicated by the bottom arrow. In Figs. 4–7, solid symbols refer to samples prepared from spermine-DNA (crossing the redissolution limit) and open symbols refer to samples prepared from Na-DNA (crossing the precipitation limit).