Fig. 15. Facilitation of purine-motif triplex formation by different polyamines. Shown are elec-trophoretic mobility shift assays of triplex formation by the oligonucleotide ODN1 in the presence of different polyamine concentrations. D, duplex DNA; S, single-stranded probe; T, triplex; W, probe DNA retained in the polyacrylamide gel well. (A) Spermine and Ws(ethyl)spermine. (B) 4444 and BE-4444. (Reprinted with permission from ref. 49. © 1997 American Chemical Society.)

Schellman (52) reported that the addition of spermidine to DNA resulted in the formation of small, roughly spherical particles. Electron microscopic examination showed the presence of rods, toroids, and spheroids when DNA was condensed by polyamines. Wilson and Bloomfield (53) applied the counterion condensation theory to calculate the amount of charge neutralization necessary to collapse DNA to nanoparticles, and found that 89-90% of the negative phosphate charges are neutralized by spermidine and spermine in aqueous medium. They also found that at least a trivalent cation is required for DNA condensation, and that mono and divalent ions effectively competed with multi-valent ions to prevent DNA condensation or decondense compacted DNA.

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