FIGURE 10.

Pictorial representation of spermine-DNA binding and collapse. (A) Electrostatic repulsion between helices is predominant at lower spermine concentrations, which prevents close approach of multiple plasmids, (B) a partially bound spermine molecule can bring 2 or more helices into close contact by reducing this repulsion and forming intermolecular bridges, and (C) at higher spermine concentrations, multiple helices collapse together. Such collapsed helices then form multimolecular “cords” packed with spermine molecules, which are then recruited into toroids.