Condensation of DNA by trivalent cations. 2. Effects of cation structure.

Electron microscopy is employed to examine DNA aggregates produced by three tripositively charged condensing agents. Spermidine, hexammine cobalt (III), and me8spermidine (in which the amine groups of spermidine are exhaustively methylated) all produce condensates. The predominant form of condensate observed is toroidal; however, me8spermidine produces a large fraction of rodlike condensates. Distributions of toroidal radii and estimated volumes suggest that the size of condensates depends on the condensing agent employed, its concentration, and the time elapsed after addition of condensing agent. While ligand charge seems to be the major factor in predicting condensing power, ligand structure influences the morphology and dimensions of the particles produced. The ability to form hydrogen bonds is not required to promote condensation, since me8spermidine has no NHs. There may be a kinetic barrier to condensation at low me8spermidine concentrations. The relative proportions of toroids and rods may depend on the energetic compensation between bending and binding in cyclic structures, or on rate-limiting formation of sharply bent or kinked regions in rods.