Polyelectrolyte effects in DNA condensation by polyamines

The conditions required for the counterion induced collapse of T7 bacteriophage DNA are briefly reviewed. Using Manning's counterion condensation theory we calculate a striking unity among collapse conditions: collapse occurs when from 89% to 90% of the DNA phosphate charges are neutralized by condensed counterions. The forces involved in collapsed DNA are investigated with emphasis on electrostatic repulsion. It is concluded that polyelectrolyte repulsion is the dominant force opposing collapse. Comparison of the results or polyelectrolyte repulsion calculation made using numerical methods and the Poisson-Boltzmann equation, with values of the attractive energies due to London dispersion interactions, leads to the conclusion that dispersion forces are probably large enough to cause collapse when the repulsions have been reduced by the presence of multivalent counterions.