Precollapse of T7 DNA by spermidine at low ionic strength: A linear dichroism and intrinsic viscosity study

At low salt ([Na + ] = 10 −3 M ), spermidine is capable of transforming DNA from a highly extended random coil to a compact particle. The transition takes place at a spermidine concentration of around 25 μ M and the compact particle has been previously studied in considerable detail for several different DNAs. The objective of the present study is to see what effect, if any, spermidine has on T7 DNA conformation prior to collapse using flow dichroism and intrinsic viscosity. We conclude that increasing the spermidine concentration from 0 to the collapse transition point (above 20 μ M ) makes DNA increasingly nondraining. Furthermore, the persistence length dropped from 785 (±42) to 560 (±32) to 445 (±26) Å on increasing the ambient spermidine concentration from 0 to 1 to 10 μ M . These results are in good agreement with counterion condensation theory and Odijk's theory of the electrostatic contribution to the persistence length of DNA. Nonetheless, it is concluded that counterion condensation is not entirely responsible for DNA collapse and that crosslinking promotes the transition to the compact state.