Ionic and structural effects on the thermal helix‐coil transition of DNA complexed with natural and synthetic polyamines

Polyamines are ubiquitous cellular components that interacts strongly with nucleic acids. Although many of the interactions of oligocations with DNA can be rationalized with polyelectrolyte theories that treat counterions as point charges, some structural effects are evident. We have explored the effects of polyamine structure on one important aspect of DNA behavior, its thermal melting transition, by using a series of spermidine analogs NH₃(CH₂)₃NH₂(CH₂)_nNH 33+, where n varies from 2 to 8 [Jorstad et al. (1980) J. Bacteriol. 141, 456–463]. For spermidine itself, n = 4. T_m for calf‐thymus DNA in the presence of each of these analogs, and the other naturally occurring polyamines putrescine²⁺ and spermine⁴⁺, was measured over a wide range of NaCl concentrations and polyamine:DNA phosphate ratios. There are modest, but significant structural effects. particularly with the shorter n = 2 and 3 derivatives, whose geometry my not allow full electrostatic interaction with DNA. Longer analogs, on the other hand, are not much different than spermidine in their effects on T_m, though a moderate maximum occurs at n = 5. Since polyamines are important in the cellular condensation and packaging of DNA, we have also delineated the critical polyamine and salt concentrations that are required to cause DNA aggregation. Here again, there are significant structural effects, which are not easily rationalized by any simple considerations.